qapi: Don't use info as witness of implicit object type

A future patch will enable error reporting from the various
QAPISchema*.check() methods. But to report an error related
to an implicit type, we'll need to associate a location with
the type (the same location as the top-level entity that is
causing the creation of the implicit type), and once we do
that, keying off of whether foo.info exists is no longer a
viable way to determine if foo is an implicit type.

Instead, add an is_implicit() method to QAPISchemaEntity, and use it.
It can be overridden later for ObjectType and EnumType, when implicit
instances of those classes gain info.

Backports commit 49823c4b4304a3e4aa5d67e089946b12d6a52d64 from qemu

qemu/scripts/qapi-types.py

@@ -230,7 +230,8 @@ class QAPISchemaGenTypeVisitor(QAPISchemaVisitor):
 
     def visit_needed(self, entity):
         # Visit everything except implicit objects
-        return not isinstance(entity, QAPISchemaObjectType) or entity.info
+        return not (entity.is_implicit() and
+                    isinstance(entity, QAPISchemaObjectType))
 
     def _gen_type_cleanup(self, name):
         self.decl += gen_type_cleanup_decl(name)

qemu/scripts/qapi-visit.py

@@ -334,7 +334,8 @@ class QAPISchemaGenVisitVisitor(QAPISchemaVisitor):
 
     def visit_needed(self, entity):
         # Visit everything except implicit objects
-        return not isinstance(entity, QAPISchemaObjectType) or entity.info
+        return not (entity.is_implicit() and
+                    isinstance(entity, QAPISchemaObjectType))
 
     def visit_enum_type(self, name, info, values, prefix):
         self.decl += gen_visit_decl(name, scalar=True)

qemu/scripts/qapi.py

@@ -798,6 +798,9 @@ class QAPISchemaEntity(object):
     def check(self, schema):
         pass
 
+    def is_implicit(self):
+        return not self.info
+
     def visit(self, visitor):
         pass
 
@@ -971,11 +974,11 @@ class QAPISchemaObjectType(QAPISchemaType):
         self.members = members
 
     def c_name(self):
-        assert self.info
+        assert not self.is_implicit()
         return QAPISchemaType.c_name(self)
 
     def c_type(self, is_param=False):
-        assert self.info
+        assert not self.is_implicit()
         return QAPISchemaType.c_type(self)
 
     def json_type(self):
@@ -1042,7 +1045,8 @@ class QAPISchemaObjectTypeVariant(QAPISchemaObjectTypeMember):
     # This function exists to support ugly simple union special cases
     # TODO get rid of them, and drop the function
     def simple_union_type(self):
-        if isinstance(self.type, QAPISchemaObjectType) and not self.type.info:
+        if (self.type.is_implicit() and
+                isinstance(self.type, QAPISchemaObjectType)):
             assert len(self.type.members) == 1
             assert not self.type.variants
             return self.type.members[0].type
@@ -1161,11 +1165,13 @@ class QAPISchema(object):
         self._def_entity(self.the_empty_object_type)
 
     def _make_implicit_enum_type(self, name, values):
-        name = name + 'Kind'
+        name = name + 'Kind'   # Use namespace reserved by add_name()
         self._def_entity(QAPISchemaEnumType(name, None, values, None))
         return name
 
     def _make_array_type(self, element_type):
+        # TODO fooList namespace is not reserved; user can create collisions,
+        # or abuse our type system with ['fooList'] for 2D array
         name = element_type + 'List'
         if not self.lookup_type(name):
             self._def_entity(QAPISchemaArrayType(name, None, element_type))