[Qemu-devel] Problem with discriminated unions with enum prefixes

["Daniel P. Berrange" <berrange@redhat.com>]

In my LUKS encryption series, I have a discriminated union for
storing options for different encryption formats. See qapi/crypto.json
in this file:

  https://lists.gnu.org/archive/html/qemu-devel/2016-02/msg03187.html

You'll notice I have the 'prefix' line for the enum commented out. When
I uncomment this, I discovered that the discriminated union visitor does
not deal with prefixes.

To save having to wade through that big series, here is a quick
reproducer

diff --git a/qapi-schema.json b/qapi-schema.json
index 8d04897..a58648d 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -4083,3 +4083,22 @@
 ##
 { 'enum': 'ReplayMode',
   'data': [ 'none', 'record', 'play' ] }
+
+
+{ 'enum': 'QDemoType',
+  'prefix': 'QDEMO_TYPE',
+  'data': ['foo', 'bar' ] }
+
+{ 'struct': 'QDemoBase',
+  'data': { 'type': 'QDemoType' } }
+
+{ 'struct': 'QDemoFoo',
+  'data': { 'eek': 'int' } }
+
+{ 'struct': 'QDemoBar',
+  'data': { 'wizz': 'str' } }
+
+{ 'union': 'QDemo',
+  'base': 'QDemoBase',
+  'discriminator': 'type',
+  'data': { 'foo': 'QDemoFoo', 'bar': 'QDemoBar' } }


Apply that and then try to build and it'll fail with:

qapi-visit.c: In function ‘visit_type_QDemo’:
qapi-visit.c:7596:10: error: ‘Q_DEMO_TYPE_FOO’ undeclared (first use in this function)
     case Q_DEMO_TYPE_FOO:
          ^
qapi-visit.c:7596:10: note: each undeclared identifier is reported only once for each function it appears in
qapi-visit.c:7599:10: error: ‘Q_DEMO_TYPE_BAR’ undeclared (first use in this function)
     case Q_DEMO_TYPE_BAR:
          ^

The issue is that we used the 'QDEMO_TYPE' custom prefix for generating the
enum, but we didn't use the prefix in the union visitor.

I know we had had previous discussions with Markus strongly wanting to kill
off the support for enum prefixes. So before I waste time trying to fix
this union visitor code to handle prefixes, I figure we should decide if
we actually want to fix it, or go with Markus' plan to kill custom prefixes
on enums.

Per previous discussions, I think the ability to have custom prefixes is
quite desirable, to get more natural enum constant names. At the end of
the day though, the default enum naming is far from the worst bit of
QEMU, so I'm not ultimately too bothered either way. We either make
custom enum prefixes work everything they need to, or remove them.

Regards,
Daniel
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