qapi: Document that input visitor semantics are prone to leaks

Most functions that can return a pointer or set an Error ** value
are decent enough to guarantee a NULL return when reporting an error.
Not so with our generated qapi visitor functions.  If the caller
is not careful to clean up partially-allocated objects on error,
then the caller suffers a memory leak.

Properly fixing it is probably complex enough to save for a later
day, so merely document it for now.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1438295587-19069-1-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi-visit.py

@@ -115,6 +115,10 @@ def generate_visit_struct_fields(name, members, base = None):
 
 
 def generate_visit_struct_body(name):
+    # FIXME: if *obj is NULL on entry, and visit_start_struct() assigns to
+    # *obj, but then visit_type_FOO_fields() fails, we should clean up *obj
+    # rather than leaving it non-NULL. As currently written, the caller must
+    # call qapi_free_FOO() to avoid a memory leak of the partial FOO.
     ret = mcgen('''
     Error *err = NULL;
 

tests/test-qmp-input-visitor.c

@@ -636,6 +636,8 @@ static void test_visitor_in_errors(TestInputVisitorData *data,
 
     visit_type_TestStruct(v, &p, NULL, &err);
     g_assert(err);
+    /* FIXME - a failed parse should not leave a partially-allocated p
+     * for us to clean up; this could cause callers to leak memory. */
     g_assert(p->string == NULL);
 
     error_free(err);