qapi: Allow anonymous base for flat union

Rather than requiring all flat unions to explicitly create
a separate base struct, we can allow the qapi schema to specify
the common members via an inline dictionary. This is similar to
how commands can specify an inline anonymous type for its 'data'.
We already have several struct types that only exist to serve as
a single flat union's base; the next commit will clean them up.
In particular, this patch's change to the BlockdevOptions example
in qapi-code-gen.txt will actually be done in the real QAPI schema.

Now that anonymous bases are legal, we need to rework the
flat-union-bad-base negative test (as previously written, it
forms what is now valid QAPI; tweak it to now provide coverage
of a new error message path), and add a positive test in
qapi-schema-test to use an anonymous base (making the integer
argument optional, for even more coverage).

Note that this patch only allows anonymous bases for flat unions;
simple unions are already enough syntactic sugar that we do not
want to burden them further. Meanwhile, while it would be easy
to also allow an anonymous base for structs, that would be quite
redundant, as the members can be put right into the struct
instead.

Backports commit ac4338f8eb783fd421aae492ca262a586918471e from qemu

qemu/scripts/qapi-types.py

@@ -73,12 +73,14 @@ struct %(c_name)s {
                  c_name=c_name(name))
 
     if base:
-        ret += mcgen('''
+        if not base.is_implicit():
+            ret += mcgen('''
     /* Members inherited from %(c_name)s: */
 ''',
-                     c_name=base.c_name())
+                         c_name=base.c_name())
         ret += gen_struct_members(base.members)
-        ret += mcgen('''
+        if not base.is_implicit():
+            ret += mcgen('''
     /* Own members: */
 ''')
     ret += gen_struct_members(members)
@@ -258,7 +260,7 @@ class QAPISchemaGenTypeVisitor(QAPISchemaVisitor):
             return
         self._fwdecl += gen_fwd_object_or_array(name)
         self.decl += gen_object(name, base, members, variants)
-        if base:
+        if base and not base.is_implicit():
             self.decl += gen_upcast(name, base)
         # TODO Worth changing the visitor signature, so we could
         # directly use rather than repeat type.is_implicit()?

qemu/scripts/qapi.py

@@ -332,6 +332,8 @@ class QAPISchemaParser(object):
 
 
 def find_base_members(base):
+    if isinstance(base, dict):
+        return base
     base_struct_define = find_struct(base)
     if not base_struct_define:
         return None
@@ -565,9 +567,10 @@ def check_union(expr, expr_info):
 
     # Else, it's a flat union.
     else:
-        # The object must have a string member 'base'.
+        # The object must have a string or dictionary 'base'.
         check_type(expr_info, "'base' for union '%s'" % name,
-                   base, allow_metas=['struct'])
+                   base, allow_dict=True, allow_optional=True,
+                   allow_metas=['struct'])
         if not base:
             raise QAPIExprError(expr_info,
                                 "Flat union '%s' must have a base"
@@ -1045,6 +1048,8 @@ class QAPISchemaMember(object):
             owner = owner[6:]
             if owner.endswith('-arg'):
                 return '(parameter of %s)' % owner[:-4]
+            elif owner.endswith('-base'):
+                return '(base of %s)' % owner[:-5]
             else:
                 assert owner.endswith('-wrapper')
                 # Unreachable and not implemented
@@ -1339,6 +1344,9 @@ class QAPISchema(object):
         base = expr.get('base')
         tag_name = expr.get('discriminator')
         tag_member = None
+        if isinstance(base, dict):
+            base = (self._make_implicit_object_type(
+                    name, info, 'base', self._make_members(base, info)))
         if tag_name:
             variants = [self._make_variant(key, value)
                         for (key, value) in data.iteritems()]