qapi: Add new clone visitor

We have a couple places in the code base that want to deep-clone
one QAPI object into another, and they were resorting to serializing
the struct out to QObject then reparsing it.  A much more efficient
version can be done by adding a new clone visitor.

Since cloning is still relatively uncommon, expose the use of the
new visitor via a QAPI_CLONE() macro that takes care of type-punning
the underlying function pointer, rather than generating lots of
unused functions for types that won't be cloned.  And yes, we're
relying on the compiler treating all pointers equally, even though
a strict C program cannot portably do so - but we're not the first
one in the qemu code base to expect it to work (hello, glib!).

The choice of adding a fourth visitor type deserves some explanation.
On the surface, the clone visitor is mostly an input visitor (it
takes arbitrary input - in this case, another QAPI object - and
creates a new QAPI object during the course of the visit).  But
ever since commit da72ab0 consolidated enum visits based on the
visitor type, using VISITOR_INPUT would cause us to run
visit_type_str(), even though for cloning there is nothing to do
(we just copy the enum value across, without regards to its mapping
to strings).   Also, since our input happens to be a QAPI object,
we can also satisfy the internal checks for VISITOR_OUTPUT.  So in
the end, I settled with a new VISITOR_CLONE, and chose its value
such that many internal checks can use 'v->type & mask', sticking
to 'v->type == value' where the difference matters.

Note that we can only clone objects (including alternates) and lists,
not built-ins or enums.  The visitor core hides integer width from
the actual visitor (since commit 04e070d), and as long as that's the
case, we can't clone top-level integers.  Then again, those can
always be cloned by direct copy, since they are not objects with
deep pointers, so it's no real loss.  And restricting cloning to
just objects and lists is cleaner than restricting it to non-integers.
As such, I documented that the clone visitor is for direct use only
by code internal to QAPI, and should not be used on incomplete objects
(other than a hack to work around the fact that we allow NULL in place
of "" in visit_type_str() in other output visitors).  Note that as
written, the clone visitor will never fail on a complete object.

Scalars (including enums) not at the root of the clone copy just fine
with no additional effort while visiting the scalar, by virtue of a
g_memdup() each time we push another struct onto the stack.  Cloning
a string requires deduplication of a pointer, which means it can also
provide the guarantee of an input visitor of never producing NULL
even when still accepting NULL in place of "" the way the QMP output
visitor does.

Cloning an 'any' type could be possible by incrementing the QObject
refcnt, but it's not obvious whether that is better than implementing
a QObject deep clone.  So for now, we document it as unsupported,
and intentionally omit the .type_any() callback to let a developer
know their usage needs implementation.

Add testsuite coverage for several different clone situations, to
ensure that the code is working.  I also tested that valgrind was
happy with the test.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1465490926-28625-14-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 38 insertions, 28 deletions

diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 00a60eaaab..fb8f4eb6ec 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -24,15 +24,16 @@
  * for doing work at each node of a QAPI graph; it can also be used
  * for a virtual walk, where there is no actual QAPI C struct.
  *
- * There are three kinds of visitor classes: input visitors (QMP,
+ * There are four kinds of visitor classes: input visitors (QMP,
  * string, and QemuOpts) parse an external representation and build
  * the corresponding QAPI graph, output visitors (QMP and string) take
- * a completed QAPI graph and generate an external representation, and
- * the dealloc visitor can take a QAPI graph (possibly partially
- * constructed) and recursively free its resources.  While the dealloc
- * and QMP input/output visitors are general, the string and QemuOpts
- * visitors have some implementation limitations; see the
- * documentation for each visitor for more details on what it
+ * a completed QAPI graph and generate an external representation, the
+ * dealloc visitor can take a QAPI graph (possibly partially
+ * constructed) and recursively free its resources, and the clone
+ * visitor performs a deep clone of one QAPI object to another.  While
+ * the dealloc and QMP input/output visitors are general, the string,
+ * QemuOpts, and clone visitors have some implementation limitations;
+ * see the documentation for each visitor for more details on what it
  * supports.  Also, see visitor-impl.h for the callback contracts
  * implemented by each visitor, and docs/qapi-code-gen.txt for more
  * about the QAPI code generator.
@@ -80,9 +81,9 @@
  *
  * If an error is detected during visit_type_FOO() with an input
  * visitor, then *@obj will be NULL for pointer types, and left
- * unchanged for scalar types.  Using an output visitor with an
- * incomplete object has undefined behavior (other than a special case
- * for visit_type_str() treating NULL like ""), while the dealloc
+ * unchanged for scalar types.  Using an output or clone visitor with
+ * an incomplete object has undefined behavior (other than a special
+ * case for visit_type_str() treating NULL like ""), while the dealloc
  * visitor safely handles incomplete objects.  Since input visitors
  * never produce an incomplete object, such an object is possible only
  * by manual construction.
@@ -102,11 +103,19 @@
  *
  * void qapi_free_FOO(FOO *obj);
  *
- * which behaves like free() in that @obj may be NULL.  Because of
- * these functions, the dealloc visitor is seldom used directly
- * outside of generated code.  QAPI types can also inherit from a base
- * class; when this happens, a function is generated for easily going
- * from the derived type to the base type:
+ * where behaves like free() in that @obj may be NULL.  Such objects
+ * may also be used with the following macro, provided alongside the
+ * clone visitor:
+ *
+ * Type *QAPI_CLONE(Type, src);
+ *
+ * in order to perform a deep clone of @src.  Because of the generated
+ * qapi_free functions and the QAPI_CLONE() macro, the clone and
+ * dealloc visitor should not be used directly outside of QAPI code.
+ *
+ * QAPI types can also inherit from a base class; when this happens, a
+ * function is generated for easily going from the derived type to the
+ * base type:
  *
  * BASE *qapi_CHILD_base(CHILD *obj);
  *
@@ -272,9 +281,9 @@ void visit_free(Visitor *v);
  * container; see the general description of @name above.
  *
  * @obj must be non-NULL for a real walk, in which case @size
- * determines how much memory an input visitor will allocate into
- * *@obj.  @obj may also be NULL for a virtual walk, in which case
- * @size is ignored.
+ * determines how much memory an input or clone visitor will allocate
+ * into *@obj.  @obj may also be NULL for a virtual walk, in which
+ * case @size is ignored.
  *
  * @errp obeys typical error usage, and reports failures such as a
  * member @name is not present, or present but not an object.  On
@@ -327,9 +336,9 @@ void visit_end_struct(Visitor *v, void **obj);
  * container; see the general description of @name above.
  *
  * @list must be non-NULL for a real walk, in which case @size
- * determines how much memory an input visitor will allocate into
- * *@list (at least sizeof(GenericList)).  Some visitors also allow
- * @list to be NULL for a virtual walk, in which case @size is
+ * determines how much memory an input or clone visitor will allocate
+ * into *@list (at least sizeof(GenericList)).  Some visitors also
+ * allow @list to be NULL for a virtual walk, in which case @size is
  * ignored.
  *
  * @errp obeys typical error usage, and reports failures such as a
@@ -386,10 +395,10 @@ void visit_end_list(Visitor *v, void **list);
  * @name expresses the relationship of this alternate to its parent
  * container; see the general description of @name above.
  *
- * @obj must not be NULL. Input visitors use @size to determine how
- * much memory to allocate into *@obj, then determine the qtype of the
- * next thing to be visited, stored in (*@obj)->type.  Other visitors
- * will leave @obj unchanged.
+ * @obj must not be NULL. Input and clone visitors use @size to
+ * determine how much memory to allocate into *@obj, then determine
+ * the qtype of the next thing to be visited, stored in (*@obj)->type.
+ * Other visitors will leave @obj unchanged.
  *
  * If @promote_int, treat integers as QTYPE_FLOAT.
  *
@@ -554,9 +563,10 @@ void visit_type_bool(Visitor *v, const char *name, bool *obj, Error **errp);
  * @name expresses the relationship of this string to its parent
  * container; see the general description of @name above.
  *
- * @obj must be non-NULL.  Input visitors set *@obj to the value
- * (never NULL).  Other visitors leave *@obj unchanged, and commonly
- * treat NULL like "".
+ * @obj must be non-NULL.  Input and clone visitors set *@obj to the
+ * value (always using "" rather than NULL for an empty string).
+ * Other visitors leave *@obj unchanged, and commonly treat NULL like
+ * "".
  *
  * It is safe to cast away const when preparing a (const char *) value
  * into @obj for use by an output visitor.