remove lib/libsqlite3, it has moved back to ports

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diff --git a/lib/libsqlite3/src/wal.c b/lib/libsqlite3/src/wal.c
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--- a/lib/libsqlite3/src/wal.c
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-/*
-** 2010 February 1
-**
-** The author disclaims copyright to this source code.  In place of
-** a legal notice, here is a blessing:
-**
-**    May you do good and not evil.
-**    May you find forgiveness for yourself and forgive others.
-**    May you share freely, never taking more than you give.
-**
-*************************************************************************
-**
-** This file contains the implementation of a write-ahead log (WAL) used in 
-** "journal_mode=WAL" mode.
-**
-** WRITE-AHEAD LOG (WAL) FILE FORMAT
-**
-** A WAL file consists of a header followed by zero or more "frames".
-** Each frame records the revised content of a single page from the
-** database file.  All changes to the database are recorded by writing
-** frames into the WAL.  Transactions commit when a frame is written that
-** contains a commit marker.  A single WAL can and usually does record 
-** multiple transactions.  Periodically, the content of the WAL is
-** transferred back into the database file in an operation called a
-** "checkpoint".
-**
-** A single WAL file can be used multiple times.  In other words, the
-** WAL can fill up with frames and then be checkpointed and then new
-** frames can overwrite the old ones.  A WAL always grows from beginning
-** toward the end.  Checksums and counters attached to each frame are
-** used to determine which frames within the WAL are valid and which
-** are leftovers from prior checkpoints.
-**
-** The WAL header is 32 bytes in size and consists of the following eight
-** big-endian 32-bit unsigned integer values:
-**
-**     0: Magic number.  0x377f0682 or 0x377f0683
-**     4: File format version.  Currently 3007000
-**     8: Database page size.  Example: 1024
-**    12: Checkpoint sequence number
-**    16: Salt-1, random integer incremented with each checkpoint
-**    20: Salt-2, a different random integer changing with each ckpt
-**    24: Checksum-1 (first part of checksum for first 24 bytes of header).
-**    28: Checksum-2 (second part of checksum for first 24 bytes of header).
-**
-** Immediately following the wal-header are zero or more frames. Each
-** frame consists of a 24-byte frame-header followed by a <page-size> bytes
-** of page data. The frame-header is six big-endian 32-bit unsigned 
-** integer values, as follows:
-**
-**     0: Page number.
-**     4: For commit records, the size of the database image in pages 
-**        after the commit. For all other records, zero.
-**     8: Salt-1 (copied from the header)
-**    12: Salt-2 (copied from the header)
-**    16: Checksum-1.
-**    20: Checksum-2.
-**
-** A frame is considered valid if and only if the following conditions are
-** true:
-**
-**    (1) The salt-1 and salt-2 values in the frame-header match
-**        salt values in the wal-header
-**
-**    (2) The checksum values in the final 8 bytes of the frame-header
-**        exactly match the checksum computed consecutively on the
-**        WAL header and the first 8 bytes and the content of all frames
-**        up to and including the current frame.
-**
-** The checksum is computed using 32-bit big-endian integers if the
-** magic number in the first 4 bytes of the WAL is 0x377f0683 and it
-** is computed using little-endian if the magic number is 0x377f0682.
-** The checksum values are always stored in the frame header in a
-** big-endian format regardless of which byte order is used to compute
-** the checksum.  The checksum is computed by interpreting the input as
-** an even number of unsigned 32-bit integers: x[0] through x[N].  The
-** algorithm used for the checksum is as follows:
-** 
-**   for i from 0 to n-1 step 2:
-**     s0 += x[i] + s1;
-**     s1 += x[i+1] + s0;
-**   endfor
-**
-** Note that s0 and s1 are both weighted checksums using fibonacci weights
-** in reverse order (the largest fibonacci weight occurs on the first element
-** of the sequence being summed.)  The s1 value spans all 32-bit 
-** terms of the sequence whereas s0 omits the final term.
-**
-** On a checkpoint, the WAL is first VFS.xSync-ed, then valid content of the
-** WAL is transferred into the database, then the database is VFS.xSync-ed.
-** The VFS.xSync operations serve as write barriers - all writes launched
-** before the xSync must complete before any write that launches after the
-** xSync begins.
-**
-** After each checkpoint, the salt-1 value is incremented and the salt-2
-** value is randomized.  This prevents old and new frames in the WAL from
-** being considered valid at the same time and being checkpointing together
-** following a crash.
-**
-** READER ALGORITHM
-**
-** To read a page from the database (call it page number P), a reader
-** first checks the WAL to see if it contains page P.  If so, then the
-** last valid instance of page P that is a followed by a commit frame
-** or is a commit frame itself becomes the value read.  If the WAL
-** contains no copies of page P that are valid and which are a commit
-** frame or are followed by a commit frame, then page P is read from
-** the database file.
-**
-** To start a read transaction, the reader records the index of the last
-** valid frame in the WAL.  The reader uses this recorded "mxFrame" value
-** for all subsequent read operations.  New transactions can be appended
-** to the WAL, but as long as the reader uses its original mxFrame value
-** and ignores the newly appended content, it will see a consistent snapshot
-** of the database from a single point in time.  This technique allows
-** multiple concurrent readers to view different versions of the database
-** content simultaneously.
-**
-** The reader algorithm in the previous paragraphs works correctly, but 
-** because frames for page P can appear anywhere within the WAL, the
-** reader has to scan the entire WAL looking for page P frames.  If the
-** WAL is large (multiple megabytes is typical) that scan can be slow,
-** and read performance suffers.  To overcome this problem, a separate
-** data structure called the wal-index is maintained to expedite the
-** search for frames of a particular page.
-** 
-** WAL-INDEX FORMAT
-**
-** Conceptually, the wal-index is shared memory, though VFS implementations
-** might choose to implement the wal-index using a mmapped file.  Because
-** the wal-index is shared memory, SQLite does not support journal_mode=WAL 
-** on a network filesystem.  All users of the database must be able to
-** share memory.
-**
-** The wal-index is transient.  After a crash, the wal-index can (and should
-** be) reconstructed from the original WAL file.  In fact, the VFS is required
-** to either truncate or zero the header of the wal-index when the last
-** connection to it closes.  Because the wal-index is transient, it can
-** use an architecture-specific format; it does not have to be cross-platform.
-** Hence, unlike the database and WAL file formats which store all values
-** as big endian, the wal-index can store multi-byte values in the native
-** byte order of the host computer.
-**
-** The purpose of the wal-index is to answer this question quickly:  Given
-** a page number P and a maximum frame index M, return the index of the 
-** last frame in the wal before frame M for page P in the WAL, or return
-** NULL if there are no frames for page P in the WAL prior to M.
-**
-** The wal-index consists of a header region, followed by an one or
-** more index blocks.  
-**
-** The wal-index header contains the total number of frames within the WAL
-** in the mxFrame field.
-**
-** Each index block except for the first contains information on 
-** HASHTABLE_NPAGE frames. The first index block contains information on
-** HASHTABLE_NPAGE_ONE frames. The values of HASHTABLE_NPAGE_ONE and 
-** HASHTABLE_NPAGE are selected so that together the wal-index header and
-** first index block are the same size as all other index blocks in the
-** wal-index.
-**
-** Each index block contains two sections, a page-mapping that contains the
-** database page number associated with each wal frame, and a hash-table 
-** that allows readers to query an index block for a specific page number.
-** The page-mapping is an array of HASHTABLE_NPAGE (or HASHTABLE_NPAGE_ONE
-** for the first index block) 32-bit page numbers. The first entry in the 
-** first index-block contains the database page number corresponding to the
-** first frame in the WAL file. The first entry in the second index block
-** in the WAL file corresponds to the (HASHTABLE_NPAGE_ONE+1)th frame in
-** the log, and so on.
-**
-** The last index block in a wal-index usually contains less than the full
-** complement of HASHTABLE_NPAGE (or HASHTABLE_NPAGE_ONE) page-numbers,
-** depending on the contents of the WAL file. This does not change the
-** allocated size of the page-mapping array - the page-mapping array merely
-** contains unused entries.
-**
-** Even without using the hash table, the last frame for page P
-** can be found by scanning the page-mapping sections of each index block
-** starting with the last index block and moving toward the first, and
-** within each index block, starting at the end and moving toward the
-** beginning.  The first entry that equals P corresponds to the frame
-** holding the content for that page.
-**
-** The hash table consists of HASHTABLE_NSLOT 16-bit unsigned integers.
-** HASHTABLE_NSLOT = 2*HASHTABLE_NPAGE, and there is one entry in the
-** hash table for each page number in the mapping section, so the hash 
-** table is never more than half full.  The expected number of collisions 
-** prior to finding a match is 1.  Each entry of the hash table is an
-** 1-based index of an entry in the mapping section of the same
-** index block.   Let K be the 1-based index of the largest entry in
-** the mapping section.  (For index blocks other than the last, K will
-** always be exactly HASHTABLE_NPAGE (4096) and for the last index block
-** K will be (mxFrame%HASHTABLE_NPAGE).)  Unused slots of the hash table
-** contain a value of 0.
-**
-** To look for page P in the hash table, first compute a hash iKey on
-** P as follows:
-**
-**      iKey = (P * 383) % HASHTABLE_NSLOT
-**
-** Then start scanning entries of the hash table, starting with iKey
-** (wrapping around to the beginning when the end of the hash table is
-** reached) until an unused hash slot is found. Let the first unused slot
-** be at index iUnused.  (iUnused might be less than iKey if there was
-** wrap-around.) Because the hash table is never more than half full,
-** the search is guaranteed to eventually hit an unused entry.  Let 
-** iMax be the value between iKey and iUnused, closest to iUnused,
-** where aHash[iMax]==P.  If there is no iMax entry (if there exists
-** no hash slot such that aHash[i]==p) then page P is not in the
-** current index block.  Otherwise the iMax-th mapping entry of the
-** current index block corresponds to the last entry that references 
-** page P.
-**
-** A hash search begins with the last index block and moves toward the
-** first index block, looking for entries corresponding to page P.  On
-** average, only two or three slots in each index block need to be
-** examined in order to either find the last entry for page P, or to
-** establish that no such entry exists in the block.  Each index block
-** holds over 4000 entries.  So two or three index blocks are sufficient
-** to cover a typical 10 megabyte WAL file, assuming 1K pages.  8 or 10
-** comparisons (on average) suffice to either locate a frame in the
-** WAL or to establish that the frame does not exist in the WAL.  This
-** is much faster than scanning the entire 10MB WAL.
-**
-** Note that entries are added in order of increasing K.  Hence, one
-** reader might be using some value K0 and a second reader that started
-** at a later time (after additional transactions were added to the WAL
-** and to the wal-index) might be using a different value K1, where K1>K0.
-** Both readers can use the same hash table and mapping section to get
-** the correct result.  There may be entries in the hash table with
-** K>K0 but to the first reader, those entries will appear to be unused
-** slots in the hash table and so the first reader will get an answer as
-** if no values greater than K0 had ever been inserted into the hash table
-** in the first place - which is what reader one wants.  Meanwhile, the
-** second reader using K1 will see additional values that were inserted
-** later, which is exactly what reader two wants.  
-**
-** When a rollback occurs, the value of K is decreased. Hash table entries
-** that correspond to frames greater than the new K value are removed
-** from the hash table at this point.
-*/
-#ifndef SQLITE_OMIT_WAL
-
-#include "wal.h"
-
-/*
-** Trace output macros
-*/
-#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
-int sqlite3WalTrace = 0;
-# define WALTRACE(X)  if(sqlite3WalTrace) sqlite3DebugPrintf X
-#else
-# define WALTRACE(X)
-#endif
-
-/*
-** The maximum (and only) versions of the wal and wal-index formats
-** that may be interpreted by this version of SQLite.
-**
-** If a client begins recovering a WAL file and finds that (a) the checksum
-** values in the wal-header are correct and (b) the version field is not
-** WAL_MAX_VERSION, recovery fails and SQLite returns SQLITE_CANTOPEN.
-**
-** Similarly, if a client successfully reads a wal-index header (i.e. the 
-** checksum test is successful) and finds that the version field is not
-** WALINDEX_MAX_VERSION, then no read-transaction is opened and SQLite
-** returns SQLITE_CANTOPEN.
-*/
-#define WAL_MAX_VERSION      3007000
-#define WALINDEX_MAX_VERSION 3007000
-
-/*
-** Indices of various locking bytes.   WAL_NREADER is the number
-** of available reader locks and should be at least 3.
-*/
-#define WAL_WRITE_LOCK         0
-#define WAL_ALL_BUT_WRITE      1
-#define WAL_CKPT_LOCK          1
-#define WAL_RECOVER_LOCK       2
-#define WAL_READ_LOCK(I)       (3+(I))
-#define WAL_NREADER            (SQLITE_SHM_NLOCK-3)
-
-
-/* Object declarations */
-typedef struct WalIndexHdr WalIndexHdr;
-typedef struct WalIterator WalIterator;
-typedef struct WalCkptInfo WalCkptInfo;
-
-
-/*
-** The following object holds a copy of the wal-index header content.
-**
-** The actual header in the wal-index consists of two copies of this
-** object.
-**
-** The szPage value can be any power of 2 between 512 and 32768, inclusive.
-** Or it can be 1 to represent a 65536-byte page.  The latter case was
-** added in 3.7.1 when support for 64K pages was added.  
-*/
-struct WalIndexHdr {
-  u32 iVersion;                   /* Wal-index version */
-  u32 unused;                     /* Unused (padding) field */
-  u32 iChange;                    /* Counter incremented each transaction */
-  u8 isInit;                      /* 1 when initialized */
-  u8 bigEndCksum;                 /* True if checksums in WAL are big-endian */
-  u16 szPage;                     /* Database page size in bytes. 1==64K */
-  u32 mxFrame;                    /* Index of last valid frame in the WAL */
-  u32 nPage;                      /* Size of database in pages */
-  u32 aFrameCksum[2];             /* Checksum of last frame in log */
-  u32 aSalt[2];                   /* Two salt values copied from WAL header */
-  u32 aCksum[2];                  /* Checksum over all prior fields */
-};
-
-/*
-** A copy of the following object occurs in the wal-index immediately
-** following the second copy of the WalIndexHdr.  This object stores
-** information used by checkpoint.
-**
-** nBackfill is the number of frames in the WAL that have been written
-** back into the database. (We call the act of moving content from WAL to
-** database "backfilling".)  The nBackfill number is never greater than
-** WalIndexHdr.mxFrame.  nBackfill can only be increased by threads
-** holding the WAL_CKPT_LOCK lock (which includes a recovery thread).
-** However, a WAL_WRITE_LOCK thread can move the value of nBackfill from
-** mxFrame back to zero when the WAL is reset.
-**
-** There is one entry in aReadMark[] for each reader lock.  If a reader
-** holds read-lock K, then the value in aReadMark[K] is no greater than
-** the mxFrame for that reader.  The value READMARK_NOT_USED (0xffffffff)
-** for any aReadMark[] means that entry is unused.  aReadMark[0] is 
-** a special case; its value is never used and it exists as a place-holder
-** to avoid having to offset aReadMark[] indexs by one.  Readers holding
-** WAL_READ_LOCK(0) always ignore the entire WAL and read all content
-** directly from the database.
-**
-** The value of aReadMark[K] may only be changed by a thread that
-** is holding an exclusive lock on WAL_READ_LOCK(K).  Thus, the value of
-** aReadMark[K] cannot changed while there is a reader is using that mark
-** since the reader will be holding a shared lock on WAL_READ_LOCK(K).
-**
-** The checkpointer may only transfer frames from WAL to database where
-** the frame numbers are less than or equal to every aReadMark[] that is
-** in use (that is, every aReadMark[j] for which there is a corresponding
-** WAL_READ_LOCK(j)).  New readers (usually) pick the aReadMark[] with the
-** largest value and will increase an unused aReadMark[] to mxFrame if there
-** is not already an aReadMark[] equal to mxFrame.  The exception to the
-** previous sentence is when nBackfill equals mxFrame (meaning that everything
-** in the WAL has been backfilled into the database) then new readers
-** will choose aReadMark[0] which has value 0 and hence such reader will
-** get all their all content directly from the database file and ignore 
-** the WAL.
-**
-** Writers normally append new frames to the end of the WAL.  However,
-** if nBackfill equals mxFrame (meaning that all WAL content has been
-** written back into the database) and if no readers are using the WAL
-** (in other words, if there are no WAL_READ_LOCK(i) where i>0) then
-** the writer will first "reset" the WAL back to the beginning and start
-** writing new content beginning at frame 1.
-**
-** We assume that 32-bit loads are atomic and so no locks are needed in
-** order to read from any aReadMark[] entries.
-*/
-struct WalCkptInfo {
-  u32 nBackfill;                  /* Number of WAL frames backfilled into DB */
-  u32 aReadMark[WAL_NREADER];     /* Reader marks */
-};
-#define READMARK_NOT_USED  0xffffffff
-
-
-/* A block of WALINDEX_LOCK_RESERVED bytes beginning at
-** WALINDEX_LOCK_OFFSET is reserved for locks. Since some systems
-** only support mandatory file-locks, we do not read or write data
-** from the region of the file on which locks are applied.
-*/
-#define WALINDEX_LOCK_OFFSET   (sizeof(WalIndexHdr)*2 + sizeof(WalCkptInfo))
-#define WALINDEX_LOCK_RESERVED 16
-#define WALINDEX_HDR_SIZE      (WALINDEX_LOCK_OFFSET+WALINDEX_LOCK_RESERVED)
-
-/* Size of header before each frame in wal */
-#define WAL_FRAME_HDRSIZE 24
-
-/* Size of write ahead log header, including checksum. */
-/* #define WAL_HDRSIZE 24 */
-#define WAL_HDRSIZE 32
-
-/* WAL magic value. Either this value, or the same value with the least
-** significant bit also set (WAL_MAGIC | 0x00000001) is stored in 32-bit
-** big-endian format in the first 4 bytes of a WAL file.
-**
-** If the LSB is set, then the checksums for each frame within the WAL
-** file are calculated by treating all data as an array of 32-bit 
-** big-endian words. Otherwise, they are calculated by interpreting 
-** all data as 32-bit little-endian words.
-*/
-#define WAL_MAGIC 0x377f0682
-
-/*
-** Return the offset of frame iFrame in the write-ahead log file, 
-** assuming a database page size of szPage bytes. The offset returned
-** is to the start of the write-ahead log frame-header.
-*/
-#define walFrameOffset(iFrame, szPage) (                               \
-  WAL_HDRSIZE + ((iFrame)-1)*(i64)((szPage)+WAL_FRAME_HDRSIZE)         \
-)
-
-/*
-** An open write-ahead log file is represented by an instance of the
-** following object.
-*/
-struct Wal {
-  sqlite3_vfs *pVfs;         /* The VFS used to create pDbFd */
-  sqlite3_file *pDbFd;       /* File handle for the database file */
-  sqlite3_file *pWalFd;      /* File handle for WAL file */
-  u32 iCallback;             /* Value to pass to log callback (or 0) */
-  i64 mxWalSize;             /* Truncate WAL to this size upon reset */
-  int nWiData;               /* Size of array apWiData */
-  int szFirstBlock;          /* Size of first block written to WAL file */
-  volatile u32 **apWiData;   /* Pointer to wal-index content in memory */
-  u32 szPage;                /* Database page size */
-  i16 readLock;              /* Which read lock is being held.  -1 for none */
-  u8 syncFlags;              /* Flags to use to sync header writes */
-  u8 exclusiveMode;          /* Non-zero if connection is in exclusive mode */
-  u8 writeLock;              /* True if in a write transaction */
-  u8 ckptLock;               /* True if holding a checkpoint lock */
-  u8 readOnly;               /* WAL_RDWR, WAL_RDONLY, or WAL_SHM_RDONLY */
-  u8 truncateOnCommit;       /* True to truncate WAL file on commit */
-  u8 syncHeader;             /* Fsync the WAL header if true */
-  u8 padToSectorBoundary;    /* Pad transactions out to the next sector */
-  WalIndexHdr hdr;           /* Wal-index header for current transaction */
-  u32 minFrame;              /* Ignore wal frames before this one */
-  const char *zWalName;      /* Name of WAL file */
-  u32 nCkpt;                 /* Checkpoint sequence counter in the wal-header */
-#ifdef SQLITE_DEBUG
-  u8 lockError;              /* True if a locking error has occurred */
-#endif
-};
-
-/*
-** Candidate values for Wal.exclusiveMode.
-*/
-#define WAL_NORMAL_MODE     0
-#define WAL_EXCLUSIVE_MODE  1     
-#define WAL_HEAPMEMORY_MODE 2
-
-/*
-** Possible values for WAL.readOnly
-*/
-#define WAL_RDWR        0    /* Normal read/write connection */
-#define WAL_RDONLY      1    /* The WAL file is readonly */
-#define WAL_SHM_RDONLY  2    /* The SHM file is readonly */
-
-/*
-** Each page of the wal-index mapping contains a hash-table made up of
-** an array of HASHTABLE_NSLOT elements of the following type.
-*/
-typedef u16 ht_slot;
-
-/*
-** This structure is used to implement an iterator that loops through
-** all frames in the WAL in database page order. Where two or more frames
-** correspond to the same database page, the iterator visits only the 
-** frame most recently written to the WAL (in other words, the frame with
-** the largest index).
-**
-** The internals of this structure are only accessed by:
-**
-**   walIteratorInit() - Create a new iterator,
-**   walIteratorNext() - Step an iterator,
-**   walIteratorFree() - Free an iterator.
-**
-** This functionality is used by the checkpoint code (see walCheckpoint()).
-*/
-struct WalIterator {
-  int iPrior;                     /* Last result returned from the iterator */
-  int nSegment;                   /* Number of entries in aSegment[] */
-  struct WalSegment {
-    int iNext;                    /* Next slot in aIndex[] not yet returned */
-    ht_slot *aIndex;              /* i0, i1, i2... such that aPgno[iN] ascend */
-    u32 *aPgno;                   /* Array of page numbers. */
-    int nEntry;                   /* Nr. of entries in aPgno[] and aIndex[] */
-    int iZero;                    /* Frame number associated with aPgno[0] */
-  } aSegment[1];                  /* One for every 32KB page in the wal-index */
-};
-
-/*
-** Define the parameters of the hash tables in the wal-index file. There
-** is a hash-table following every HASHTABLE_NPAGE page numbers in the
-** wal-index.
-**
-** Changing any of these constants will alter the wal-index format and
-** create incompatibilities.
-*/
-#define HASHTABLE_NPAGE      4096                 /* Must be power of 2 */
-#define HASHTABLE_HASH_1     383                  /* Should be prime */
-#define HASHTABLE_NSLOT      (HASHTABLE_NPAGE*2)  /* Must be a power of 2 */
-
-/* 
-** The block of page numbers associated with the first hash-table in a
-** wal-index is smaller than usual. This is so that there is a complete
-** hash-table on each aligned 32KB page of the wal-index.
-*/
-#define HASHTABLE_NPAGE_ONE  (HASHTABLE_NPAGE - (WALINDEX_HDR_SIZE/sizeof(u32)))
-
-/* The wal-index is divided into pages of WALINDEX_PGSZ bytes each. */
-#define WALINDEX_PGSZ   (                                         \
-    sizeof(ht_slot)*HASHTABLE_NSLOT + HASHTABLE_NPAGE*sizeof(u32) \
-)
-
-/*
-** Obtain a pointer to the iPage'th page of the wal-index. The wal-index
-** is broken into pages of WALINDEX_PGSZ bytes. Wal-index pages are
-** numbered from zero.
-**
-** If this call is successful, *ppPage is set to point to the wal-index
-** page and SQLITE_OK is returned. If an error (an OOM or VFS error) occurs,
-** then an SQLite error code is returned and *ppPage is set to 0.
-*/
-static int walIndexPage(Wal *pWal, int iPage, volatile u32 **ppPage){
-  int rc = SQLITE_OK;
-
-  /* Enlarge the pWal->apWiData[] array if required */
-  if( pWal->nWiData<=iPage ){
-    int nByte = sizeof(u32*)*(iPage+1);
-    volatile u32 **apNew;
-    apNew = (volatile u32 **)sqlite3_realloc64((void *)pWal->apWiData, nByte);
-    if( !apNew ){
-      *ppPage = 0;
-      return SQLITE_NOMEM;
-    }
-    memset((void*)&apNew[pWal->nWiData], 0,
-           sizeof(u32*)*(iPage+1-pWal->nWiData));
-    pWal->apWiData = apNew;
-    pWal->nWiData = iPage+1;
-  }
-
-  /* Request a pointer to the required page from the VFS */
-  if( pWal->apWiData[iPage]==0 ){
-    if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE ){
-      pWal->apWiData[iPage] = (u32 volatile *)sqlite3MallocZero(WALINDEX_PGSZ);
-      if( !pWal->apWiData[iPage] ) rc = SQLITE_NOMEM;
-    }else{
-      rc = sqlite3OsShmMap(pWal->pDbFd, iPage, WALINDEX_PGSZ, 
-          pWal->writeLock, (void volatile **)&pWal->apWiData[iPage]
-      );
-      if( rc==SQLITE_READONLY ){
-        pWal->readOnly |= WAL_SHM_RDONLY;
-        rc = SQLITE_OK;
-      }
-    }
-  }
-
-  *ppPage = pWal->apWiData[iPage];
-  assert( iPage==0 || *ppPage || rc!=SQLITE_OK );
-  return rc;
-}
-
-/*
-** Return a pointer to the WalCkptInfo structure in the wal-index.
-*/
-static volatile WalCkptInfo *walCkptInfo(Wal *pWal){
-  assert( pWal->nWiData>0 && pWal->apWiData[0] );
-  return (volatile WalCkptInfo*)&(pWal->apWiData[0][sizeof(WalIndexHdr)/2]);
-}
-
-/*
-** Return a pointer to the WalIndexHdr structure in the wal-index.
-*/
-static volatile WalIndexHdr *walIndexHdr(Wal *pWal){
-  assert( pWal->nWiData>0 && pWal->apWiData[0] );
-  return (volatile WalIndexHdr*)pWal->apWiData[0];
-}
-
-/*
-** The argument to this macro must be of type u32. On a little-endian
-** architecture, it returns the u32 value that results from interpreting
-** the 4 bytes as a big-endian value. On a big-endian architecture, it
-** returns the value that would be produced by interpreting the 4 bytes
-** of the input value as a little-endian integer.
-*/
-#define BYTESWAP32(x) ( \
-    (((x)&0x000000FF)<<24) + (((x)&0x0000FF00)<<8)  \
-  + (((x)&0x00FF0000)>>8)  + (((x)&0xFF000000)>>24) \
-)
-
-/*
-** Generate or extend an 8 byte checksum based on the data in 
-** array aByte[] and the initial values of aIn[0] and aIn[1] (or
-** initial values of 0 and 0 if aIn==NULL).
-**
-** The checksum is written back into aOut[] before returning.
-**
-** nByte must be a positive multiple of 8.
-*/
-static void walChecksumBytes(
-  int nativeCksum, /* True for native byte-order, false for non-native */
-  u8 *a,           /* Content to be checksummed */
-  int nByte,       /* Bytes of content in a[].  Must be a multiple of 8. */
-  const u32 *aIn,  /* Initial checksum value input */
-  u32 *aOut        /* OUT: Final checksum value output */
-){
-  u32 s1, s2;
-  u32 *aData = (u32 *)a;
-  u32 *aEnd = (u32 *)&a[nByte];
-
-  if( aIn ){
-    s1 = aIn[0];
-    s2 = aIn[1];
-  }else{
-    s1 = s2 = 0;
-  }
-
-  assert( nByte>=8 );
-  assert( (nByte&0x00000007)==0 );
-
-  if( nativeCksum ){
-    do {
-      s1 += *aData++ + s2;
-      s2 += *aData++ + s1;
-    }while( aData<aEnd );
-  }else{
-    do {
-      s1 += BYTESWAP32(aData[0]) + s2;
-      s2 += BYTESWAP32(aData[1]) + s1;
-      aData += 2;
-    }while( aData<aEnd );
-  }
-
-  aOut[0] = s1;
-  aOut[1] = s2;
-}
-
-static void walShmBarrier(Wal *pWal){
-  if( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE ){
-    sqlite3OsShmBarrier(pWal->pDbFd);
-  }
-}
-
-/*
-** Write the header information in pWal->hdr into the wal-index.
-**
-** The checksum on pWal->hdr is updated before it is written.
-*/
-static void walIndexWriteHdr(Wal *pWal){
-  volatile WalIndexHdr *aHdr = walIndexHdr(pWal);
-  const int nCksum = offsetof(WalIndexHdr, aCksum);
-
-  assert( pWal->writeLock );
-  pWal->hdr.isInit = 1;
-  pWal->hdr.iVersion = WALINDEX_MAX_VERSION;
-  walChecksumBytes(1, (u8*)&pWal->hdr, nCksum, 0, pWal->hdr.aCksum);
-  memcpy((void*)&aHdr[1], (const void*)&pWal->hdr, sizeof(WalIndexHdr));
-  walShmBarrier(pWal);
-  memcpy((void*)&aHdr[0], (const void*)&pWal->hdr, sizeof(WalIndexHdr));
-}
-
-/*
-** This function encodes a single frame header and writes it to a buffer
-** supplied by the caller. A frame-header is made up of a series of 
-** 4-byte big-endian integers, as follows:
-**
-**     0: Page number.
-**     4: For commit records, the size of the database image in pages 
-**        after the commit. For all other records, zero.
-**     8: Salt-1 (copied from the wal-header)
-**    12: Salt-2 (copied from the wal-header)
-**    16: Checksum-1.
-**    20: Checksum-2.
-*/
-static void walEncodeFrame(
-  Wal *pWal,                      /* The write-ahead log */
-  u32 iPage,                      /* Database page number for frame */
-  u32 nTruncate,                  /* New db size (or 0 for non-commit frames) */
-  u8 *aData,                      /* Pointer to page data */
-  u8 *aFrame                      /* OUT: Write encoded frame here */
-){
-  int nativeCksum;                /* True for native byte-order checksums */
-  u32 *aCksum = pWal->hdr.aFrameCksum;
-  assert( WAL_FRAME_HDRSIZE==24 );
-  sqlite3Put4byte(&aFrame[0], iPage);
-  sqlite3Put4byte(&aFrame[4], nTruncate);
-  memcpy(&aFrame[8], pWal->hdr.aSalt, 8);
-
-  nativeCksum = (pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN);
-  walChecksumBytes(nativeCksum, aFrame, 8, aCksum, aCksum);
-  walChecksumBytes(nativeCksum, aData, pWal->szPage, aCksum, aCksum);
-
-  sqlite3Put4byte(&aFrame[16], aCksum[0]);
-  sqlite3Put4byte(&aFrame[20], aCksum[1]);
-}
-
-/*
-** Check to see if the frame with header in aFrame[] and content
-** in aData[] is valid.  If it is a valid frame, fill *piPage and
-** *pnTruncate and return true.  Return if the frame is not valid.
-*/
-static int walDecodeFrame(
-  Wal *pWal,                      /* The write-ahead log */
-  u32 *piPage,                    /* OUT: Database page number for frame */
-  u32 *pnTruncate,                /* OUT: New db size (or 0 if not commit) */
-  u8 *aData,                      /* Pointer to page data (for checksum) */
-  u8 *aFrame                      /* Frame data */
-){
-  int nativeCksum;                /* True for native byte-order checksums */
-  u32 *aCksum = pWal->hdr.aFrameCksum;
-  u32 pgno;                       /* Page number of the frame */
-  assert( WAL_FRAME_HDRSIZE==24 );
-
-  /* A frame is only valid if the salt values in the frame-header
-  ** match the salt values in the wal-header. 
-  */
-  if( memcmp(&pWal->hdr.aSalt, &aFrame[8], 8)!=0 ){
-    return 0;
-  }
-
-  /* A frame is only valid if the page number is creater than zero.
-  */
-  pgno = sqlite3Get4byte(&aFrame[0]);
-  if( pgno==0 ){
-    return 0;
-  }
-
-  /* A frame is only valid if a checksum of the WAL header,
-  ** all prior frams, the first 16 bytes of this frame-header, 
-  ** and the frame-data matches the checksum in the last 8 
-  ** bytes of this frame-header.
-  */
-  nativeCksum = (pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN);
-  walChecksumBytes(nativeCksum, aFrame, 8, aCksum, aCksum);
-  walChecksumBytes(nativeCksum, aData, pWal->szPage, aCksum, aCksum);
-  if( aCksum[0]!=sqlite3Get4byte(&aFrame[16]) 
-   || aCksum[1]!=sqlite3Get4byte(&aFrame[20]) 
-  ){
-    /* Checksum failed. */
-    return 0;
-  }
-
-  /* If we reach this point, the frame is valid.  Return the page number
-  ** and the new database size.
-  */
-  *piPage = pgno;
-  *pnTruncate = sqlite3Get4byte(&aFrame[4]);
-  return 1;
-}
-
-
-#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
-/*
-** Names of locks.  This routine is used to provide debugging output and is not
-** a part of an ordinary build.
-*/
-static const char *walLockName(int lockIdx){
-  if( lockIdx==WAL_WRITE_LOCK ){
-    return "WRITE-LOCK";
-  }else if( lockIdx==WAL_CKPT_LOCK ){
-    return "CKPT-LOCK";
-  }else if( lockIdx==WAL_RECOVER_LOCK ){
-    return "RECOVER-LOCK";
-  }else{
-    static char zName[15];
-    sqlite3_snprintf(sizeof(zName), zName, "READ-LOCK[%d]",
-                     lockIdx-WAL_READ_LOCK(0));
-    return zName;
-  }
-}
-#endif /*defined(SQLITE_TEST) || defined(SQLITE_DEBUG) */
-    
-
-/*
-** Set or release locks on the WAL.  Locks are either shared or exclusive.
-** A lock cannot be moved directly between shared and exclusive - it must go
-** through the unlocked state first.
-**
-** In locking_mode=EXCLUSIVE, all of these routines become no-ops.
-*/
-static int walLockShared(Wal *pWal, int lockIdx){
-  int rc;
-  if( pWal->exclusiveMode ) return SQLITE_OK;
-  rc = sqlite3OsShmLock(pWal->pDbFd, lockIdx, 1,
-                        SQLITE_SHM_LOCK | SQLITE_SHM_SHARED);
-  WALTRACE(("WAL%p: acquire SHARED-%s %s\n", pWal,
-            walLockName(lockIdx), rc ? "failed" : "ok"));
-  VVA_ONLY( pWal->lockError = (u8)(rc!=SQLITE_OK && rc!=SQLITE_BUSY); )
-  return rc;
-}
-static void walUnlockShared(Wal *pWal, int lockIdx){
-  if( pWal->exclusiveMode ) return;
-  (void)sqlite3OsShmLock(pWal->pDbFd, lockIdx, 1,
-                         SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED);
-  WALTRACE(("WAL%p: release SHARED-%s\n", pWal, walLockName(lockIdx)));
-}
-static int walLockExclusive(Wal *pWal, int lockIdx, int n, int fBlock){
-  int rc;
-  if( pWal->exclusiveMode ) return SQLITE_OK;
-  if( fBlock ) sqlite3OsFileControl(pWal->pDbFd, SQLITE_FCNTL_WAL_BLOCK, 0);
-  rc = sqlite3OsShmLock(pWal->pDbFd, lockIdx, n,
-                        SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE);
-  WALTRACE(("WAL%p: acquire EXCLUSIVE-%s cnt=%d %s\n", pWal,
-            walLockName(lockIdx), n, rc ? "failed" : "ok"));
-  VVA_ONLY( pWal->lockError = (u8)(rc!=SQLITE_OK && rc!=SQLITE_BUSY); )
-  return rc;
-}
-static void walUnlockExclusive(Wal *pWal, int lockIdx, int n){
-  if( pWal->exclusiveMode ) return;
-  (void)sqlite3OsShmLock(pWal->pDbFd, lockIdx, n,
-                         SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE);
-  WALTRACE(("WAL%p: release EXCLUSIVE-%s cnt=%d\n", pWal,
-             walLockName(lockIdx), n));
-}
-
-/*
-** Compute a hash on a page number.  The resulting hash value must land
-** between 0 and (HASHTABLE_NSLOT-1).  The walHashNext() function advances
-** the hash to the next value in the event of a collision.
-*/
-static int walHash(u32 iPage){
-  assert( iPage>0 );
-  assert( (HASHTABLE_NSLOT & (HASHTABLE_NSLOT-1))==0 );
-  return (iPage*HASHTABLE_HASH_1) & (HASHTABLE_NSLOT-1);
-}
-static int walNextHash(int iPriorHash){
-  return (iPriorHash+1)&(HASHTABLE_NSLOT-1);
-}
-
-/* 
-** Return pointers to the hash table and page number array stored on
-** page iHash of the wal-index. The wal-index is broken into 32KB pages
-** numbered starting from 0.
-**
-** Set output variable *paHash to point to the start of the hash table
-** in the wal-index file. Set *piZero to one less than the frame 
-** number of the first frame indexed by this hash table. If a
-** slot in the hash table is set to N, it refers to frame number 
-** (*piZero+N) in the log.
-**
-** Finally, set *paPgno so that *paPgno[1] is the page number of the
-** first frame indexed by the hash table, frame (*piZero+1).
-*/
-static int walHashGet(
-  Wal *pWal,                      /* WAL handle */
-  int iHash,                      /* Find the iHash'th table */
-  volatile ht_slot **paHash,      /* OUT: Pointer to hash index */
-  volatile u32 **paPgno,          /* OUT: Pointer to page number array */
-  u32 *piZero                     /* OUT: Frame associated with *paPgno[0] */
-){
-  int rc;                         /* Return code */
-  volatile u32 *aPgno;
-
-  rc = walIndexPage(pWal, iHash, &aPgno);
-  assert( rc==SQLITE_OK || iHash>0 );
-
-  if( rc==SQLITE_OK ){
-    u32 iZero;
-    volatile ht_slot *aHash;
-
-    aHash = (volatile ht_slot *)&aPgno[HASHTABLE_NPAGE];
-    if( iHash==0 ){
-      aPgno = &aPgno[WALINDEX_HDR_SIZE/sizeof(u32)];
-      iZero = 0;
-    }else{
-      iZero = HASHTABLE_NPAGE_ONE + (iHash-1)*HASHTABLE_NPAGE;
-    }
-  
-    *paPgno = &aPgno[-1];
-    *paHash = aHash;
-    *piZero = iZero;
-  }
-  return rc;
-}
-
-/*
-** Return the number of the wal-index page that contains the hash-table
-** and page-number array that contain entries corresponding to WAL frame
-** iFrame. The wal-index is broken up into 32KB pages. Wal-index pages 
-** are numbered starting from 0.
-*/
-static int walFramePage(u32 iFrame){
-  int iHash = (iFrame+HASHTABLE_NPAGE-HASHTABLE_NPAGE_ONE-1) / HASHTABLE_NPAGE;
-  assert( (iHash==0 || iFrame>HASHTABLE_NPAGE_ONE)
-       && (iHash>=1 || iFrame<=HASHTABLE_NPAGE_ONE)
-       && (iHash<=1 || iFrame>(HASHTABLE_NPAGE_ONE+HASHTABLE_NPAGE))
-       && (iHash>=2 || iFrame<=HASHTABLE_NPAGE_ONE+HASHTABLE_NPAGE)
-       && (iHash<=2 || iFrame>(HASHTABLE_NPAGE_ONE+2*HASHTABLE_NPAGE))
-  );
-  return iHash;
-}
-
-/*
-** Return the page number associated with frame iFrame in this WAL.
-*/
-static u32 walFramePgno(Wal *pWal, u32 iFrame){
-  int iHash = walFramePage(iFrame);
-  if( iHash==0 ){
-    return pWal->apWiData[0][WALINDEX_HDR_SIZE/sizeof(u32) + iFrame - 1];
-  }
-  return pWal->apWiData[iHash][(iFrame-1-HASHTABLE_NPAGE_ONE)%HASHTABLE_NPAGE];
-}
-
-/*
-** Remove entries from the hash table that point to WAL slots greater
-** than pWal->hdr.mxFrame.
-**
-** This function is called whenever pWal->hdr.mxFrame is decreased due
-** to a rollback or savepoint.
-**
-** At most only the hash table containing pWal->hdr.mxFrame needs to be
-** updated.  Any later hash tables will be automatically cleared when
-** pWal->hdr.mxFrame advances to the point where those hash tables are
-** actually needed.
-*/
-static void walCleanupHash(Wal *pWal){
-  volatile ht_slot *aHash = 0;    /* Pointer to hash table to clear */
-  volatile u32 *aPgno = 0;        /* Page number array for hash table */
-  u32 iZero = 0;                  /* frame == (aHash[x]+iZero) */
-  int iLimit = 0;                 /* Zero values greater than this */
-  int nByte;                      /* Number of bytes to zero in aPgno[] */
-  int i;                          /* Used to iterate through aHash[] */
-
-  assert( pWal->writeLock );
-  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE-1 );
-  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE );
-  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE+1 );
-
-  if( pWal->hdr.mxFrame==0 ) return;
-
-  /* Obtain pointers to the hash-table and page-number array containing 
-  ** the entry that corresponds to frame pWal->hdr.mxFrame. It is guaranteed
-  ** that the page said hash-table and array reside on is already mapped.
-  */
-  assert( pWal->nWiData>walFramePage(pWal->hdr.mxFrame) );
-  assert( pWal->apWiData[walFramePage(pWal->hdr.mxFrame)] );
-  walHashGet(pWal, walFramePage(pWal->hdr.mxFrame), &aHash, &aPgno, &iZero);
-
-  /* Zero all hash-table entries that correspond to frame numbers greater
-  ** than pWal->hdr.mxFrame.
-  */
-  iLimit = pWal->hdr.mxFrame - iZero;
-  assert( iLimit>0 );
-  for(i=0; i<HASHTABLE_NSLOT; i++){
-    if( aHash[i]>iLimit ){
-      aHash[i] = 0;
-    }
-  }
-  
-  /* Zero the entries in the aPgno array that correspond to frames with
-  ** frame numbers greater than pWal->hdr.mxFrame. 
-  */
-  nByte = (int)((char *)aHash - (char *)&aPgno[iLimit+1]);
-  memset((void *)&aPgno[iLimit+1], 0, nByte);
-
-#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
-  /* Verify that the every entry in the mapping region is still reachable
-  ** via the hash table even after the cleanup.
-  */
-  if( iLimit ){
-    int j;           /* Loop counter */
-    int iKey;        /* Hash key */
-    for(j=1; j<=iLimit; j++){
-      for(iKey=walHash(aPgno[j]); aHash[iKey]; iKey=walNextHash(iKey)){
-        if( aHash[iKey]==j ) break;
-      }
-      assert( aHash[iKey]==j );
-    }
-  }
-#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
-}
-
-
-/*
-** Set an entry in the wal-index that will map database page number
-** pPage into WAL frame iFrame.
-*/
-static int walIndexAppend(Wal *pWal, u32 iFrame, u32 iPage){
-  int rc;                         /* Return code */
-  u32 iZero = 0;                  /* One less than frame number of aPgno[1] */
-  volatile u32 *aPgno = 0;        /* Page number array */
-  volatile ht_slot *aHash = 0;    /* Hash table */
-
-  rc = walHashGet(pWal, walFramePage(iFrame), &aHash, &aPgno, &iZero);
-
-  /* Assuming the wal-index file was successfully mapped, populate the
-  ** page number array and hash table entry.
-  */
-  if( rc==SQLITE_OK ){
-    int iKey;                     /* Hash table key */
-    int idx;                      /* Value to write to hash-table slot */
-    int nCollide;                 /* Number of hash collisions */
-
-    idx = iFrame - iZero;
-    assert( idx <= HASHTABLE_NSLOT/2 + 1 );
-    
-    /* If this is the first entry to be added to this hash-table, zero the
-    ** entire hash table and aPgno[] array before proceeding. 
-    */
-    if( idx==1 ){
-      int nByte = (int)((u8 *)&aHash[HASHTABLE_NSLOT] - (u8 *)&aPgno[1]);
-      memset((void*)&aPgno[1], 0, nByte);
-    }
-
-    /* If the entry in aPgno[] is already set, then the previous writer
-    ** must have exited unexpectedly in the middle of a transaction (after
-    ** writing one or more dirty pages to the WAL to free up memory). 
-    ** Remove the remnants of that writers uncommitted transaction from 
-    ** the hash-table before writing any new entries.
-    */
-    if( aPgno[idx] ){
-      walCleanupHash(pWal);
-      assert( !aPgno[idx] );
-    }
-
-    /* Write the aPgno[] array entry and the hash-table slot. */
-    nCollide = idx;
-    for(iKey=walHash(iPage); aHash[iKey]; iKey=walNextHash(iKey)){
-      if( (nCollide--)==0 ) return SQLITE_CORRUPT_BKPT;
-    }
-    aPgno[idx] = iPage;
-    aHash[iKey] = (ht_slot)idx;
-
-#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
-    /* Verify that the number of entries in the hash table exactly equals
-    ** the number of entries in the mapping region.
-    */
-    {
-      int i;           /* Loop counter */
-      int nEntry = 0;  /* Number of entries in the hash table */
-      for(i=0; i<HASHTABLE_NSLOT; i++){ if( aHash[i] ) nEntry++; }
-      assert( nEntry==idx );
-    }
-
-    /* Verify that the every entry in the mapping region is reachable
-    ** via the hash table.  This turns out to be a really, really expensive
-    ** thing to check, so only do this occasionally - not on every
-    ** iteration.
-    */
-    if( (idx&0x3ff)==0 ){
-      int i;           /* Loop counter */
-      for(i=1; i<=idx; i++){
-        for(iKey=walHash(aPgno[i]); aHash[iKey]; iKey=walNextHash(iKey)){
-          if( aHash[iKey]==i ) break;
-        }
-        assert( aHash[iKey]==i );
-      }
-    }
-#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
-  }
-
-
-  return rc;
-}
-
-
-/*
-** Recover the wal-index by reading the write-ahead log file. 
-**
-** This routine first tries to establish an exclusive lock on the
-** wal-index to prevent other threads/processes from doing anything
-** with the WAL or wal-index while recovery is running.  The
-** WAL_RECOVER_LOCK is also held so that other threads will know
-** that this thread is running recovery.  If unable to establish
-** the necessary locks, this routine returns SQLITE_BUSY.
-*/
-static int walIndexRecover(Wal *pWal){
-  int rc;                         /* Return Code */
-  i64 nSize;                      /* Size of log file */
-  u32 aFrameCksum[2] = {0, 0};
-  int iLock;                      /* Lock offset to lock for checkpoint */
-  int nLock;                      /* Number of locks to hold */
-
-  /* Obtain an exclusive lock on all byte in the locking range not already
-  ** locked by the caller. The caller is guaranteed to have locked the
-  ** WAL_WRITE_LOCK byte, and may have also locked the WAL_CKPT_LOCK byte.
-  ** If successful, the same bytes that are locked here are unlocked before
-  ** this function returns.
-  */
-  assert( pWal->ckptLock==1 || pWal->ckptLock==0 );
-  assert( WAL_ALL_BUT_WRITE==WAL_WRITE_LOCK+1 );
-  assert( WAL_CKPT_LOCK==WAL_ALL_BUT_WRITE );
-  assert( pWal->writeLock );
-  iLock = WAL_ALL_BUT_WRITE + pWal->ckptLock;
-  nLock = SQLITE_SHM_NLOCK - iLock;
-  rc = walLockExclusive(pWal, iLock, nLock, 0);
-  if( rc ){
-    return rc;
-  }
-  WALTRACE(("WAL%p: recovery begin...\n", pWal));
-
-  memset(&pWal->hdr, 0, sizeof(WalIndexHdr));
-
-  rc = sqlite3OsFileSize(pWal->pWalFd, &nSize);
-  if( rc!=SQLITE_OK ){
-    goto recovery_error;
-  }
-
-  if( nSize>WAL_HDRSIZE ){
-    u8 aBuf[WAL_HDRSIZE];         /* Buffer to load WAL header into */
-    u8 *aFrame = 0;               /* Malloc'd buffer to load entire frame */
-    int szFrame;                  /* Number of bytes in buffer aFrame[] */
-    u8 *aData;                    /* Pointer to data part of aFrame buffer */
-    int iFrame;                   /* Index of last frame read */
-    i64 iOffset;                  /* Next offset to read from log file */
-    int szPage;                   /* Page size according to the log */
-    u32 magic;                    /* Magic value read from WAL header */
-    u32 version;                  /* Magic value read from WAL header */
-    int isValid;                  /* True if this frame is valid */
-
-    /* Read in the WAL header. */
-    rc = sqlite3OsRead(pWal->pWalFd, aBuf, WAL_HDRSIZE, 0);
-    if( rc!=SQLITE_OK ){
-      goto recovery_error;
-    }
-
-    /* If the database page size is not a power of two, or is greater than
-    ** SQLITE_MAX_PAGE_SIZE, conclude that the WAL file contains no valid 
-    ** data. Similarly, if the 'magic' value is invalid, ignore the whole
-    ** WAL file.
-    */
-    magic = sqlite3Get4byte(&aBuf[0]);
-    szPage = sqlite3Get4byte(&aBuf[8]);
-    if( (magic&0xFFFFFFFE)!=WAL_MAGIC 
-     || szPage&(szPage-1) 
-     || szPage>SQLITE_MAX_PAGE_SIZE 
-     || szPage<512 
-    ){
-      goto finished;
-    }
-    pWal->hdr.bigEndCksum = (u8)(magic&0x00000001);
-    pWal->szPage = szPage;
-    pWal->nCkpt = sqlite3Get4byte(&aBuf[12]);
-    memcpy(&pWal->hdr.aSalt, &aBuf[16], 8);
-
-    /* Verify that the WAL header checksum is correct */
-    walChecksumBytes(pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN, 
-        aBuf, WAL_HDRSIZE-2*4, 0, pWal->hdr.aFrameCksum
-    );
-    if( pWal->hdr.aFrameCksum[0]!=sqlite3Get4byte(&aBuf[24])
-     || pWal->hdr.aFrameCksum[1]!=sqlite3Get4byte(&aBuf[28])
-    ){
-      goto finished;
-    }
-
-    /* Verify that the version number on the WAL format is one that
-    ** are able to understand */
-    version = sqlite3Get4byte(&aBuf[4]);
-    if( version!=WAL_MAX_VERSION ){
-      rc = SQLITE_CANTOPEN_BKPT;
-      goto finished;
-    }
-
-    /* Malloc a buffer to read frames into. */
-    szFrame = szPage + WAL_FRAME_HDRSIZE;
-    aFrame = (u8 *)sqlite3_malloc64(szFrame);
-    if( !aFrame ){
-      rc = SQLITE_NOMEM;
-      goto recovery_error;
-    }
-    aData = &aFrame[WAL_FRAME_HDRSIZE];
-
-    /* Read all frames from the log file. */
-    iFrame = 0;
-    for(iOffset=WAL_HDRSIZE; (iOffset+szFrame)<=nSize; iOffset+=szFrame){
-      u32 pgno;                   /* Database page number for frame */
-      u32 nTruncate;              /* dbsize field from frame header */
-
-      /* Read and decode the next log frame. */
-      iFrame++;
-      rc = sqlite3OsRead(pWal->pWalFd, aFrame, szFrame, iOffset);
-      if( rc!=SQLITE_OK ) break;
-      isValid = walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame);
-      if( !isValid ) break;
-      rc = walIndexAppend(pWal, iFrame, pgno);
-      if( rc!=SQLITE_OK ) break;
-
-      /* If nTruncate is non-zero, this is a commit record. */
-      if( nTruncate ){
-        pWal->hdr.mxFrame = iFrame;
-        pWal->hdr.nPage = nTruncate;
-        pWal->hdr.szPage = (u16)((szPage&0xff00) | (szPage>>16));
-        testcase( szPage<=32768 );
-        testcase( szPage>=65536 );
-        aFrameCksum[0] = pWal->hdr.aFrameCksum[0];
-        aFrameCksum[1] = pWal->hdr.aFrameCksum[1];
-      }
-    }
-
-    sqlite3_free(aFrame);
-  }
-
-finished:
-  if( rc==SQLITE_OK ){
-    volatile WalCkptInfo *pInfo;
-    int i;
-    pWal->hdr.aFrameCksum[0] = aFrameCksum[0];
-    pWal->hdr.aFrameCksum[1] = aFrameCksum[1];
-    walIndexWriteHdr(pWal);
-
-    /* Reset the checkpoint-header. This is safe because this thread is 
-    ** currently holding locks that exclude all other readers, writers and
-    ** checkpointers.
-    */
-    pInfo = walCkptInfo(pWal);
-    pInfo->nBackfill = 0;
-    pInfo->aReadMark[0] = 0;
-    for(i=1; i<WAL_NREADER; i++) pInfo->aReadMark[i] = READMARK_NOT_USED;
-    if( pWal->hdr.mxFrame ) pInfo->aReadMark[1] = pWal->hdr.mxFrame;
-
-    /* If more than one frame was recovered from the log file, report an
-    ** event via sqlite3_log(). This is to help with identifying performance
-    ** problems caused by applications routinely shutting down without
-    ** checkpointing the log file.
-    */
-    if( pWal->hdr.nPage ){
-      sqlite3_log(SQLITE_NOTICE_RECOVER_WAL,
-          "recovered %d frames from WAL file %s",
-          pWal->hdr.mxFrame, pWal->zWalName
-      );
-    }
-  }
-
-recovery_error:
-  WALTRACE(("WAL%p: recovery %s\n", pWal, rc ? "failed" : "ok"));
-  walUnlockExclusive(pWal, iLock, nLock);
-  return rc;
-}
-
-/*
-** Close an open wal-index.
-*/
-static void walIndexClose(Wal *pWal, int isDelete){
-  if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE ){
-    int i;
-    for(i=0; i<pWal->nWiData; i++){
-      sqlite3_free((void *)pWal->apWiData[i]);
-      pWal->apWiData[i] = 0;
-    }
-  }else{
-    sqlite3OsShmUnmap(pWal->pDbFd, isDelete);
-  }
-}
-
-/* 
-** Open a connection to the WAL file zWalName. The database file must 
-** already be opened on connection pDbFd. The buffer that zWalName points
-** to must remain valid for the lifetime of the returned Wal* handle.
-**
-** A SHARED lock should be held on the database file when this function
-** is called. The purpose of this SHARED lock is to prevent any other
-** client from unlinking the WAL or wal-index file. If another process
-** were to do this just after this client opened one of these files, the
-** system would be badly broken.
-**
-** If the log file is successfully opened, SQLITE_OK is returned and 
-** *ppWal is set to point to a new WAL handle. If an error occurs,
-** an SQLite error code is returned and *ppWal is left unmodified.
-*/
-int sqlite3WalOpen(
-  sqlite3_vfs *pVfs,              /* vfs module to open wal and wal-index */
-  sqlite3_file *pDbFd,            /* The open database file */
-  const char *zWalName,           /* Name of the WAL file */
-  int bNoShm,                     /* True to run in heap-memory mode */
-  i64 mxWalSize,                  /* Truncate WAL to this size on reset */
-  Wal **ppWal                     /* OUT: Allocated Wal handle */
-){
-  int rc;                         /* Return Code */
-  Wal *pRet;                      /* Object to allocate and return */
-  int flags;                      /* Flags passed to OsOpen() */
-
-  assert( zWalName && zWalName[0] );
-  assert( pDbFd );
-
-  /* In the amalgamation, the os_unix.c and os_win.c source files come before
-  ** this source file.  Verify that the #defines of the locking byte offsets
-  ** in os_unix.c and os_win.c agree with the WALINDEX_LOCK_OFFSET value.
-  */
-#ifdef WIN_SHM_BASE
-  assert( WIN_SHM_BASE==WALINDEX_LOCK_OFFSET );
-#endif
-#ifdef UNIX_SHM_BASE
-  assert( UNIX_SHM_BASE==WALINDEX_LOCK_OFFSET );
-#endif
-
-
-  /* Allocate an instance of struct Wal to return. */
-  *ppWal = 0;
-  pRet = (Wal*)sqlite3MallocZero(sizeof(Wal) + pVfs->szOsFile);
-  if( !pRet ){
-    return SQLITE_NOMEM;
-  }
-
-  pRet->pVfs = pVfs;
-  pRet->pWalFd = (sqlite3_file *)&pRet[1];
-  pRet->pDbFd = pDbFd;
-  pRet->readLock = -1;
-  pRet->mxWalSize = mxWalSize;
-  pRet->zWalName = zWalName;
-  pRet->syncHeader = 1;
-  pRet->padToSectorBoundary = 1;
-  pRet->exclusiveMode = (bNoShm ? WAL_HEAPMEMORY_MODE: WAL_NORMAL_MODE);
-
-  /* Open file handle on the write-ahead log file. */
-  flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_WAL);
-  rc = sqlite3OsOpen(pVfs, zWalName, pRet->pWalFd, flags, &flags);
-  if( rc==SQLITE_OK && flags&SQLITE_OPEN_READONLY ){
-    pRet->readOnly = WAL_RDONLY;
-  }
-
-  if( rc!=SQLITE_OK ){
-    walIndexClose(pRet, 0);
-    sqlite3OsClose(pRet->pWalFd);
-    sqlite3_free(pRet);
-  }else{
-    int iDC = sqlite3OsDeviceCharacteristics(pDbFd);
-    if( iDC & SQLITE_IOCAP_SEQUENTIAL ){ pRet->syncHeader = 0; }
-    if( iDC & SQLITE_IOCAP_POWERSAFE_OVERWRITE ){
-      pRet->padToSectorBoundary = 0;
-    }
-    *ppWal = pRet;
-    WALTRACE(("WAL%d: opened\n", pRet));
-  }
-  return rc;
-}
-
-/*
-** Change the size to which the WAL file is trucated on each reset.
-*/
-void sqlite3WalLimit(Wal *pWal, i64 iLimit){
-  if( pWal ) pWal->mxWalSize = iLimit;
-}
-
-/*
-** Find the smallest page number out of all pages held in the WAL that
-** has not been returned by any prior invocation of this method on the
-** same WalIterator object.   Write into *piFrame the frame index where
-** that page was last written into the WAL.  Write into *piPage the page
-** number.
-**
-** Return 0 on success.  If there are no pages in the WAL with a page
-** number larger than *piPage, then return 1.
-*/
-static int walIteratorNext(
-  WalIterator *p,               /* Iterator */
-  u32 *piPage,                  /* OUT: The page number of the next page */
-  u32 *piFrame                  /* OUT: Wal frame index of next page */
-){
-  u32 iMin;                     /* Result pgno must be greater than iMin */
-  u32 iRet = 0xFFFFFFFF;        /* 0xffffffff is never a valid page number */
-  int i;                        /* For looping through segments */
-
-  iMin = p->iPrior;
-  assert( iMin<0xffffffff );
-  for(i=p->nSegment-1; i>=0; i--){
-    struct WalSegment *pSegment = &p->aSegment[i];
-    while( pSegment->iNext<pSegment->nEntry ){
-      u32 iPg = pSegment->aPgno[pSegment->aIndex[pSegment->iNext]];
-      if( iPg>iMin ){
-        if( iPg<iRet ){
-          iRet = iPg;
-          *piFrame = pSegment->iZero + pSegment->aIndex[pSegment->iNext];
-        }
-        break;
-      }
-      pSegment->iNext++;
-    }
-  }
-
-  *piPage = p->iPrior = iRet;
-  return (iRet==0xFFFFFFFF);
-}
-
-/*
-** This function merges two sorted lists into a single sorted list.
-**
-** aLeft[] and aRight[] are arrays of indices.  The sort key is
-** aContent[aLeft[]] and aContent[aRight[]].  Upon entry, the following
-** is guaranteed for all J<K:
-**
-**        aContent[aLeft[J]] < aContent[aLeft[K]]
-**        aContent[aRight[J]] < aContent[aRight[K]]
-**
-** This routine overwrites aRight[] with a new (probably longer) sequence
-** of indices such that the aRight[] contains every index that appears in
-** either aLeft[] or the old aRight[] and such that the second condition
-** above is still met.
-**
-** The aContent[aLeft[X]] values will be unique for all X.  And the
-** aContent[aRight[X]] values will be unique too.  But there might be
-** one or more combinations of X and Y such that
-**
-**      aLeft[X]!=aRight[Y]  &&  aContent[aLeft[X]] == aContent[aRight[Y]]
-**
-** When that happens, omit the aLeft[X] and use the aRight[Y] index.
-*/
-static void walMerge(
-  const u32 *aContent,            /* Pages in wal - keys for the sort */
-  ht_slot *aLeft,                 /* IN: Left hand input list */
-  int nLeft,                      /* IN: Elements in array *paLeft */
-  ht_slot **paRight,              /* IN/OUT: Right hand input list */
-  int *pnRight,                   /* IN/OUT: Elements in *paRight */
-  ht_slot *aTmp                   /* Temporary buffer */
-){
-  int iLeft = 0;                  /* Current index in aLeft */
-  int iRight = 0;                 /* Current index in aRight */
-  int iOut = 0;                   /* Current index in output buffer */
-  int nRight = *pnRight;
-  ht_slot *aRight = *paRight;
-
-  assert( nLeft>0 && nRight>0 );
-  while( iRight<nRight || iLeft<nLeft ){
-    ht_slot logpage;
-    Pgno dbpage;
-
-    if( (iLeft<nLeft) 
-     && (iRight>=nRight || aContent[aLeft[iLeft]]<aContent[aRight[iRight]])
-    ){
-      logpage = aLeft[iLeft++];
-    }else{
-      logpage = aRight[iRight++];
-    }
-    dbpage = aContent[logpage];
-
-    aTmp[iOut++] = logpage;
-    if( iLeft<nLeft && aContent[aLeft[iLeft]]==dbpage ) iLeft++;
-
-    assert( iLeft>=nLeft || aContent[aLeft[iLeft]]>dbpage );
-    assert( iRight>=nRight || aContent[aRight[iRight]]>dbpage );
-  }
-
-  *paRight = aLeft;
-  *pnRight = iOut;
-  memcpy(aLeft, aTmp, sizeof(aTmp[0])*iOut);
-}
-
-/*
-** Sort the elements in list aList using aContent[] as the sort key.
-** Remove elements with duplicate keys, preferring to keep the
-** larger aList[] values.
-**
-** The aList[] entries are indices into aContent[].  The values in
-** aList[] are to be sorted so that for all J<K:
-**
-**      aContent[aList[J]] < aContent[aList[K]]
-**
-** For any X and Y such that
-**
-**      aContent[aList[X]] == aContent[aList[Y]]
-**
-** Keep the larger of the two values aList[X] and aList[Y] and discard
-** the smaller.
-*/
-static void walMergesort(
-  const u32 *aContent,            /* Pages in wal */
-  ht_slot *aBuffer,               /* Buffer of at least *pnList items to use */
-  ht_slot *aList,                 /* IN/OUT: List to sort */
-  int *pnList                     /* IN/OUT: Number of elements in aList[] */
-){
-  struct Sublist {
-    int nList;                    /* Number of elements in aList */
-    ht_slot *aList;               /* Pointer to sub-list content */
-  };
-
-  const int nList = *pnList;      /* Size of input list */
-  int nMerge = 0;                 /* Number of elements in list aMerge */
-  ht_slot *aMerge = 0;            /* List to be merged */
-  int iList;                      /* Index into input list */
-  u32 iSub = 0;                   /* Index into aSub array */
-  struct Sublist aSub[13];        /* Array of sub-lists */
-
-  memset(aSub, 0, sizeof(aSub));
-  assert( nList<=HASHTABLE_NPAGE && nList>0 );
-  assert( HASHTABLE_NPAGE==(1<<(ArraySize(aSub)-1)) );
-
-  for(iList=0; iList<nList; iList++){
-    nMerge = 1;
-    aMerge = &aList[iList];
-    for(iSub=0; iList & (1<<iSub); iSub++){
-      struct Sublist *p;
-      assert( iSub<ArraySize(aSub) );
-      p = &aSub[iSub];
-      assert( p->aList && p->nList<=(1<<iSub) );
-      assert( p->aList==&aList[iList&~((2<<iSub)-1)] );
-      walMerge(aContent, p->aList, p->nList, &aMerge, &nMerge, aBuffer);
-    }
-    aSub[iSub].aList = aMerge;
-    aSub[iSub].nList = nMerge;
-  }
-
-  for(iSub++; iSub<ArraySize(aSub); iSub++){
-    if( nList & (1<<iSub) ){
-      struct Sublist *p;
-      assert( iSub<ArraySize(aSub) );
-      p = &aSub[iSub];
-      assert( p->nList<=(1<<iSub) );
-      assert( p->aList==&aList[nList&~((2<<iSub)-1)] );
-      walMerge(aContent, p->aList, p->nList, &aMerge, &nMerge, aBuffer);
-    }
-  }
-  assert( aMerge==aList );
-  *pnList = nMerge;
-
-#ifdef SQLITE_DEBUG
-  {
-    int i;
-    for(i=1; i<*pnList; i++){
-      assert( aContent[aList[i]] > aContent[aList[i-1]] );
-    }
-  }
-#endif
-}
-
-/* 
-** Free an iterator allocated by walIteratorInit().
-*/
-static void walIteratorFree(WalIterator *p){
-  sqlite3_free(p);
-}
-
-/*
-** Construct a WalInterator object that can be used to loop over all 
-** pages in the WAL in ascending order. The caller must hold the checkpoint
-** lock.
-**
-** On success, make *pp point to the newly allocated WalInterator object
-** return SQLITE_OK. Otherwise, return an error code. If this routine
-** returns an error, the value of *pp is undefined.
-**
-** The calling routine should invoke walIteratorFree() to destroy the
-** WalIterator object when it has finished with it.
-*/
-static int walIteratorInit(Wal *pWal, WalIterator **pp){
-  WalIterator *p;                 /* Return value */
-  int nSegment;                   /* Number of segments to merge */
-  u32 iLast;                      /* Last frame in log */
-  int nByte;                      /* Number of bytes to allocate */
-  int i;                          /* Iterator variable */
-  ht_slot *aTmp;                  /* Temp space used by merge-sort */
-  int rc = SQLITE_OK;             /* Return Code */
-
-  /* This routine only runs while holding the checkpoint lock. And
-  ** it only runs if there is actually content in the log (mxFrame>0).
-  */
-  assert( pWal->ckptLock && pWal->hdr.mxFrame>0 );
-  iLast = pWal->hdr.mxFrame;
-
-  /* Allocate space for the WalIterator object. */
-  nSegment = walFramePage(iLast) + 1;
-  nByte = sizeof(WalIterator) 
-        + (nSegment-1)*sizeof(struct WalSegment)
-        + iLast*sizeof(ht_slot);
-  p = (WalIterator *)sqlite3_malloc64(nByte);
-  if( !p ){
-    return SQLITE_NOMEM;
-  }
-  memset(p, 0, nByte);
-  p->nSegment = nSegment;
-
-  /* Allocate temporary space used by the merge-sort routine. This block
-  ** of memory will be freed before this function returns.
-  */
-  aTmp = (ht_slot *)sqlite3_malloc64(
-      sizeof(ht_slot) * (iLast>HASHTABLE_NPAGE?HASHTABLE_NPAGE:iLast)
-  );
-  if( !aTmp ){
-    rc = SQLITE_NOMEM;
-  }
-
-  for(i=0; rc==SQLITE_OK && i<nSegment; i++){
-    volatile ht_slot *aHash;
-    u32 iZero;
-    volatile u32 *aPgno;
-
-    rc = walHashGet(pWal, i, &aHash, &aPgno, &iZero);
-    if( rc==SQLITE_OK ){
-      int j;                      /* Counter variable */
-      int nEntry;                 /* Number of entries in this segment */
-      ht_slot *aIndex;            /* Sorted index for this segment */
-
-      aPgno++;
-      if( (i+1)==nSegment ){
-        nEntry = (int)(iLast - iZero);
-      }else{
-        nEntry = (int)((u32*)aHash - (u32*)aPgno);
-      }
-      aIndex = &((ht_slot *)&p->aSegment[p->nSegment])[iZero];
-      iZero++;
-  
-      for(j=0; j<nEntry; j++){
-        aIndex[j] = (ht_slot)j;
-      }
-      walMergesort((u32 *)aPgno, aTmp, aIndex, &nEntry);
-      p->aSegment[i].iZero = iZero;
-      p->aSegment[i].nEntry = nEntry;
-      p->aSegment[i].aIndex = aIndex;
-      p->aSegment[i].aPgno = (u32 *)aPgno;
-    }
-  }
-  sqlite3_free(aTmp);
-
-  if( rc!=SQLITE_OK ){
-    walIteratorFree(p);
-  }
-  *pp = p;
-  return rc;
-}
-
-/*
-** Attempt to obtain the exclusive WAL lock defined by parameters lockIdx and
-** n. If the attempt fails and parameter xBusy is not NULL, then it is a
-** busy-handler function. Invoke it and retry the lock until either the
-** lock is successfully obtained or the busy-handler returns 0.
-*/
-static int walBusyLock(
-  Wal *pWal,                      /* WAL connection */
-  int (*xBusy)(void*),            /* Function to call when busy */
-  void *pBusyArg,                 /* Context argument for xBusyHandler */
-  int lockIdx,                    /* Offset of first byte to lock */
-  int n                           /* Number of bytes to lock */
-){
-  int rc;
-  do {
-    rc = walLockExclusive(pWal, lockIdx, n, 0);
-  }while( xBusy && rc==SQLITE_BUSY && xBusy(pBusyArg) );
-  return rc;
-}
-
-/*
-** The cache of the wal-index header must be valid to call this function.
-** Return the page-size in bytes used by the database.
-*/
-static int walPagesize(Wal *pWal){
-  return (pWal->hdr.szPage&0xfe00) + ((pWal->hdr.szPage&0x0001)<<16);
-}
-
-/*
-** The following is guaranteed when this function is called:
-**
-**   a) the WRITER lock is held,
-**   b) the entire log file has been checkpointed, and
-**   c) any existing readers are reading exclusively from the database
-**      file - there are no readers that may attempt to read a frame from
-**      the log file.
-**
-** This function updates the shared-memory structures so that the next
-** client to write to the database (which may be this one) does so by
-** writing frames into the start of the log file.
-**
-** The value of parameter salt1 is used as the aSalt[1] value in the 
-** new wal-index header. It should be passed a pseudo-random value (i.e. 
-** one obtained from sqlite3_randomness()).
-*/
-static void walRestartHdr(Wal *pWal, u32 salt1){
-  volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
-  int i;                          /* Loop counter */
-  u32 *aSalt = pWal->hdr.aSalt;   /* Big-endian salt values */
-  pWal->nCkpt++;
-  pWal->hdr.mxFrame = 0;
-  sqlite3Put4byte((u8*)&aSalt[0], 1 + sqlite3Get4byte((u8*)&aSalt[0]));
-  memcpy(&pWal->hdr.aSalt[1], &salt1, 4);
-  walIndexWriteHdr(pWal);
-  pInfo->nBackfill = 0;
-  pInfo->aReadMark[1] = 0;
-  for(i=2; i<WAL_NREADER; i++) pInfo->aReadMark[i] = READMARK_NOT_USED;
-  assert( pInfo->aReadMark[0]==0 );
-}
-
-/*
-** Copy as much content as we can from the WAL back into the database file
-** in response to an sqlite3_wal_checkpoint() request or the equivalent.
-**
-** The amount of information copies from WAL to database might be limited
-** by active readers.  This routine will never overwrite a database page
-** that a concurrent reader might be using.
-**
-** All I/O barrier operations (a.k.a fsyncs) occur in this routine when
-** SQLite is in WAL-mode in synchronous=NORMAL.  That means that if 
-** checkpoints are always run by a background thread or background 
-** process, foreground threads will never block on a lengthy fsync call.
-**
-** Fsync is called on the WAL before writing content out of the WAL and
-** into the database.  This ensures that if the new content is persistent
-** in the WAL and can be recovered following a power-loss or hard reset.
-**
-** Fsync is also called on the database file if (and only if) the entire
-** WAL content is copied into the database file.  This second fsync makes
-** it safe to delete the WAL since the new content will persist in the
-** database file.
-**
-** This routine uses and updates the nBackfill field of the wal-index header.
-** This is the only routine that will increase the value of nBackfill.  
-** (A WAL reset or recovery will revert nBackfill to zero, but not increase
-** its value.)
-**
-** The caller must be holding sufficient locks to ensure that no other
-** checkpoint is running (in any other thread or process) at the same
-** time.
-*/
-static int walCheckpoint(
-  Wal *pWal,                      /* Wal connection */
-  int eMode,                      /* One of PASSIVE, FULL or RESTART */
-  int (*xBusy)(void*),            /* Function to call when busy */
-  void *pBusyArg,                 /* Context argument for xBusyHandler */
-  int sync_flags,                 /* Flags for OsSync() (or 0) */
-  u8 *zBuf                        /* Temporary buffer to use */
-){
-  int rc = SQLITE_OK;             /* Return code */
-  int szPage;                     /* Database page-size */
-  WalIterator *pIter = 0;         /* Wal iterator context */
-  u32 iDbpage = 0;                /* Next database page to write */
-  u32 iFrame = 0;                 /* Wal frame containing data for iDbpage */
-  u32 mxSafeFrame;                /* Max frame that can be backfilled */
-  u32 mxPage;                     /* Max database page to write */
-  int i;                          /* Loop counter */
-  volatile WalCkptInfo *pInfo;    /* The checkpoint status information */
-
-  szPage = walPagesize(pWal);
-  testcase( szPage<=32768 );
-  testcase( szPage>=65536 );
-  pInfo = walCkptInfo(pWal);
-  if( pInfo->nBackfill<pWal->hdr.mxFrame ){
-
-    /* Allocate the iterator */
-    rc = walIteratorInit(pWal, &pIter);
-    if( rc!=SQLITE_OK ){
-      return rc;
-    }
-    assert( pIter );
-
-    /* EVIDENCE-OF: R-62920-47450 The busy-handler callback is never invoked
-    ** in the SQLITE_CHECKPOINT_PASSIVE mode. */
-    assert( eMode!=SQLITE_CHECKPOINT_PASSIVE || xBusy==0 );
-
-    /* Compute in mxSafeFrame the index of the last frame of the WAL that is
-    ** safe to write into the database.  Frames beyond mxSafeFrame might
-    ** overwrite database pages that are in use by active readers and thus
-    ** cannot be backfilled from the WAL.
-    */
-    mxSafeFrame = pWal->hdr.mxFrame;
-    mxPage = pWal->hdr.nPage;
-    for(i=1; i<WAL_NREADER; i++){
-      /* Thread-sanitizer reports that the following is an unsafe read,
-      ** as some other thread may be in the process of updating the value
-      ** of the aReadMark[] slot. The assumption here is that if that is
-      ** happening, the other client may only be increasing the value,
-      ** not decreasing it. So assuming either that either the "old" or
-      ** "new" version of the value is read, and not some arbitrary value
-      ** that would never be written by a real client, things are still 
-      ** safe.  */
-      u32 y = pInfo->aReadMark[i];
-      if( mxSafeFrame>y ){
-        assert( y<=pWal->hdr.mxFrame );
-        rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(i), 1);
-        if( rc==SQLITE_OK ){
-          pInfo->aReadMark[i] = (i==1 ? mxSafeFrame : READMARK_NOT_USED);
-          walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
-        }else if( rc==SQLITE_BUSY ){
-          mxSafeFrame = y;
-          xBusy = 0;
-        }else{
-          goto walcheckpoint_out;
-        }
-      }
-    }
-
-    if( pInfo->nBackfill<mxSafeFrame
-     && (rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(0),1))==SQLITE_OK
-    ){
-      i64 nSize;                    /* Current size of database file */
-      u32 nBackfill = pInfo->nBackfill;
-
-      /* Sync the WAL to disk */
-      if( sync_flags ){
-        rc = sqlite3OsSync(pWal->pWalFd, sync_flags);
-      }
-
-      /* If the database may grow as a result of this checkpoint, hint
-      ** about the eventual size of the db file to the VFS layer.
-      */
-      if( rc==SQLITE_OK ){
-        i64 nReq = ((i64)mxPage * szPage);
-        rc = sqlite3OsFileSize(pWal->pDbFd, &nSize);
-        if( rc==SQLITE_OK && nSize<nReq ){
-          sqlite3OsFileControlHint(pWal->pDbFd, SQLITE_FCNTL_SIZE_HINT, &nReq);
-        }
-      }
-
-
-      /* Iterate through the contents of the WAL, copying data to the db file */
-      while( rc==SQLITE_OK && 0==walIteratorNext(pIter, &iDbpage, &iFrame) ){
-        i64 iOffset;
-        assert( walFramePgno(pWal, iFrame)==iDbpage );
-        if( iFrame<=nBackfill || iFrame>mxSafeFrame || iDbpage>mxPage ){
-          continue;
-        }
-        iOffset = walFrameOffset(iFrame, szPage) + WAL_FRAME_HDRSIZE;
-        /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL file */
-        rc = sqlite3OsRead(pWal->pWalFd, zBuf, szPage, iOffset);
-        if( rc!=SQLITE_OK ) break;
-        iOffset = (iDbpage-1)*(i64)szPage;
-        testcase( IS_BIG_INT(iOffset) );
-        rc = sqlite3OsWrite(pWal->pDbFd, zBuf, szPage, iOffset);
-        if( rc!=SQLITE_OK ) break;
-      }
-
-      /* If work was actually accomplished... */
-      if( rc==SQLITE_OK ){
-        if( mxSafeFrame==walIndexHdr(pWal)->mxFrame ){
-          i64 szDb = pWal->hdr.nPage*(i64)szPage;
-          testcase( IS_BIG_INT(szDb) );
-          rc = sqlite3OsTruncate(pWal->pDbFd, szDb);
-          if( rc==SQLITE_OK && sync_flags ){
-            rc = sqlite3OsSync(pWal->pDbFd, sync_flags);
-          }
-        }
-        if( rc==SQLITE_OK ){
-          pInfo->nBackfill = mxSafeFrame;
-        }
-      }
-
-      /* Release the reader lock held while backfilling */
-      walUnlockExclusive(pWal, WAL_READ_LOCK(0), 1);
-    }
-
-    if( rc==SQLITE_BUSY ){
-      /* Reset the return code so as not to report a checkpoint failure
-      ** just because there are active readers.  */
-      rc = SQLITE_OK;
-    }
-  }
-
-  /* If this is an SQLITE_CHECKPOINT_RESTART or TRUNCATE operation, and the
-  ** entire wal file has been copied into the database file, then block 
-  ** until all readers have finished using the wal file. This ensures that 
-  ** the next process to write to the database restarts the wal file.
-  */
-  if( rc==SQLITE_OK && eMode!=SQLITE_CHECKPOINT_PASSIVE ){
-    assert( pWal->writeLock );
-    if( pInfo->nBackfill<pWal->hdr.mxFrame ){
-      rc = SQLITE_BUSY;
-    }else if( eMode>=SQLITE_CHECKPOINT_RESTART ){
-      u32 salt1;
-      sqlite3_randomness(4, &salt1);
-      assert( pInfo->nBackfill==pWal->hdr.mxFrame );
-      rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(1), WAL_NREADER-1);
-      if( rc==SQLITE_OK ){
-        if( eMode==SQLITE_CHECKPOINT_TRUNCATE ){
-          /* IMPLEMENTATION-OF: R-44699-57140 This mode works the same way as
-          ** SQLITE_CHECKPOINT_RESTART with the addition that it also
-          ** truncates the log file to zero bytes just prior to a
-          ** successful return.
-          **
-          ** In theory, it might be safe to do this without updating the
-          ** wal-index header in shared memory, as all subsequent reader or
-          ** writer clients should see that the entire log file has been
-          ** checkpointed and behave accordingly. This seems unsafe though,
-          ** as it would leave the system in a state where the contents of
-          ** the wal-index header do not match the contents of the 
-          ** file-system. To avoid this, update the wal-index header to
-          ** indicate that the log file contains zero valid frames.  */
-          walRestartHdr(pWal, salt1);
-          rc = sqlite3OsTruncate(pWal->pWalFd, 0);
-        }
-        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
-      }
-    }
-  }
-
- walcheckpoint_out:
-  walIteratorFree(pIter);
-  return rc;
-}
-
-/*
-** If the WAL file is currently larger than nMax bytes in size, truncate
-** it to exactly nMax bytes. If an error occurs while doing so, ignore it.
-*/
-static void walLimitSize(Wal *pWal, i64 nMax){
-  i64 sz;
-  int rx;
-  sqlite3BeginBenignMalloc();
-  rx = sqlite3OsFileSize(pWal->pWalFd, &sz);
-  if( rx==SQLITE_OK && (sz > nMax ) ){
-    rx = sqlite3OsTruncate(pWal->pWalFd, nMax);
-  }
-  sqlite3EndBenignMalloc();
-  if( rx ){
-    sqlite3_log(rx, "cannot limit WAL size: %s", pWal->zWalName);
-  }
-}
-
-/*
-** Close a connection to a log file.
-*/
-int sqlite3WalClose(
-  Wal *pWal,                      /* Wal to close */
-  int sync_flags,                 /* Flags to pass to OsSync() (or 0) */
-  int nBuf,
-  u8 *zBuf                        /* Buffer of at least nBuf bytes */
-){
-  int rc = SQLITE_OK;
-  if( pWal ){
-    int isDelete = 0;             /* True to unlink wal and wal-index files */
-
-    /* If an EXCLUSIVE lock can be obtained on the database file (using the
-    ** ordinary, rollback-mode locking methods, this guarantees that the
-    ** connection associated with this log file is the only connection to
-    ** the database. In this case checkpoint the database and unlink both
-    ** the wal and wal-index files.
-    **
-    ** The EXCLUSIVE lock is not released before returning.
-    */
-    rc = sqlite3OsLock(pWal->pDbFd, SQLITE_LOCK_EXCLUSIVE);
-    if( rc==SQLITE_OK ){
-      if( pWal->exclusiveMode==WAL_NORMAL_MODE ){
-        pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
-      }
-      rc = sqlite3WalCheckpoint(
-          pWal, SQLITE_CHECKPOINT_PASSIVE, 0, 0, sync_flags, nBuf, zBuf, 0, 0
-      );
-      if( rc==SQLITE_OK ){
-        int bPersist = -1;
-        sqlite3OsFileControlHint(
-            pWal->pDbFd, SQLITE_FCNTL_PERSIST_WAL, &bPersist
-        );
-        if( bPersist!=1 ){
-          /* Try to delete the WAL file if the checkpoint completed and
-          ** fsyned (rc==SQLITE_OK) and if we are not in persistent-wal
-          ** mode (!bPersist) */
-          isDelete = 1;
-        }else if( pWal->mxWalSize>=0 ){
-          /* Try to truncate the WAL file to zero bytes if the checkpoint
-          ** completed and fsynced (rc==SQLITE_OK) and we are in persistent
-          ** WAL mode (bPersist) and if the PRAGMA journal_size_limit is a
-          ** non-negative value (pWal->mxWalSize>=0).  Note that we truncate
-          ** to zero bytes as truncating to the journal_size_limit might
-          ** leave a corrupt WAL file on disk. */
-          walLimitSize(pWal, 0);
-        }
-      }
-    }
-
-    walIndexClose(pWal, isDelete);
-    sqlite3OsClose(pWal->pWalFd);
-    if( isDelete ){
-      sqlite3BeginBenignMalloc();
-      sqlite3OsDelete(pWal->pVfs, pWal->zWalName, 0);
-      sqlite3EndBenignMalloc();
-    }
-    WALTRACE(("WAL%p: closed\n", pWal));
-    sqlite3_free((void *)pWal->apWiData);
-    sqlite3_free(pWal);
-  }
-  return rc;
-}
-
-/*
-** Try to read the wal-index header.  Return 0 on success and 1 if
-** there is a problem.
-**
-** The wal-index is in shared memory.  Another thread or process might
-** be writing the header at the same time this procedure is trying to
-** read it, which might result in inconsistency.  A dirty read is detected
-** by verifying that both copies of the header are the same and also by
-** a checksum on the header.
-**
-** If and only if the read is consistent and the header is different from
-** pWal->hdr, then pWal->hdr is updated to the content of the new header
-** and *pChanged is set to 1.
-**
-** If the checksum cannot be verified return non-zero. If the header
-** is read successfully and the checksum verified, return zero.
-*/
-static int walIndexTryHdr(Wal *pWal, int *pChanged){
-  u32 aCksum[2];                  /* Checksum on the header content */
-  WalIndexHdr h1, h2;             /* Two copies of the header content */
-  WalIndexHdr volatile *aHdr;     /* Header in shared memory */
-
-  /* The first page of the wal-index must be mapped at this point. */
-  assert( pWal->nWiData>0 && pWal->apWiData[0] );
-
-  /* Read the header. This might happen concurrently with a write to the
-  ** same area of shared memory on a different CPU in a SMP,
-  ** meaning it is possible that an inconsistent snapshot is read
-  ** from the file. If this happens, return non-zero.
-  **
-  ** There are two copies of the header at the beginning of the wal-index.
-  ** When reading, read [0] first then [1].  Writes are in the reverse order.
-  ** Memory barriers are used to prevent the compiler or the hardware from
-  ** reordering the reads and writes.
-  */
-  aHdr = walIndexHdr(pWal);
-  memcpy(&h1, (void *)&aHdr[0], sizeof(h1));
-  walShmBarrier(pWal);
-  memcpy(&h2, (void *)&aHdr[1], sizeof(h2));
-
-  if( memcmp(&h1, &h2, sizeof(h1))!=0 ){
-    return 1;   /* Dirty read */
-  }  
-  if( h1.isInit==0 ){
-    return 1;   /* Malformed header - probably all zeros */
-  }
-  walChecksumBytes(1, (u8*)&h1, sizeof(h1)-sizeof(h1.aCksum), 0, aCksum);
-  if( aCksum[0]!=h1.aCksum[0] || aCksum[1]!=h1.aCksum[1] ){
-    return 1;   /* Checksum does not match */
-  }
-
-  if( memcmp(&pWal->hdr, &h1, sizeof(WalIndexHdr)) ){
-    *pChanged = 1;
-    memcpy(&pWal->hdr, &h1, sizeof(WalIndexHdr));
-    pWal->szPage = (pWal->hdr.szPage&0xfe00) + ((pWal->hdr.szPage&0x0001)<<16);
-    testcase( pWal->szPage<=32768 );
-    testcase( pWal->szPage>=65536 );
-  }
-
-  /* The header was successfully read. Return zero. */
-  return 0;
-}
-
-/*
-** Read the wal-index header from the wal-index and into pWal->hdr.
-** If the wal-header appears to be corrupt, try to reconstruct the
-** wal-index from the WAL before returning.
-**
-** Set *pChanged to 1 if the wal-index header value in pWal->hdr is
-** changed by this operation.  If pWal->hdr is unchanged, set *pChanged
-** to 0.
-**
-** If the wal-index header is successfully read, return SQLITE_OK. 
-** Otherwise an SQLite error code.
-*/
-static int walIndexReadHdr(Wal *pWal, int *pChanged){
-  int rc;                         /* Return code */
-  int badHdr;                     /* True if a header read failed */
-  volatile u32 *page0;            /* Chunk of wal-index containing header */
-
-  /* Ensure that page 0 of the wal-index (the page that contains the 
-  ** wal-index header) is mapped. Return early if an error occurs here.
-  */
-  assert( pChanged );
-  rc = walIndexPage(pWal, 0, &page0);
-  if( rc!=SQLITE_OK ){
-    return rc;
-  };
-  assert( page0 || pWal->writeLock==0 );
-
-  /* If the first page of the wal-index has been mapped, try to read the
-  ** wal-index header immediately, without holding any lock. This usually
-  ** works, but may fail if the wal-index header is corrupt or currently 
-  ** being modified by another thread or process.
-  */
-  badHdr = (page0 ? walIndexTryHdr(pWal, pChanged) : 1);
-
-  /* If the first attempt failed, it might have been due to a race
-  ** with a writer.  So get a WRITE lock and try again.
-  */
-  assert( badHdr==0 || pWal->writeLock==0 );
-  if( badHdr ){
-    if( pWal->readOnly & WAL_SHM_RDONLY ){
-      if( SQLITE_OK==(rc = walLockShared(pWal, WAL_WRITE_LOCK)) ){
-        walUnlockShared(pWal, WAL_WRITE_LOCK);
-        rc = SQLITE_READONLY_RECOVERY;
-      }
-    }else if( SQLITE_OK==(rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1, 1)) ){
-      pWal->writeLock = 1;
-      if( SQLITE_OK==(rc = walIndexPage(pWal, 0, &page0)) ){
-        badHdr = walIndexTryHdr(pWal, pChanged);
-        if( badHdr ){
-          /* If the wal-index header is still malformed even while holding
-          ** a WRITE lock, it can only mean that the header is corrupted and
-          ** needs to be reconstructed.  So run recovery to do exactly that.
-          */
-          rc = walIndexRecover(pWal);
-          *pChanged = 1;
-        }
-      }
-      pWal->writeLock = 0;
-      walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
-    }
-  }
-
-  /* If the header is read successfully, check the version number to make
-  ** sure the wal-index was not constructed with some future format that
-  ** this version of SQLite cannot understand.
-  */
-  if( badHdr==0 && pWal->hdr.iVersion!=WALINDEX_MAX_VERSION ){
-    rc = SQLITE_CANTOPEN_BKPT;
-  }
-
-  return rc;
-}
-
-/*
-** This is the value that walTryBeginRead returns when it needs to
-** be retried.
-*/
-#define WAL_RETRY  (-1)
-
-/*
-** Attempt to start a read transaction.  This might fail due to a race or
-** other transient condition.  When that happens, it returns WAL_RETRY to
-** indicate to the caller that it is safe to retry immediately.
-**
-** On success return SQLITE_OK.  On a permanent failure (such an
-** I/O error or an SQLITE_BUSY because another process is running
-** recovery) return a positive error code.
-**
-** The useWal parameter is true to force the use of the WAL and disable
-** the case where the WAL is bypassed because it has been completely
-** checkpointed.  If useWal==0 then this routine calls walIndexReadHdr() 
-** to make a copy of the wal-index header into pWal->hdr.  If the 
-** wal-index header has changed, *pChanged is set to 1 (as an indication 
-** to the caller that the local paget cache is obsolete and needs to be 
-** flushed.)  When useWal==1, the wal-index header is assumed to already
-** be loaded and the pChanged parameter is unused.
-**
-** The caller must set the cnt parameter to the number of prior calls to
-** this routine during the current read attempt that returned WAL_RETRY.
-** This routine will start taking more aggressive measures to clear the
-** race conditions after multiple WAL_RETRY returns, and after an excessive
-** number of errors will ultimately return SQLITE_PROTOCOL.  The
-** SQLITE_PROTOCOL return indicates that some other process has gone rogue
-** and is not honoring the locking protocol.  There is a vanishingly small
-** chance that SQLITE_PROTOCOL could be returned because of a run of really
-** bad luck when there is lots of contention for the wal-index, but that
-** possibility is so small that it can be safely neglected, we believe.
-**
-** On success, this routine obtains a read lock on 
-** WAL_READ_LOCK(pWal->readLock).  The pWal->readLock integer is
-** in the range 0 <= pWal->readLock < WAL_NREADER.  If pWal->readLock==(-1)
-** that means the Wal does not hold any read lock.  The reader must not
-** access any database page that is modified by a WAL frame up to and
-** including frame number aReadMark[pWal->readLock].  The reader will
-** use WAL frames up to and including pWal->hdr.mxFrame if pWal->readLock>0
-** Or if pWal->readLock==0, then the reader will ignore the WAL
-** completely and get all content directly from the database file.
-** If the useWal parameter is 1 then the WAL will never be ignored and
-** this routine will always set pWal->readLock>0 on success.
-** When the read transaction is completed, the caller must release the
-** lock on WAL_READ_LOCK(pWal->readLock) and set pWal->readLock to -1.
-**
-** This routine uses the nBackfill and aReadMark[] fields of the header
-** to select a particular WAL_READ_LOCK() that strives to let the
-** checkpoint process do as much work as possible.  This routine might
-** update values of the aReadMark[] array in the header, but if it does
-** so it takes care to hold an exclusive lock on the corresponding
-** WAL_READ_LOCK() while changing values.
-*/
-static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
-  volatile WalCkptInfo *pInfo;    /* Checkpoint information in wal-index */
-  u32 mxReadMark;                 /* Largest aReadMark[] value */
-  int mxI;                        /* Index of largest aReadMark[] value */
-  int i;                          /* Loop counter */
-  int rc = SQLITE_OK;             /* Return code  */
-
-  assert( pWal->readLock<0 );     /* Not currently locked */
-
-  /* Take steps to avoid spinning forever if there is a protocol error.
-  **
-  ** Circumstances that cause a RETRY should only last for the briefest
-  ** instances of time.  No I/O or other system calls are done while the
-  ** locks are held, so the locks should not be held for very long. But 
-  ** if we are unlucky, another process that is holding a lock might get
-  ** paged out or take a page-fault that is time-consuming to resolve, 
-  ** during the few nanoseconds that it is holding the lock.  In that case,
-  ** it might take longer than normal for the lock to free.
-  **
-  ** After 5 RETRYs, we begin calling sqlite3OsSleep().  The first few
-  ** calls to sqlite3OsSleep() have a delay of 1 microsecond.  Really this
-  ** is more of a scheduler yield than an actual delay.  But on the 10th
-  ** an subsequent retries, the delays start becoming longer and longer, 
-  ** so that on the 100th (and last) RETRY we delay for 323 milliseconds.
-  ** The total delay time before giving up is less than 10 seconds.
-  */
-  if( cnt>5 ){
-    int nDelay = 1;                      /* Pause time in microseconds */
-    if( cnt>100 ){
-      VVA_ONLY( pWal->lockError = 1; )
-      return SQLITE_PROTOCOL;
-    }
-    if( cnt>=10 ) nDelay = (cnt-9)*(cnt-9)*39;
-    sqlite3OsSleep(pWal->pVfs, nDelay);
-  }
-
-  if( !useWal ){
-    rc = walIndexReadHdr(pWal, pChanged);
-    if( rc==SQLITE_BUSY ){
-      /* If there is not a recovery running in another thread or process
-      ** then convert BUSY errors to WAL_RETRY.  If recovery is known to
-      ** be running, convert BUSY to BUSY_RECOVERY.  There is a race here
-      ** which might cause WAL_RETRY to be returned even if BUSY_RECOVERY
-      ** would be technically correct.  But the race is benign since with
-      ** WAL_RETRY this routine will be called again and will probably be
-      ** right on the second iteration.
-      */
-      if( pWal->apWiData[0]==0 ){
-        /* This branch is taken when the xShmMap() method returns SQLITE_BUSY.
-        ** We assume this is a transient condition, so return WAL_RETRY. The
-        ** xShmMap() implementation used by the default unix and win32 VFS 
-        ** modules may return SQLITE_BUSY due to a race condition in the 
-        ** code that determines whether or not the shared-memory region 
-        ** must be zeroed before the requested page is returned.
-        */
-        rc = WAL_RETRY;
-      }else if( SQLITE_OK==(rc = walLockShared(pWal, WAL_RECOVER_LOCK)) ){
-        walUnlockShared(pWal, WAL_RECOVER_LOCK);
-        rc = WAL_RETRY;
-      }else if( rc==SQLITE_BUSY ){
-        rc = SQLITE_BUSY_RECOVERY;
-      }
-    }
-    if( rc!=SQLITE_OK ){
-      return rc;
-    }
-  }
-
-  pInfo = walCkptInfo(pWal);
-  if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame ){
-    /* The WAL has been completely backfilled (or it is empty).
-    ** and can be safely ignored.
-    */
-    rc = walLockShared(pWal, WAL_READ_LOCK(0));
-    walShmBarrier(pWal);
-    if( rc==SQLITE_OK ){
-      if( memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr)) ){
-        /* It is not safe to allow the reader to continue here if frames
-        ** may have been appended to the log before READ_LOCK(0) was obtained.
-        ** When holding READ_LOCK(0), the reader ignores the entire log file,
-        ** which implies that the database file contains a trustworthy
-        ** snapshot. Since holding READ_LOCK(0) prevents a checkpoint from
-        ** happening, this is usually correct.
-        **
-        ** However, if frames have been appended to the log (or if the log 
-        ** is wrapped and written for that matter) before the READ_LOCK(0)
-        ** is obtained, that is not necessarily true. A checkpointer may
-        ** have started to backfill the appended frames but crashed before
-        ** it finished. Leaving a corrupt image in the database file.
-        */
-        walUnlockShared(pWal, WAL_READ_LOCK(0));
-        return WAL_RETRY;
-      }
-      pWal->readLock = 0;
-      return SQLITE_OK;
-    }else if( rc!=SQLITE_BUSY ){
-      return rc;
-    }
-  }
-
-  /* If we get this far, it means that the reader will want to use
-  ** the WAL to get at content from recent commits.  The job now is
-  ** to select one of the aReadMark[] entries that is closest to
-  ** but not exceeding pWal->hdr.mxFrame and lock that entry.
-  */
-  mxReadMark = 0;
-  mxI = 0;
-  for(i=1; i<WAL_NREADER; i++){
-    u32 thisMark = pInfo->aReadMark[i];
-    if( mxReadMark<=thisMark && thisMark<=pWal->hdr.mxFrame ){
-      assert( thisMark!=READMARK_NOT_USED );
-      mxReadMark = thisMark;
-      mxI = i;
-    }
-  }
-  /* There was once an "if" here. The extra "{" is to preserve indentation. */
-  {
-    if( (pWal->readOnly & WAL_SHM_RDONLY)==0
-     && (mxReadMark<pWal->hdr.mxFrame || mxI==0)
-    ){
-      for(i=1; i<WAL_NREADER; i++){
-        rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1, 0);
-        if( rc==SQLITE_OK ){
-          mxReadMark = pInfo->aReadMark[i] = pWal->hdr.mxFrame;
-          mxI = i;
-          walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
-          break;
-        }else if( rc!=SQLITE_BUSY ){
-          return rc;
-        }
-      }
-    }
-    if( mxI==0 ){
-      assert( rc==SQLITE_BUSY || (pWal->readOnly & WAL_SHM_RDONLY)!=0 );
-      return rc==SQLITE_BUSY ? WAL_RETRY : SQLITE_READONLY_CANTLOCK;
-    }
-
-    rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
-    if( rc ){
-      return rc==SQLITE_BUSY ? WAL_RETRY : rc;
-    }
-    /* Now that the read-lock has been obtained, check that neither the
-    ** value in the aReadMark[] array or the contents of the wal-index
-    ** header have changed.
-    **
-    ** It is necessary to check that the wal-index header did not change
-    ** between the time it was read and when the shared-lock was obtained
-    ** on WAL_READ_LOCK(mxI) was obtained to account for the possibility
-    ** that the log file may have been wrapped by a writer, or that frames
-    ** that occur later in the log than pWal->hdr.mxFrame may have been
-    ** copied into the database by a checkpointer. If either of these things
-    ** happened, then reading the database with the current value of
-    ** pWal->hdr.mxFrame risks reading a corrupted snapshot. So, retry
-    ** instead.
-    **
-    ** Before checking that the live wal-index header has not changed
-    ** since it was read, set Wal.minFrame to the first frame in the wal
-    ** file that has not yet been checkpointed. This client will not need
-    ** to read any frames earlier than minFrame from the wal file - they
-    ** can be safely read directly from the database file.
-    **
-    ** Because a ShmBarrier() call is made between taking the copy of 
-    ** nBackfill and checking that the wal-header in shared-memory still
-    ** matches the one cached in pWal->hdr, it is guaranteed that the 
-    ** checkpointer that set nBackfill was not working with a wal-index
-    ** header newer than that cached in pWal->hdr. If it were, that could
-    ** cause a problem. The checkpointer could omit to checkpoint
-    ** a version of page X that lies before pWal->minFrame (call that version
-    ** A) on the basis that there is a newer version (version B) of the same
-    ** page later in the wal file. But if version B happens to like past
-    ** frame pWal->hdr.mxFrame - then the client would incorrectly assume
-    ** that it can read version A from the database file. However, since
-    ** we can guarantee that the checkpointer that set nBackfill could not
-    ** see any pages past pWal->hdr.mxFrame, this problem does not come up.
-    */
-    pWal->minFrame = pInfo->nBackfill+1;
-    walShmBarrier(pWal);
-    if( pInfo->aReadMark[mxI]!=mxReadMark
-     || memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr))
-    ){
-      walUnlockShared(pWal, WAL_READ_LOCK(mxI));
-      return WAL_RETRY;
-    }else{
-      assert( mxReadMark<=pWal->hdr.mxFrame );
-      pWal->readLock = (i16)mxI;
-    }
-  }
-  return rc;
-}
-
-/*
-** Begin a read transaction on the database.
-**
-** This routine used to be called sqlite3OpenSnapshot() and with good reason:
-** it takes a snapshot of the state of the WAL and wal-index for the current
-** instant in time.  The current thread will continue to use this snapshot.
-** Other threads might append new content to the WAL and wal-index but
-** that extra content is ignored by the current thread.
-**
-** If the database contents have changes since the previous read
-** transaction, then *pChanged is set to 1 before returning.  The
-** Pager layer will use this to know that is cache is stale and
-** needs to be flushed.
-*/
-int sqlite3WalBeginReadTransaction(Wal *pWal, int *pChanged){
-  int rc;                         /* Return code */
-  int cnt = 0;                    /* Number of TryBeginRead attempts */
-
-  do{
-    rc = walTryBeginRead(pWal, pChanged, 0, ++cnt);
-  }while( rc==WAL_RETRY );
-  testcase( (rc&0xff)==SQLITE_BUSY );
-  testcase( (rc&0xff)==SQLITE_IOERR );
-  testcase( rc==SQLITE_PROTOCOL );
-  testcase( rc==SQLITE_OK );
-  return rc;
-}
-
-/*
-** Finish with a read transaction.  All this does is release the
-** read-lock.
-*/
-void sqlite3WalEndReadTransaction(Wal *pWal){
-  sqlite3WalEndWriteTransaction(pWal);
-  if( pWal->readLock>=0 ){
-    walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));
-    pWal->readLock = -1;
-  }
-}
-
-/*
-** Search the wal file for page pgno. If found, set *piRead to the frame that
-** contains the page. Otherwise, if pgno is not in the wal file, set *piRead
-** to zero.
-**
-** Return SQLITE_OK if successful, or an error code if an error occurs. If an
-** error does occur, the final value of *piRead is undefined.
-*/
-int sqlite3WalFindFrame(
-  Wal *pWal,                      /* WAL handle */
-  Pgno pgno,                      /* Database page number to read data for */
-  u32 *piRead                     /* OUT: Frame number (or zero) */
-){
-  u32 iRead = 0;                  /* If !=0, WAL frame to return data from */
-  u32 iLast = pWal->hdr.mxFrame;  /* Last page in WAL for this reader */
-  int iHash;                      /* Used to loop through N hash tables */
-  int iMinHash;
-
-  /* This routine is only be called from within a read transaction. */
-  assert( pWal->readLock>=0 || pWal->lockError );
-
-  /* If the "last page" field of the wal-index header snapshot is 0, then
-  ** no data will be read from the wal under any circumstances. Return early
-  ** in this case as an optimization.  Likewise, if pWal->readLock==0, 
-  ** then the WAL is ignored by the reader so return early, as if the 
-  ** WAL were empty.
-  */
-  if( iLast==0 || pWal->readLock==0 ){
-    *piRead = 0;
-    return SQLITE_OK;
-  }
-
-  /* Search the hash table or tables for an entry matching page number
-  ** pgno. Each iteration of the following for() loop searches one
-  ** hash table (each hash table indexes up to HASHTABLE_NPAGE frames).
-  **
-  ** This code might run concurrently to the code in walIndexAppend()
-  ** that adds entries to the wal-index (and possibly to this hash 
-  ** table). This means the value just read from the hash 
-  ** slot (aHash[iKey]) may have been added before or after the 
-  ** current read transaction was opened. Values added after the
-  ** read transaction was opened may have been written incorrectly -
-  ** i.e. these slots may contain garbage data. However, we assume
-  ** that any slots written before the current read transaction was
-  ** opened remain unmodified.
-  **
-  ** For the reasons above, the if(...) condition featured in the inner
-  ** loop of the following block is more stringent that would be required 
-  ** if we had exclusive access to the hash-table:
-  **
-  **   (aPgno[iFrame]==pgno): 
-  **     This condition filters out normal hash-table collisions.
-  **
-  **   (iFrame<=iLast): 
-  **     This condition filters out entries that were added to the hash
-  **     table after the current read-transaction had started.
-  */
-  iMinHash = walFramePage(pWal->minFrame);
-  for(iHash=walFramePage(iLast); iHash>=iMinHash && iRead==0; iHash--){
-    volatile ht_slot *aHash;      /* Pointer to hash table */
-    volatile u32 *aPgno;          /* Pointer to array of page numbers */
-    u32 iZero;                    /* Frame number corresponding to aPgno[0] */
-    int iKey;                     /* Hash slot index */
-    int nCollide;                 /* Number of hash collisions remaining */
-    int rc;                       /* Error code */
-
-    rc = walHashGet(pWal, iHash, &aHash, &aPgno, &iZero);
-    if( rc!=SQLITE_OK ){
-      return rc;
-    }
-    nCollide = HASHTABLE_NSLOT;
-    for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){
-      u32 iFrame = aHash[iKey] + iZero;
-      if( iFrame<=iLast && iFrame>=pWal->minFrame && aPgno[aHash[iKey]]==pgno ){
-        assert( iFrame>iRead || CORRUPT_DB );
-        iRead = iFrame;
-      }
-      if( (nCollide--)==0 ){
-        return SQLITE_CORRUPT_BKPT;
-      }
-    }
-  }
-
-#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
-  /* If expensive assert() statements are available, do a linear search
-  ** of the wal-index file content. Make sure the results agree with the
-  ** result obtained using the hash indexes above.  */
-  {
-    u32 iRead2 = 0;
-    u32 iTest;
-    assert( pWal->minFrame>0 );
-    for(iTest=iLast; iTest>=pWal->minFrame; iTest--){
-      if( walFramePgno(pWal, iTest)==pgno ){
-        iRead2 = iTest;
-        break;
-      }
-    }
-    assert( iRead==iRead2 );
-  }
-#endif
-
-  *piRead = iRead;
-  return SQLITE_OK;
-}
-
-/*
-** Read the contents of frame iRead from the wal file into buffer pOut
-** (which is nOut bytes in size). Return SQLITE_OK if successful, or an
-** error code otherwise.
-*/
-int sqlite3WalReadFrame(
-  Wal *pWal,                      /* WAL handle */
-  u32 iRead,                      /* Frame to read */
-  int nOut,                       /* Size of buffer pOut in bytes */
-  u8 *pOut                        /* Buffer to write page data to */
-){
-  int sz;
-  i64 iOffset;
-  sz = pWal->hdr.szPage;
-  sz = (sz&0xfe00) + ((sz&0x0001)<<16);
-  testcase( sz<=32768 );
-  testcase( sz>=65536 );
-  iOffset = walFrameOffset(iRead, sz) + WAL_FRAME_HDRSIZE;
-  /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL */
-  return sqlite3OsRead(pWal->pWalFd, pOut, (nOut>sz ? sz : nOut), iOffset);
-}
-
-/* 
-** Return the size of the database in pages (or zero, if unknown).
-*/
-Pgno sqlite3WalDbsize(Wal *pWal){
-  if( pWal && ALWAYS(pWal->readLock>=0) ){
-    return pWal->hdr.nPage;
-  }
-  return 0;
-}
-
-
-/* 
-** This function starts a write transaction on the WAL.
-**
-** A read transaction must have already been started by a prior call
-** to sqlite3WalBeginReadTransaction().
-**
-** If another thread or process has written into the database since
-** the read transaction was started, then it is not possible for this
-** thread to write as doing so would cause a fork.  So this routine
-** returns SQLITE_BUSY in that case and no write transaction is started.
-**
-** There can only be a single writer active at a time.
-*/
-int sqlite3WalBeginWriteTransaction(Wal *pWal){
-  int rc;
-
-  /* Cannot start a write transaction without first holding a read
-  ** transaction. */
-  assert( pWal->readLock>=0 );
-
-  if( pWal->readOnly ){
-    return SQLITE_READONLY;
-  }
-
-  /* Only one writer allowed at a time.  Get the write lock.  Return
-  ** SQLITE_BUSY if unable.
-  */
-  rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1, 0);
-  if( rc ){
-    return rc;
-  }
-  pWal->writeLock = 1;
-
-  /* If another connection has written to the database file since the
-  ** time the read transaction on this connection was started, then
-  ** the write is disallowed.
-  */
-  if( memcmp(&pWal->hdr, (void *)walIndexHdr(pWal), sizeof(WalIndexHdr))!=0 ){
-    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
-    pWal->writeLock = 0;
-    rc = SQLITE_BUSY_SNAPSHOT;
-  }
-
-  return rc;
-}
-
-/*
-** End a write transaction.  The commit has already been done.  This
-** routine merely releases the lock.
-*/
-int sqlite3WalEndWriteTransaction(Wal *pWal){
-  if( pWal->writeLock ){
-    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
-    pWal->writeLock = 0;
-    pWal->truncateOnCommit = 0;
-  }
-  return SQLITE_OK;
-}
-
-/*
-** If any data has been written (but not committed) to the log file, this
-** function moves the write-pointer back to the start of the transaction.
-**
-** Additionally, the callback function is invoked for each frame written
-** to the WAL since the start of the transaction. If the callback returns
-** other than SQLITE_OK, it is not invoked again and the error code is
-** returned to the caller.
-**
-** Otherwise, if the callback function does not return an error, this
-** function returns SQLITE_OK.
-*/
-int sqlite3WalUndo(Wal *pWal, int (*xUndo)(void *, Pgno), void *pUndoCtx){
-  int rc = SQLITE_OK;
-  if( ALWAYS(pWal->writeLock) ){
-    Pgno iMax = pWal->hdr.mxFrame;
-    Pgno iFrame;
-  
-    /* Restore the clients cache of the wal-index header to the state it
-    ** was in before the client began writing to the database. 
-    */
-    memcpy(&pWal->hdr, (void *)walIndexHdr(pWal), sizeof(WalIndexHdr));
-
-    for(iFrame=pWal->hdr.mxFrame+1; 
-        ALWAYS(rc==SQLITE_OK) && iFrame<=iMax; 
-        iFrame++
-    ){
-      /* This call cannot fail. Unless the page for which the page number
-      ** is passed as the second argument is (a) in the cache and 
-      ** (b) has an outstanding reference, then xUndo is either a no-op
-      ** (if (a) is false) or simply expels the page from the cache (if (b)
-      ** is false).
-      **
-      ** If the upper layer is doing a rollback, it is guaranteed that there
-      ** are no outstanding references to any page other than page 1. And
-      ** page 1 is never written to the log until the transaction is
-      ** committed. As a result, the call to xUndo may not fail.
-      */
-      assert( walFramePgno(pWal, iFrame)!=1 );
-      rc = xUndo(pUndoCtx, walFramePgno(pWal, iFrame));
-    }
-    if( iMax!=pWal->hdr.mxFrame ) walCleanupHash(pWal);
-  }
-  return rc;
-}
-
-/* 
-** Argument aWalData must point to an array of WAL_SAVEPOINT_NDATA u32 
-** values. This function populates the array with values required to 
-** "rollback" the write position of the WAL handle back to the current 
-** point in the event of a savepoint rollback (via WalSavepointUndo()).
-*/
-void sqlite3WalSavepoint(Wal *pWal, u32 *aWalData){
-  assert( pWal->writeLock );
-  aWalData[0] = pWal->hdr.mxFrame;
-  aWalData[1] = pWal->hdr.aFrameCksum[0];
-  aWalData[2] = pWal->hdr.aFrameCksum[1];
-  aWalData[3] = pWal->nCkpt;
-}
-
-/* 
-** Move the write position of the WAL back to the point identified by
-** the values in the aWalData[] array. aWalData must point to an array
-** of WAL_SAVEPOINT_NDATA u32 values that has been previously populated
-** by a call to WalSavepoint().
-*/
-int sqlite3WalSavepointUndo(Wal *pWal, u32 *aWalData){
-  int rc = SQLITE_OK;
-
-  assert( pWal->writeLock );
-  assert( aWalData[3]!=pWal->nCkpt || aWalData[0]<=pWal->hdr.mxFrame );
-
-  if( aWalData[3]!=pWal->nCkpt ){
-    /* This savepoint was opened immediately after the write-transaction
-    ** was started. Right after that, the writer decided to wrap around
-    ** to the start of the log. Update the savepoint values to match.
-    */
-    aWalData[0] = 0;
-    aWalData[3] = pWal->nCkpt;
-  }
-
-  if( aWalData[0]<pWal->hdr.mxFrame ){
-    pWal->hdr.mxFrame = aWalData[0];
-    pWal->hdr.aFrameCksum[0] = aWalData[1];
-    pWal->hdr.aFrameCksum[1] = aWalData[2];
-    walCleanupHash(pWal);
-  }
-
-  return rc;
-}
-
-/*
-** This function is called just before writing a set of frames to the log
-** file (see sqlite3WalFrames()). It checks to see if, instead of appending
-** to the current log file, it is possible to overwrite the start of the
-** existing log file with the new frames (i.e. "reset" the log). If so,
-** it sets pWal->hdr.mxFrame to 0. Otherwise, pWal->hdr.mxFrame is left
-** unchanged.
-**
-** SQLITE_OK is returned if no error is encountered (regardless of whether
-** or not pWal->hdr.mxFrame is modified). An SQLite error code is returned
-** if an error occurs.
-*/
-static int walRestartLog(Wal *pWal){
-  int rc = SQLITE_OK;
-  int cnt;
-
-  if( pWal->readLock==0 ){
-    volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
-    assert( pInfo->nBackfill==pWal->hdr.mxFrame );
-    if( pInfo->nBackfill>0 ){
-      u32 salt1;
-      sqlite3_randomness(4, &salt1);
-      rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1, 0);
-      if( rc==SQLITE_OK ){
-        /* If all readers are using WAL_READ_LOCK(0) (in other words if no
-        ** readers are currently using the WAL), then the transactions
-        ** frames will overwrite the start of the existing log. Update the
-        ** wal-index header to reflect this.
-        **
-        ** In theory it would be Ok to update the cache of the header only
-        ** at this point. But updating the actual wal-index header is also
-        ** safe and means there is no special case for sqlite3WalUndo()
-        ** to handle if this transaction is rolled back.  */
-        walRestartHdr(pWal, salt1);
-        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
-      }else if( rc!=SQLITE_BUSY ){
-        return rc;
-      }
-    }
-    walUnlockShared(pWal, WAL_READ_LOCK(0));
-    pWal->readLock = -1;
-    cnt = 0;
-    do{
-      int notUsed;
-      rc = walTryBeginRead(pWal, &notUsed, 1, ++cnt);
-    }while( rc==WAL_RETRY );
-    assert( (rc&0xff)!=SQLITE_BUSY ); /* BUSY not possible when useWal==1 */
-    testcase( (rc&0xff)==SQLITE_IOERR );
-    testcase( rc==SQLITE_PROTOCOL );
-    testcase( rc==SQLITE_OK );
-  }
-  return rc;
-}
-
-/*
-** Information about the current state of the WAL file and where
-** the next fsync should occur - passed from sqlite3WalFrames() into
-** walWriteToLog().
-*/
-typedef struct WalWriter {
-  Wal *pWal;                   /* The complete WAL information */
-  sqlite3_file *pFd;           /* The WAL file to which we write */
-  sqlite3_int64 iSyncPoint;    /* Fsync at this offset */
-  int syncFlags;               /* Flags for the fsync */
-  int szPage;                  /* Size of one page */
-} WalWriter;
-
-/*
-** Write iAmt bytes of content into the WAL file beginning at iOffset.
-** Do a sync when crossing the p->iSyncPoint boundary.
-**
-** In other words, if iSyncPoint is in between iOffset and iOffset+iAmt,
-** first write the part before iSyncPoint, then sync, then write the
-** rest.
-*/
-static int walWriteToLog(
-  WalWriter *p,              /* WAL to write to */
-  void *pContent,            /* Content to be written */
-  int iAmt,                  /* Number of bytes to write */
-  sqlite3_int64 iOffset      /* Start writing at this offset */
-){
-  int rc;
-  if( iOffset<p->iSyncPoint && iOffset+iAmt>=p->iSyncPoint ){
-    int iFirstAmt = (int)(p->iSyncPoint - iOffset);
-    rc = sqlite3OsWrite(p->pFd, pContent, iFirstAmt, iOffset);
-    if( rc ) return rc;
-    iOffset += iFirstAmt;
-    iAmt -= iFirstAmt;
-    pContent = (void*)(iFirstAmt + (char*)pContent);
-    assert( p->syncFlags & (SQLITE_SYNC_NORMAL|SQLITE_SYNC_FULL) );
-    rc = sqlite3OsSync(p->pFd, p->syncFlags & SQLITE_SYNC_MASK);
-    if( iAmt==0 || rc ) return rc;
-  }
-  rc = sqlite3OsWrite(p->pFd, pContent, iAmt, iOffset);
-  return rc;
-}
-
-/*
-** Write out a single frame of the WAL
-*/
-static int walWriteOneFrame(
-  WalWriter *p,               /* Where to write the frame */
-  PgHdr *pPage,               /* The page of the frame to be written */
-  int nTruncate,              /* The commit flag.  Usually 0.  >0 for commit */
-  sqlite3_int64 iOffset       /* Byte offset at which to write */
-){
-  int rc;                         /* Result code from subfunctions */
-  void *pData;                    /* Data actually written */
-  u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-header in */
-#if defined(SQLITE_HAS_CODEC)
-  if( (pData = sqlite3PagerCodec(pPage))==0 ) return SQLITE_NOMEM;
-#else
-  pData = pPage->pData;
-#endif
-  walEncodeFrame(p->pWal, pPage->pgno, nTruncate, pData, aFrame);
-  rc = walWriteToLog(p, aFrame, sizeof(aFrame), iOffset);
-  if( rc ) return rc;
-  /* Write the page data */
-  rc = walWriteToLog(p, pData, p->szPage, iOffset+sizeof(aFrame));
-  return rc;
-}
-
-/* 
-** Write a set of frames to the log. The caller must hold the write-lock
-** on the log file (obtained using sqlite3WalBeginWriteTransaction()).
-*/
-int sqlite3WalFrames(
-  Wal *pWal,                      /* Wal handle to write to */
-  int szPage,                     /* Database page-size in bytes */
-  PgHdr *pList,                   /* List of dirty pages to write */
-  Pgno nTruncate,                 /* Database size after this commit */
-  int isCommit,                   /* True if this is a commit */
-  int sync_flags                  /* Flags to pass to OsSync() (or 0) */
-){
-  int rc;                         /* Used to catch return codes */
-  u32 iFrame;                     /* Next frame address */
-  PgHdr *p;                       /* Iterator to run through pList with. */
-  PgHdr *pLast = 0;               /* Last frame in list */
-  int nExtra = 0;                 /* Number of extra copies of last page */
-  int szFrame;                    /* The size of a single frame */
-  i64 iOffset;                    /* Next byte to write in WAL file */
-  WalWriter w;                    /* The writer */
-
-  assert( pList );
-  assert( pWal->writeLock );
-
-  /* If this frame set completes a transaction, then nTruncate>0.  If
-  ** nTruncate==0 then this frame set does not complete the transaction. */
-  assert( (isCommit!=0)==(nTruncate!=0) );
-
-#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
-  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
-    WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
-              pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
-  }
-#endif
-
-  /* See if it is possible to write these frames into the start of the
-  ** log file, instead of appending to it at pWal->hdr.mxFrame.
-  */
-  if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){
-    return rc;
-  }
-
-  /* If this is the first frame written into the log, write the WAL
-  ** header to the start of the WAL file. See comments at the top of
-  ** this source file for a description of the WAL header format.
-  */
-  iFrame = pWal->hdr.mxFrame;
-  if( iFrame==0 ){
-    u8 aWalHdr[WAL_HDRSIZE];      /* Buffer to assemble wal-header in */
-    u32 aCksum[2];                /* Checksum for wal-header */
-
-    sqlite3Put4byte(&aWalHdr[0], (WAL_MAGIC | SQLITE_BIGENDIAN));
-    sqlite3Put4byte(&aWalHdr[4], WAL_MAX_VERSION);
-    sqlite3Put4byte(&aWalHdr[8], szPage);
-    sqlite3Put4byte(&aWalHdr[12], pWal->nCkpt);
-    if( pWal->nCkpt==0 ) sqlite3_randomness(8, pWal->hdr.aSalt);
-    memcpy(&aWalHdr[16], pWal->hdr.aSalt, 8);
-    walChecksumBytes(1, aWalHdr, WAL_HDRSIZE-2*4, 0, aCksum);
-    sqlite3Put4byte(&aWalHdr[24], aCksum[0]);
-    sqlite3Put4byte(&aWalHdr[28], aCksum[1]);
-    
-    pWal->szPage = szPage;
-    pWal->hdr.bigEndCksum = SQLITE_BIGENDIAN;
-    pWal->hdr.aFrameCksum[0] = aCksum[0];
-    pWal->hdr.aFrameCksum[1] = aCksum[1];
-    pWal->truncateOnCommit = 1;
-
-    rc = sqlite3OsWrite(pWal->pWalFd, aWalHdr, sizeof(aWalHdr), 0);
-    WALTRACE(("WAL%p: wal-header write %s\n", pWal, rc ? "failed" : "ok"));
-    if( rc!=SQLITE_OK ){
-      return rc;
-    }
-
-    /* Sync the header (unless SQLITE_IOCAP_SEQUENTIAL is true or unless
-    ** all syncing is turned off by PRAGMA synchronous=OFF).  Otherwise
-    ** an out-of-order write following a WAL restart could result in
-    ** database corruption.  See the ticket:
-    **
-    **     http://localhost:591/sqlite/info/ff5be73dee
-    */
-    if( pWal->syncHeader && sync_flags ){
-      rc = sqlite3OsSync(pWal->pWalFd, sync_flags & SQLITE_SYNC_MASK);
-      if( rc ) return rc;
-    }
-  }
-  assert( (int)pWal->szPage==szPage );
-
-  /* Setup information needed to write frames into the WAL */
-  w.pWal = pWal;
-  w.pFd = pWal->pWalFd;
-  w.iSyncPoint = 0;
-  w.syncFlags = sync_flags;
-  w.szPage = szPage;
-  iOffset = walFrameOffset(iFrame+1, szPage);
-  szFrame = szPage + WAL_FRAME_HDRSIZE;
-
-  /* Write all frames into the log file exactly once */
-  for(p=pList; p; p=p->pDirty){
-    int nDbSize;   /* 0 normally.  Positive == commit flag */
-    iFrame++;
-    assert( iOffset==walFrameOffset(iFrame, szPage) );
-    nDbSize = (isCommit && p->pDirty==0) ? nTruncate : 0;
-    rc = walWriteOneFrame(&w, p, nDbSize, iOffset);
-    if( rc ) return rc;
-    pLast = p;
-    iOffset += szFrame;
-  }
-
-  /* If this is the end of a transaction, then we might need to pad
-  ** the transaction and/or sync the WAL file.
-  **
-  ** Padding and syncing only occur if this set of frames complete a
-  ** transaction and if PRAGMA synchronous=FULL.  If synchronous==NORMAL
-  ** or synchronous==OFF, then no padding or syncing are needed.
-  **
-  ** If SQLITE_IOCAP_POWERSAFE_OVERWRITE is defined, then padding is not
-  ** needed and only the sync is done.  If padding is needed, then the
-  ** final frame is repeated (with its commit mark) until the next sector
-  ** boundary is crossed.  Only the part of the WAL prior to the last
-  ** sector boundary is synced; the part of the last frame that extends
-  ** past the sector boundary is written after the sync.
-  */
-  if( isCommit && (sync_flags & WAL_SYNC_TRANSACTIONS)!=0 ){
-    if( pWal->padToSectorBoundary ){
-      int sectorSize = sqlite3SectorSize(pWal->pWalFd);
-      w.iSyncPoint = ((iOffset+sectorSize-1)/sectorSize)*sectorSize;
-      while( iOffset<w.iSyncPoint ){
-        rc = walWriteOneFrame(&w, pLast, nTruncate, iOffset);
-        if( rc ) return rc;
-        iOffset += szFrame;
-        nExtra++;
-      }
-    }else{
-      rc = sqlite3OsSync(w.pFd, sync_flags & SQLITE_SYNC_MASK);
-    }
-  }
-
-  /* If this frame set completes the first transaction in the WAL and
-  ** if PRAGMA journal_size_limit is set, then truncate the WAL to the
-  ** journal size limit, if possible.
-  */
-  if( isCommit && pWal->truncateOnCommit && pWal->mxWalSize>=0 ){
-    i64 sz = pWal->mxWalSize;
-    if( walFrameOffset(iFrame+nExtra+1, szPage)>pWal->mxWalSize ){
-      sz = walFrameOffset(iFrame+nExtra+1, szPage);
-    }
-    walLimitSize(pWal, sz);
-    pWal->truncateOnCommit = 0;
-  }
-
-  /* Append data to the wal-index. It is not necessary to lock the 
-  ** wal-index to do this as the SQLITE_SHM_WRITE lock held on the wal-index
-  ** guarantees that there are no other writers, and no data that may
-  ** be in use by existing readers is being overwritten.
-  */
-  iFrame = pWal->hdr.mxFrame;
-  for(p=pList; p && rc==SQLITE_OK; p=p->pDirty){
-    iFrame++;
-    rc = walIndexAppend(pWal, iFrame, p->pgno);
-  }
-  while( rc==SQLITE_OK && nExtra>0 ){
-    iFrame++;
-    nExtra--;
-    rc = walIndexAppend(pWal, iFrame, pLast->pgno);
-  }
-
-  if( rc==SQLITE_OK ){
-    /* Update the private copy of the header. */
-    pWal->hdr.szPage = (u16)((szPage&0xff00) | (szPage>>16));
-    testcase( szPage<=32768 );
-    testcase( szPage>=65536 );
-    pWal->hdr.mxFrame = iFrame;
-    if( isCommit ){
-      pWal->hdr.iChange++;
-      pWal->hdr.nPage = nTruncate;
-    }
-    /* If this is a commit, update the wal-index header too. */
-    if( isCommit ){
-      walIndexWriteHdr(pWal);
-      pWal->iCallback = iFrame;
-    }
-  }
-
-  WALTRACE(("WAL%p: frame write %s\n", pWal, rc ? "failed" : "ok"));
-  return rc;
-}
-
-/* 
-** This routine is called to implement sqlite3_wal_checkpoint() and
-** related interfaces.
-**
-** Obtain a CHECKPOINT lock and then backfill as much information as
-** we can from WAL into the database.
-**
-** If parameter xBusy is not NULL, it is a pointer to a busy-handler
-** callback. In this case this function runs a blocking checkpoint.
-*/
-int sqlite3WalCheckpoint(
-  Wal *pWal,                      /* Wal connection */
-  int eMode,                      /* PASSIVE, FULL, RESTART, or TRUNCATE */
-  int (*xBusy)(void*),            /* Function to call when busy */
-  void *pBusyArg,                 /* Context argument for xBusyHandler */
-  int sync_flags,                 /* Flags to sync db file with (or 0) */
-  int nBuf,                       /* Size of temporary buffer */
-  u8 *zBuf,                       /* Temporary buffer to use */
-  int *pnLog,                     /* OUT: Number of frames in WAL */
-  int *pnCkpt                     /* OUT: Number of backfilled frames in WAL */
-){
-  int rc;                         /* Return code */
-  int isChanged = 0;              /* True if a new wal-index header is loaded */
-  int eMode2 = eMode;             /* Mode to pass to walCheckpoint() */
-  int (*xBusy2)(void*) = xBusy;   /* Busy handler for eMode2 */
-
-  assert( pWal->ckptLock==0 );
-  assert( pWal->writeLock==0 );
-
-  /* EVIDENCE-OF: R-62920-47450 The busy-handler callback is never invoked
-  ** in the SQLITE_CHECKPOINT_PASSIVE mode. */
-  assert( eMode!=SQLITE_CHECKPOINT_PASSIVE || xBusy==0 );
-
-  if( pWal->readOnly ) return SQLITE_READONLY;
-  WALTRACE(("WAL%p: checkpoint begins\n", pWal));
-
-  /* IMPLEMENTATION-OF: R-62028-47212 All calls obtain an exclusive 
-  ** "checkpoint" lock on the database file. */
-  rc = walLockExclusive(pWal, WAL_CKPT_LOCK, 1, 0);
-  if( rc ){
-    /* EVIDENCE-OF: R-10421-19736 If any other process is running a
-    ** checkpoint operation at the same time, the lock cannot be obtained and
-    ** SQLITE_BUSY is returned.
-    ** EVIDENCE-OF: R-53820-33897 Even if there is a busy-handler configured,
-    ** it will not be invoked in this case.
-    */
-    testcase( rc==SQLITE_BUSY );
-    testcase( xBusy!=0 );
-    return rc;
-  }
-  pWal->ckptLock = 1;
-
-  /* IMPLEMENTATION-OF: R-59782-36818 The SQLITE_CHECKPOINT_FULL, RESTART and
-  ** TRUNCATE modes also obtain the exclusive "writer" lock on the database
-  ** file.
-  **
-  ** EVIDENCE-OF: R-60642-04082 If the writer lock cannot be obtained
-  ** immediately, and a busy-handler is configured, it is invoked and the
-  ** writer lock retried until either the busy-handler returns 0 or the
-  ** lock is successfully obtained.
-  */
-  if( eMode!=SQLITE_CHECKPOINT_PASSIVE ){
-    rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_WRITE_LOCK, 1);
-    if( rc==SQLITE_OK ){
-      pWal->writeLock = 1;
-    }else if( rc==SQLITE_BUSY ){
-      eMode2 = SQLITE_CHECKPOINT_PASSIVE;
-      xBusy2 = 0;
-      rc = SQLITE_OK;
-    }
-  }
-
-  /* Read the wal-index header. */
-  if( rc==SQLITE_OK ){
-    rc = walIndexReadHdr(pWal, &isChanged);
-    if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
-      sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
-    }
-  }
-
-  /* Copy data from the log to the database file. */
-  if( rc==SQLITE_OK ){
-    if( pWal->hdr.mxFrame && walPagesize(pWal)!=nBuf ){
-      rc = SQLITE_CORRUPT_BKPT;
-    }else{
-      rc = walCheckpoint(pWal, eMode2, xBusy2, pBusyArg, sync_flags, zBuf);
-    }
-
-    /* If no error occurred, set the output variables. */
-    if( rc==SQLITE_OK || rc==SQLITE_BUSY ){
-      if( pnLog ) *pnLog = (int)pWal->hdr.mxFrame;
-      if( pnCkpt ) *pnCkpt = (int)(walCkptInfo(pWal)->nBackfill);
-    }
-  }
-
-  if( isChanged ){
-    /* If a new wal-index header was loaded before the checkpoint was 
-    ** performed, then the pager-cache associated with pWal is now
-    ** out of date. So zero the cached wal-index header to ensure that
-    ** next time the pager opens a snapshot on this database it knows that
-    ** the cache needs to be reset.
-    */
-    memset(&pWal->hdr, 0, sizeof(WalIndexHdr));
-  }
-
-  /* Release the locks. */
-  sqlite3WalEndWriteTransaction(pWal);
-  walUnlockExclusive(pWal, WAL_CKPT_LOCK, 1);
-  pWal->ckptLock = 0;
-  WALTRACE(("WAL%p: checkpoint %s\n", pWal, rc ? "failed" : "ok"));
-  return (rc==SQLITE_OK && eMode!=eMode2 ? SQLITE_BUSY : rc);
-}
-
-/* Return the value to pass to a sqlite3_wal_hook callback, the
-** number of frames in the WAL at the point of the last commit since
-** sqlite3WalCallback() was called.  If no commits have occurred since
-** the last call, then return 0.
-*/
-int sqlite3WalCallback(Wal *pWal){
-  u32 ret = 0;
-  if( pWal ){
-    ret = pWal->iCallback;
-    pWal->iCallback = 0;
-  }
-  return (int)ret;
-}
-
-/*
-** This function is called to change the WAL subsystem into or out
-** of locking_mode=EXCLUSIVE.
-**
-** If op is zero, then attempt to change from locking_mode=EXCLUSIVE
-** into locking_mode=NORMAL.  This means that we must acquire a lock
-** on the pWal->readLock byte.  If the WAL is already in locking_mode=NORMAL
-** or if the acquisition of the lock fails, then return 0.  If the
-** transition out of exclusive-mode is successful, return 1.  This
-** operation must occur while the pager is still holding the exclusive
-** lock on the main database file.
-**
-** If op is one, then change from locking_mode=NORMAL into 
-** locking_mode=EXCLUSIVE.  This means that the pWal->readLock must
-** be released.  Return 1 if the transition is made and 0 if the
-** WAL is already in exclusive-locking mode - meaning that this
-** routine is a no-op.  The pager must already hold the exclusive lock
-** on the main database file before invoking this operation.
-**
-** If op is negative, then do a dry-run of the op==1 case but do
-** not actually change anything. The pager uses this to see if it
-** should acquire the database exclusive lock prior to invoking
-** the op==1 case.
-*/
-int sqlite3WalExclusiveMode(Wal *pWal, int op){
-  int rc;
-  assert( pWal->writeLock==0 );
-  assert( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE || op==-1 );
-
-  /* pWal->readLock is usually set, but might be -1 if there was a 
-  ** prior error while attempting to acquire are read-lock. This cannot 
-  ** happen if the connection is actually in exclusive mode (as no xShmLock
-  ** locks are taken in this case). Nor should the pager attempt to
-  ** upgrade to exclusive-mode following such an error.
-  */
-  assert( pWal->readLock>=0 || pWal->lockError );
-  assert( pWal->readLock>=0 || (op<=0 && pWal->exclusiveMode==0) );
-
-  if( op==0 ){
-    if( pWal->exclusiveMode ){
-      pWal->exclusiveMode = 0;
-      if( walLockShared(pWal, WAL_READ_LOCK(pWal->readLock))!=SQLITE_OK ){
-        pWal->exclusiveMode = 1;
-      }
-      rc = pWal->exclusiveMode==0;
-    }else{
-      /* Already in locking_mode=NORMAL */
-      rc = 0;
-    }
-  }else if( op>0 ){
-    assert( pWal->exclusiveMode==0 );
-    assert( pWal->readLock>=0 );
-    walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));
-    pWal->exclusiveMode = 1;
-    rc = 1;
-  }else{
-    rc = pWal->exclusiveMode==0;
-  }
-  return rc;
-}
-
-/* 
-** Return true if the argument is non-NULL and the WAL module is using
-** heap-memory for the wal-index. Otherwise, if the argument is NULL or the
-** WAL module is using shared-memory, return false. 
-*/
-int sqlite3WalHeapMemory(Wal *pWal){
-  return (pWal && pWal->exclusiveMode==WAL_HEAPMEMORY_MODE );
-}
-
-#ifdef SQLITE_ENABLE_ZIPVFS
-/*
-** If the argument is not NULL, it points to a Wal object that holds a
-** read-lock. This function returns the database page-size if it is known,
-** or zero if it is not (or if pWal is NULL).
-*/
-int sqlite3WalFramesize(Wal *pWal){
-  assert( pWal==0 || pWal->readLock>=0 );
-  return (pWal ? pWal->szPage : 0);
-}
-#endif
-
-#endif /* #ifndef SQLITE_OMIT_WAL */