qapi: Emit structs used as variants in topological order

Right now, we emit the branches of union types as a boxed pointer,
and it suffices to have a forward declaration of the type. However,
a future patch will swap things to directly use the branch type,
instead of hiding it behind a pointer. For this to work, the
compiler needs the full definition of the type, not just a forward
declaration, prior to the union that is including the branch type.
This patch just adds topological sorting to hoist all types
mentioned in a branch of a union to be fully declared before the
union itself. The sort is always possible, because we do not
allow circular union types that include themselves as a direct
branch (it is, however, still possible to include a branch type
that itself has a pointer to the union, for a type that can
indirectly recursively nest itself - that remains safe, because
that the member of the branch type will remain a pointer, and the
QMP representation of such a type adds another {} for each recurring
layer of the union type).

Backports commit 1de5d4ca0752138034305f3d4e8fe17ef6503569 from qemu

qemu/scripts/qapi-types.py

@@ -2,7 +2,7 @@
 # QAPI types generator
 #
 # Copyright IBM, Corp. 2011
-# Copyright (c) 2013-2015 Red Hat Inc.
+# Copyright (c) 2013-2016 Red Hat Inc.
 #
 # Authors:
 #  Anthony Liguori <aliguori@us.ibm.com>
@@ -13,6 +13,10 @@
 
 from qapi import *
 
+# variants must be emitted before their container; track what has already
+# been output
+objects_seen = set()
+
 
 def gen_fwd_object_or_array(name):
     return mcgen('''
@@ -51,7 +55,19 @@ def gen_struct_fields(members):
 
 
 def gen_object(name, base, members, variants):
-    ret = mcgen('''
+    if name in objects_seen:
+        return ''
+    objects_seen.add(name)
+
+    ret = ''
+    if variants:
+        for v in variants.variants:
+            if (isinstance(v.type, QAPISchemaObjectType) and
+                    not v.type.is_implicit()):
+                ret += gen_object(v.type.name, v.type.base,
+                                  v.type.local_members, v.type.variants)
+
+    ret += mcgen('''
 
 struct %(c_name)s {
 ''',