qapi: Drop support for boxed alternate arguments

Commands and events can define their argument type inline (default) or
by referring to another type ('boxed': true, since commit c818408e44
"qapi: Implement boxed types for commands/events", v2.7.0).  The
unboxed inline definition is an (anonymous) struct type.  The boxed
type may be a struct, union, or alternate type.

The latter is problematic: docs/interop/qemu-spec.txt requires the
value of the 'data' key to be a json-object, but any non-degenerate
alternate type has at least one branch that isn't.

Fortunately, we haven't made use of alternates in this context outside
tests/.  Drop support for them.

QAPISchemaAlternateType.is_empty() is now unused.  Drop it, too.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-4-armbru@redhat.com>

docs/devel/qapi-code-gen.txt

@@ -612,9 +612,9 @@ the command.  Normally, 'data' is a dictionary for an anonymous type,
 or names a struct type (possibly empty, but not a union), and its
 members are passed as separate arguments to this function.  If the
 command definition includes a key 'boxed' with the boolean value true,
-then 'data' is instead the name of any non-empty complex type
-(struct, union, or alternate), and a pointer to that QAPI type is
-passed as a single argument.
+then 'data' is instead the name of any non-empty complex type (struct
+or union), and a pointer to that QAPI type is passed as a single
+argument.
 
 The generator also emits a marshalling function that extracts
 arguments for the user's function out of an input QDict, calls the
@@ -714,9 +714,9 @@ The generator emits a function to send the event.  Normally, 'data' is
 a dictionary for an anonymous type, or names a struct type (possibly
 empty, but not a union), and its members are passed as separate
 arguments to this function.  If the event definition includes a key
-'boxed' with the boolean value true, then 'data' is instead the name of
-any non-empty complex type (struct, union, or alternate), and a
-pointer to that QAPI type is passed as a single argument.
+'boxed' with the boolean value true, then 'data' is instead the name
+of any non-empty complex type (struct or union), and a pointer to that
+QAPI type is passed as a single argument.
 
 
 === Features ===

scripts/qapi/common.py

@@ -840,7 +840,7 @@ def check_command(expr, info):
 
     args_meta = ['struct']
     if boxed:
-        args_meta += ['union', 'alternate']
+        args_meta += ['union']
     check_type(info, "'data' for command '%s'" % name,
                expr.get('data'), allow_dict=not boxed,
                allow_metas=args_meta)
@@ -858,7 +858,7 @@ def check_event(expr, info):
 
     meta = ['struct']
     if boxed:
-        meta += ['union', 'alternate']
+        meta += ['union']
     check_type(info, "'data' for event '%s'" % name,
                expr.get('data'), allow_dict=not boxed,
                allow_metas=meta)
@@ -1690,9 +1690,6 @@ class QAPISchemaAlternateType(QAPISchemaType):
         visitor.visit_alternate_type(self.name, self.info, self.ifcond,
                                      self.variants)
 
-    def is_empty(self):
-        return False
-
 
 class QAPISchemaCommand(QAPISchemaEntity):
     def __init__(self, name, info, doc, ifcond, arg_type, ret_type,
@@ -1714,15 +1711,13 @@ class QAPISchemaCommand(QAPISchemaEntity):
         QAPISchemaEntity.check(self, schema)
         if self._arg_type_name:
             self.arg_type = schema.lookup_type(self._arg_type_name)
-            assert (isinstance(self.arg_type, QAPISchemaObjectType) or
-                    isinstance(self.arg_type, QAPISchemaAlternateType))
+            assert isinstance(self.arg_type, QAPISchemaObjectType)
             self.arg_type.check(schema)
             if self.boxed:
                 if self.arg_type.is_empty():
                     raise QAPISemError(self.info,
                                        "Cannot use 'boxed' with empty type")
             else:
-                assert not isinstance(self.arg_type, QAPISchemaAlternateType)
                 assert not self.arg_type.variants
         elif self.boxed:
             raise QAPISemError(self.info, "Use of 'boxed' requires 'data'")
@@ -1750,15 +1745,13 @@ class QAPISchemaEvent(QAPISchemaEntity):
         QAPISchemaEntity.check(self, schema)
         if self._arg_type_name:
             self.arg_type = schema.lookup_type(self._arg_type_name)
-            assert (isinstance(self.arg_type, QAPISchemaObjectType) or
-                    isinstance(self.arg_type, QAPISchemaAlternateType))
+            assert isinstance(self.arg_type, QAPISchemaObjectType)
             self.arg_type.check(schema)
             if self.boxed:
                 if self.arg_type.is_empty():
                     raise QAPISemError(self.info,
                                        "Cannot use 'boxed' with empty type")
             else:
-                assert not isinstance(self.arg_type, QAPISchemaAlternateType)
                 assert not self.arg_type.variants
         elif self.boxed:
             raise QAPISemError(self.info, "Use of 'boxed' requires 'data'")

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

@@ -180,7 +180,7 @@
 { 'event': 'EVENT_D',
   'data': { 'a' : 'EventStructOne', 'b' : 'str', '*c': 'str', '*enum3': 'EnumOne' } }
 { 'event': 'EVENT_E', 'boxed': true, 'data': 'UserDefZero' }
-{ 'event': 'EVENT_F', 'boxed': true, 'data': 'UserDefAlternate' }
+{ 'event': 'EVENT_F', 'boxed': true, 'data': 'UserDefFlatUnion' }
 
 # test that we correctly compile downstream extensions, as well as munge
 # ticklish names

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

@@ -252,7 +252,7 @@ event EVENT_D q_obj_EVENT_D-arg
    boxed=False
 event EVENT_E UserDefZero
    boxed=True
-event EVENT_F UserDefAlternate
+event EVENT_F UserDefFlatUnion
    boxed=True
 enum __org.qemu_x-Enum
     member __org.qemu_x-value