Fix '-L' file handling. Mixing idioms in previous cleanups ended up

fopen()'ing the the file with a mode of "a" (so watchers don't detect
changes until the file is re-written as part of interface
configuration) but using rewind() to start writing from the beginning
of the file.

Use ftruncate() and overwrite the date as intended, rather than
appending new data.

Problem reported and diffs tested by Mike via tech@. Thanks!

1 files changed, 21 insertions, 11 deletions

diff --git a/sbin/dhclient/dhclient.c b/sbin/dhclient/dhclient.c
index 7c26729a00d..ba1543cddce 100644
--- a/sbin/dhclient/dhclient.c
+++ b/sbin/dhclient/dhclient.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: dhclient.c,v 1.517 2017/10/27 16:00:47 jmc Exp $	*/
+/*	$OpenBSD: dhclient.c,v 1.518 2017/10/30 16:44:18 krw Exp $	*/
 
 /*
  * Copyright 2004 Henning Brauer <henning@openbsd.org>
@@ -636,6 +636,12 @@ main(int argc, char *argv[])
 	}
 
 	if (strlen(path_option_db) != 0) {
+		/*
+		 * Open 'a' so file is not truncated. The truncation
+		 * is done when new data is about to be written to the
+		 * file. This avoids false notifications to watchers that
+		 * network configuration changes have occurred.
+		 */
 		if ((optionDB = fopen(path_option_db, "a")) == NULL)
 			fatal("fopen(%s)", path_option_db);
 	}
@@ -1763,25 +1769,29 @@ rewrite_option_db(char *name, struct client_lease *offered,
 	if (optionDB == NULL)
 		return;
 
-	rewind(optionDB);
+	if (ftruncate(fileno(optionDB), 0) == -1) {
+		log_warn("optionDB ftruncate()");
+		return;
+	}
 
 	leasestr = lease_as_string(name, "offered", offered);
-	if (leasestr != NULL)
-		fprintf(optionDB, "%s", leasestr);
-	else
+	if (leasestr == NULL)
 		log_warnx("%s: cannot make offered lease into string",
 		    log_procname);
+	else if (fprintf(optionDB, "%s", leasestr) == -1)
+		log_warn("optionDB 'offered' fprintf()");
 
 	leasestr = lease_as_string(name, "effective", effective);
-	if (leasestr != NULL)
-		fprintf(optionDB, "%s", leasestr);
-	else
+	if (leasestr == NULL)
 		log_warnx("%s: cannot make effective lease into string",
 		    log_procname);
+	else if (fprintf(optionDB, "%s", leasestr) == -1)
+		log_warn("optionDB 'effective' fprintf()");
 
-	fflush(optionDB);
-	ftruncate(fileno(optionDB), ftello(optionDB));
-	fsync(fileno(optionDB));
+	if (fflush(optionDB) == -1)
+		log_warn("optionDB fflush()");
+	else if (fsync(fileno(optionDB)) == -1)
+		log_warn("optionDB fsync()");
 }
 
 void