[Qemu-devel] [RFC PATCH 12.6/47] qapi: Document shortcoming with union 'data' branch

[Eric Blake]

Add a FIXME to remind us to fully audit whether removing the
'void *data' branch of each qapi union type can be done safely.

Signed-off-by: Eric Blake <address@hidden>
---

Another potential doc FIXME patch, this time based on 7/47
(https://lists.gnu.org/archive/html/qemu-devel/2015-07/msg06101.html)

 scripts/qapi-types.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index c6c2786..b3434b9 100644
--- a/scripts/qapi-types.py
+++ b/scripts/qapi-types.py
@@ -217,6 +217,14 @@ struct %(name)s
 ''',
                      discriminator_type_name=c_name(discriminator_type_name))

+    # FIXME: What purpose does data serve, besides preventing a union that
+    # has a branch named 'data'? We use it in qapi-visit.py to decide
+    # whether to bypass the switch statement if visiting the discriminator
+    # failed; but since we 0-initialize structs, and cannot tell what
+    # branch of the union is in use if the discriminator is invalid, there
+    # should not be any data leaks even without a data pointer.  Or, if
+    # 'data' is merely added to guarantee we don't have an empty union,
+    # shouldn't we enforce that at .json parse time?
     ret += mcgen('''
     union {
         void *data;
-- 
2.4.3