qapi: allow empty branches in flat unions

It often happens that just a few discriminator values imply extra data in
a flat union. Existing checks did not make possible to leave other values
uncovered. Such cases had to be worked around by either stating a dummy
(empty) type or introducing another (subset) discriminator enumeration.

Both options create redundant entities in qapi files for little profit.

With this patch it is not necessary anymore to add designated union
fields for every possible value of a discriminator enumeration.

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Message-Id: <1529311206-76847-2-git-send-email-anton.nefedov@virtuozzo.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

tests/qapi-schema/flat-union-incomplete-branch.err

@@ -1 +0,0 @@
-tests/qapi-schema/flat-union-incomplete-branch.json:6: Union 'TestUnion' data missing 'value2' branch

tests/qapi-schema/flat-union-incomplete-branch.exit

@@ -1 +0,0 @@
-1

tests/qapi-schema/flat-union-incomplete-branch.json

@@ -1,9 +0,0 @@
-# we require all branches of the union to be covered
-{ 'enum': 'TestEnum',
-  'data': [ 'value1', 'value2' ] }
-{ 'struct': 'TestTypeA',
-  'data': { 'string': 'str' } }
-{ 'union': 'TestUnion',
-  'base': { 'type': 'TestEnum' },
-  'discriminator': 'type',
-  'data': { 'value1': 'TestTypeA' } }

tests/qapi-schema/qapi-schema-test.json

@@ -39,7 +39,7 @@
             '*enum1': 'EnumOne' } }   # intentional forward reference
 
 { 'enum': 'EnumOne',
-  'data': [ 'value1', 'value2', 'value3' ] }
+  'data': [ 'value1', 'value2', 'value3', 'value4' ] }
 
 { 'struct': 'UserDefZero',
   'data': { 'integer': 'int' } }
@@ -76,7 +76,9 @@
   'discriminator': 'enum1',
   'data': { 'value1' : 'UserDefA',
             'value2' : 'UserDefB',
-            'value3' : 'UserDefB' } }
+            'value3' : 'UserDefB'
+            # 'value4' defaults to empty
+  } }
 
 { 'struct': 'UserDefUnionBase',
   'base': 'UserDefZero',

tests/qapi-schema/qapi-schema-test.out

@@ -23,7 +23,7 @@ object UserDefOne
     base UserDefZero
     member string: str optional=False
     member enum1: EnumOne optional=True
-enum EnumOne ['value1', 'value2', 'value3']
+enum EnumOne ['value1', 'value2', 'value3', 'value4']
 object UserDefZero
     member integer: int optional=False
 object UserDefTwoDictDict
@@ -52,6 +52,7 @@ object UserDefFlatUnion
     case value1: UserDefA
     case value2: UserDefB
     case value3: UserDefB
+    case value4: q_empty
 object UserDefUnionBase
     base UserDefZero
     member string: str optional=False