qapi: Permit omitting all flat union branches

Absent flat union branches default to the empty struct (since commit
800877bb16 "qapi: allow empty branches in flat unions").  But an
attempt to omit all of them is rejected with "Union 'FOO' has no
branches".  Harmless oddity, but it's easy to avoid, so do that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-11-armbru@redhat.com>
[Commit message typo fixed]

docs/devel/qapi-code-gen.txt

@@ -436,8 +436,7 @@ Union types are used to let the user choose between several different
 variants for an object.  There are two flavors: simple (no
 discriminator or base), and flat (both discriminator and base).  A union
 type is defined using a data dictionary as explained in the following
-paragraphs.  The data dictionary for either type of union must not
-be empty.
+paragraphs.  Unions must have at least one branch.
 
 A simple union type defines a mapping from automatic discriminator
 values to data types like in this example:

scripts/qapi/common.py

@@ -852,7 +852,7 @@ def check_union(expr, info):
 
     # With no discriminator it is a simple union.
     if discriminator is None:
-        enum_define = None
+        enum_values = members.keys()
         allow_metas = ['built-in', 'union', 'alternate', 'struct', 'enum']
         if base is not None:
             raise QAPISemError(info, "Simple union '%s' must not have a base" %
@@ -885,16 +885,17 @@ def check_union(expr, info):
                                'must not be conditional' %
                                (base, discriminator, name))
         enum_define = enum_types.get(discriminator_value['type'])
-        allow_metas = ['struct']
         # Do not allow string discriminator
         if not enum_define:
             raise QAPISemError(info,
                                "Discriminator '%s' must be of enumeration "
                                "type" % discriminator)
+        enum_values = enum_get_names(enum_define)
+        allow_metas = ['struct']
+
+    if (len(enum_values) == 0):
+        raise QAPISemError(info, "Union '%s' has no branches" % name)
 
-    # Check every branch; don't allow an empty union
-    if len(members) == 0:
-        raise QAPISemError(info, "Union '%s' cannot have empty 'data'" % name)
     for (key, value) in members.items():
         check_name(info, "Member of union '%s'" % name, key)
 
@@ -907,8 +908,8 @@ def check_union(expr, info):
 
         # If the discriminator names an enum type, then all members
         # of 'data' must also be members of the enum type.
-        if enum_define:
-            if key not in enum_get_names(enum_define):
+        if discriminator is not None:
+            if key not in enum_values:
                 raise QAPISemError(info,
                                    "Discriminator value '%s' is not found in "
                                    "enum '%s'"
@@ -1578,7 +1579,6 @@ def __init__(self, tag_name, tag_member, variants):
         assert bool(tag_member) != bool(tag_name)
         assert (isinstance(tag_name, str) or
                 isinstance(tag_member, QAPISchemaObjectTypeMember))
-        assert len(variants) > 0
         for v in variants:
             assert isinstance(v, QAPISchemaObjectTypeVariant)
         self._tag_name = tag_name

tests/qapi-schema/flat-union-empty.err

@@ -1 +1 @@
-tests/qapi-schema/flat-union-empty.json:4: Union 'Union' cannot have empty 'data'
+tests/qapi-schema/flat-union-empty.json:4: Union 'Union' has no branches

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

@@ -1,4 +1,4 @@
-# flat unions cannot be empty
+# flat union discriminator cannot be empty
 { 'enum': 'Empty', 'data': [ ] }
 { 'struct': 'Base', 'data': { 'type': 'Empty' } }
 { 'union': 'Union', 'base': 'Base', 'discriminator': 'type', 'data': { } }

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

@@ -25,6 +25,11 @@
 { 'struct': 'Empty1', 'data': { } }
 { 'struct': 'Empty2', 'base': 'Empty1', 'data': { } }
 
+# Likewise for an empty flat union
+{ 'union': 'Union',
+  'base': { 'type': 'EnumOne' }, 'discriminator': 'type',
+  'data': { } }
+
 { 'command': 'user_def_cmd0', 'data': 'Empty2', 'returns': 'Empty2' }
 
 # for testing override of default naming heuristic

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

@@ -23,6 +23,15 @@ enum MyEnum
 object Empty1
 object Empty2
     base Empty1
+object q_obj_Union-base
+    member type: EnumOne optional=False
+object Union
+    base q_obj_Union-base
+    tag type
+    case value1: q_empty
+    case value2: q_empty
+    case value3: q_empty
+    case value4: q_empty
 command user_def_cmd0 Empty2 -> Empty2
    gen=True success_response=True boxed=False oob=False preconfig=False
 enum QEnumTwo

tests/qapi-schema/union-empty.err

@@ -1 +1 @@
-tests/qapi-schema/union-empty.json:2: Union 'Union' cannot have empty 'data'
+tests/qapi-schema/union-empty.json:2: Union 'Union' has no branches

tests/qapi-schema/union-empty.json

@@ -1,2 +1,2 @@
-# unions cannot be empty
+# simple unions cannot be empty
 { 'union': 'Union', 'data': { } }