qemu/tests/qapi-schema/union-empty.json

# union discriminator enum cannot be empty
{ 'enum': 'Empty', 'data': [ ] }
{ 'struct': 'Base', 'data': { 'type': 'Empty' } }
{ 'union': 'Union', 'base': 'Base', 'discriminator': 'type', 'data': { } }
qapi: Drop simple unions Simple unions predate flat unions. Having both complicates the QAPI schema language and the QAPI generator. We haven't been using simple unions in new code for a long time, because they are less flexible and somewhat awkward on the wire. The previous commits eliminated simple union from the tree. Now drop them from the QAPI schema language entirely, and update mentions of "flat union" to just "union". Signed-off-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Message-Id: <20210917143134.412106-22-armbru@redhat.com> 2021-09-17 14:31:32 +00:00			`# union discriminator enum cannot be empty`
qapi: Add tests for empty unions The documentation claims that alternates are useful for allowing two or more types, although nothing enforces this. Meanwhile, it is silent on whether empty unions are allowed. In practice, the generated code will compile, in part because we have a 'void *data' branch; but attempting to visit such a type will cause an abort(). While there's no technical reason that a degenerate union could not be made to work, it's harder to justify the time spent in chasing known (the current abort() during visit) and unknown corner cases, than it would be to just outlaw them. A future patch will probably take the approach of forbidding them; in the meantime, we can at least add testsuite coverage to make it obvious where things stand. In addition to adding tests to expose the problems, we also need to adjust existing tests that are meant to test something else, but which could fail for the wrong reason if we reject degenerate alternates/unions. Note that empty structs are explicitly supported (for example, right now they are the only way to specify that one branch of a flat union adds no additional members), and empty enums are covered by the testsuite as working (even if they do not seem to have much use). Signed-off-by: Eric Blake <eblake@redhat.com> Message-Id: <1443565276-4535-8-git-send-email-eblake@redhat.com> Signed-off-by: Markus Armbruster <armbru@redhat.com> 2015-09-29 22:21:05 +00:00			`{ 'enum': 'Empty', 'data': [ ] }`
			`{ 'struct': 'Base', 'data': { 'type': 'Empty' } }`
			`{ 'union': 'Union', 'base': 'Base', 'discriminator': 'type', 'data': { } }`