qapi: Fix excessive QAPISchemaEntity.check() recursion

Entity checking goes back to commit ac88219a6c "qapi: New QAPISchema
intermediate representation", v2.5.0.  It's designed to work as
follows: QAPISchema.check() calls .check() for all the schema's
entities.  An entity's .check() recurses into another entity's
.check() only if the C struct generated for the former contains the C
struct generated for the latter (pointers don't count).  This is used
to detect "object contains itself".

There are two instances of this:

* An object's C struct contains its base's C struct

  QAPISchemaObjectType.check() calls self.base.check()

* An object's C struct contains its variants' C structs

  QAPISchemaObjectTypeVariants().check calls v.type.check().  Since
  commit b807a1e1e3 "qapi: Check for QAPI collisions involving variant
  members", v2.6.0.

Thus, only object types can participate in recursion.

QAPISchemaObjectType.check() is made for that: it checks @self when
called the first time, recursing into base and variants, it reports an
"contains itself" error when this recursion reaches an object being
checked, and does nothing it reaches an object that has been checked
already.

The other .check() may safely assume they get called exactly once.

Sadly, this design has since eroded:

* QAPISchemaCommand.check() and QAPISchemaEvent.check() call
  .args_type.check().  Since commit c818408e44 "qapi: Implement boxed
  types for commands/events", v2.7.0.  Harmless, since args_type can
  only be an object type.

* QAPISchemaEntity.check() calls ._ifcond.check() when inheriting the
  condition from another type.  Since commit 4fca21c1b0 qapi: leave
  the ifcond attribute undefined until check(), v3.0.0.  This makes
  simple union wrapper types recurse into the wrapped type (nothing
  else uses this condition inheritance).  The .check() of types used
  as simple union branch type get called multiple times.

* QAPISchemaObjectType.check() calls its super type's .check()
  *before* the conditional handling multiple calls.  Also since commit
  4fca21c1b0.  QAPISchemaObjectType.check()'s guard against multiple
  checking doesn't protect QAPISchemaEntity.check().

* QAPISchemaArrayType.check() calls .element_type.check().  Also since
  commit 4fca21c1b0.  The .check() of types used as array element
  types get called multiple times.

  Commit 56a4689582 "qapi: Fix array first used in a different module"
  (v4.0.0) added more code relying on this .element_type.check().

The absence of explosions suggests the .check() involved happen to be
effectively idempotent.

Fix the unwanted recursion anyway:

* QAPISchemaCommand.check() and QAPISchemaEvent.check() calling
  .args_type.check() is unnecessary.  Delete the calls.

* Fix QAPISchemaObjectType.check() to call its super type's .check()
  after the conditional handling multiple calls.

* A QAPISchemaEntity's .ifcond becomes valid at .check().  This is due
  to arrays and simple unions.

  Most types get ifcond and info passed to their constructor.

  Array types don't: they get it from their element type, which
  becomes known only in .element_type.check().

  The implicit wrapper object types for simple union branches don't:
  they get it from the wrapped type, which might be an array.

  Ditch the idea to set .ifcond in .check().  Instead, turn it into a
  property and compute it on demand.  Safe because it's only used
  after the schema has been checked.

  Most types simply return the ifcond passed to their constructor.

  Array types forward to their .element_type instead, and the wrapper
  types forward to the wrapped type.

* A QAPISchemaEntity's .module becomes valid at .check().  This is
  because computing it needs info and schema.fname, and because array
  types get it from their element type instead.

  Make it a property just like .ifcond.

Additionally, have QAPISchemaEntity.check() assert it gets called at
most once, so the design invariant will stick this time.  Neglecting
that was entirely my fault.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-19-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message tidied up]

0 files changed, 0 insertions, 0 deletions