qapi: Create simple union type member earlier

For simple unions, we were creating the implicit 'type' tag
member during the QAPISchemaObjectTypeVariants constructor.
This is different from every other implicit QAPISchemaEntity
object, which get created by QAPISchema methods.  Hoist the
creation to the caller (renaming _make_tag_enum() to
_make_implicit_tag()), and pass the entity rather than the
string name, so that we have the nice property that no
entities are created as a side effect within a different
entity.  A later patch will then have an easier time of
associating location info with each entity creation.

No change to generated code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1444710158-8723-10-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi.py

diff --git a/scripts/qapi.py b/scripts/qapi.py
index 9e01705..471bbfc 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -1010,18 +1010,18 @@
 
 
 class QAPISchemaObjectTypeVariants(object):
-    def __init__(self, tag_name, tag_enum, variants):
-        assert tag_name is None or isinstance(tag_name, str)
-        assert tag_enum is None or isinstance(tag_enum, str)
+    def __init__(self, tag_name, tag_member, variants):
+        # Flat unions pass tag_name but not tag_member.
+        # Simple unions and alternates pass tag_member but not tag_name.
+        # After check(), tag_member is always set, and tag_name remains
+        # a reliable witness of being used by a flat union.
+        assert bool(tag_member) != bool(tag_name)
+        assert (isinstance(tag_name, str) or
+                isinstance(tag_member, QAPISchemaObjectTypeMember))
         for v in variants:
             assert isinstance(v, QAPISchemaObjectTypeVariant)
         self.tag_name = tag_name
-        if tag_name:
-            assert not tag_enum
-            self.tag_member = None
-        else:
-            self.tag_member = QAPISchemaObjectTypeMember('type', tag_enum,
-                                                         False)
+        self.tag_member = tag_member
         self.variants = variants
 
     def check(self, schema, members, seen):
@@ -1231,28 +1231,29 @@
                                               [self._make_member('data', typ)])
         return QAPISchemaObjectTypeVariant(case, typ)
 
-    def _make_tag_enum(self, type_name, variants):
-        return self._make_implicit_enum_type(type_name,
-                                             [v.name for v in variants])
+    def _make_implicit_tag(self, type_name, variants):
+        typ = self._make_implicit_enum_type(type_name,
+                                            [v.name for v in variants])
+        return QAPISchemaObjectTypeMember('type', typ, False)
 
     def _def_union_type(self, expr, info):
         name = expr['union']
         data = expr['data']
         base = expr.get('base')
         tag_name = expr.get('discriminator')
-        tag_enum = None
+        tag_member = None
         if tag_name:
             variants = [self._make_variant(key, value)
                         for (key, value) in data.iteritems()]
         else:
             variants = [self._make_simple_variant(key, value)
                         for (key, value) in data.iteritems()]
-            tag_enum = self._make_tag_enum(name, variants)
+            tag_member = self._make_implicit_tag(name, variants)
         self._def_entity(
             QAPISchemaObjectType(name, info, base,
                                  self._make_members(OrderedDict()),
                                  QAPISchemaObjectTypeVariants(tag_name,
-                                                              tag_enum,
+                                                              tag_member,
                                                               variants)))
 
     def _def_alternate_type(self, expr, info):
@@ -1260,11 +1261,11 @@
         data = expr['data']
         variants = [self._make_variant(key, value)
                     for (key, value) in data.iteritems()]
-        tag_enum = self._make_tag_enum(name, variants)
+        tag_member = self._make_implicit_tag(name, variants)
         self._def_entity(
             QAPISchemaAlternateType(name, info,
                                     QAPISchemaObjectTypeVariants(None,
-                                                                 tag_enum,
+                                                                 tag_member,
                                                                  variants)))
 
     def _def_command(self, expr, info):