qapi: Use predicate callback to determine visit filtering

Previously, qapi-types and qapi-visit filtered out implicit
objects during visit_object_type() by using 'info' (works since
implicit objects do not [yet] have associated info); meanwhile
qapi-introspect filtered out all schema types on the first pass
by returning a python type from visit_begin(), which was then
used at a distance in QAPISchema.visit() to do the filtering.

Rather than keeping these ad hoc approaches, add a new visitor
callback visit_needed() which returns False to skip a given
entity, and which defaults to True unless overridden. Use the
new mechanism to simplify all three filtering visitors.

No change to the generated code.

Backports commit 25a0d9c977c2f5db914b0a1619759fd77d97b016 from qemu

qemu/scripts/qapi-types.py

@@ -228,6 +228,10 @@ class QAPISchemaGenTypeVisitor(QAPISchemaVisitor):
         self.decl = self._btin + self.decl
         self._btin = None
 
+    def visit_needed(self, entity):
+        # Visit everything except implicit objects
+        return not isinstance(entity, QAPISchemaObjectType) or entity.info
+
     def _gen_type_cleanup(self, name):
         self.decl += gen_type_cleanup_decl(name)
         self.defn += gen_type_cleanup(name)
@@ -249,14 +253,13 @@ class QAPISchemaGenTypeVisitor(QAPISchemaVisitor):
             self._gen_type_cleanup(name)
 
     def visit_object_type(self, name, info, base, members, variants):
-        if info:
-            self._fwdecl += gen_fwd_object_or_array(name)
-            if variants:
-                assert not members      # not implemented
-                self.decl += gen_union(name, base, variants)
-            else:
-                self.decl += gen_struct(name, base, members)
-            self._gen_type_cleanup(name)
+        self._fwdecl += gen_fwd_object_or_array(name)
+        if variants:
+            assert not members      # not implemented
+            self.decl += gen_union(name, base, variants)
+        else:
+            self.decl += gen_struct(name, base, members)
+        self._gen_type_cleanup(name)
 
     def visit_alternate_type(self, name, info, variants):
         self._fwdecl += gen_fwd_object_or_array(name)

qemu/scripts/qapi-visit.py

@@ -332,6 +332,10 @@ class QAPISchemaGenVisitVisitor(QAPISchemaVisitor):
         self.decl = self._btin + self.decl
         self._btin = None
 
+    def visit_needed(self, entity):
+        # Visit everything except implicit objects
+        return not isinstance(entity, QAPISchemaObjectType) or entity.info
+
     def visit_enum_type(self, name, info, values, prefix):
         self.decl += gen_visit_decl(name, scalar=True)
         self.defn += gen_visit_enum(name)
@@ -348,13 +352,12 @@ class QAPISchemaGenVisitVisitor(QAPISchemaVisitor):
             self.defn += defn
 
     def visit_object_type(self, name, info, base, members, variants):
-        if info:
-            self.decl += gen_visit_decl(name)
-            if variants:
-                assert not members      # not implemented
-                self.defn += gen_visit_union(name, base, variants)
-            else:
-                self.defn += gen_visit_struct(name, base, members)
+        self.decl += gen_visit_decl(name)
+        if variants:
+            assert not members      # not implemented
+            self.defn += gen_visit_union(name, base, variants)
+        else:
+            self.defn += gen_visit_struct(name, base, members)
 
     def visit_alternate_type(self, name, info, variants):
         self.decl += gen_visit_decl(name)

qemu/scripts/qapi.py

@@ -808,6 +808,10 @@ class QAPISchemaVisitor(object):
     def visit_end(self):
         pass
 
+    def visit_needed(self, entity):
+        # Default to visiting everything
+        return True
+
     def visit_builtin_type(self, name, info, json_type):
         pass
 
@@ -1300,10 +1304,10 @@ class QAPISchema(object):
             ent.check(self)
 
     def visit(self, visitor):
-        ignore = visitor.visit_begin(self)
-        for name in sorted(self._entity_dict.keys()):
-            if not ignore or not isinstance(self._entity_dict[name], ignore):
-                self._entity_dict[name].visit(visitor)
+        visitor.visit_begin(self)
+        for (name, entity) in sorted(self._entity_dict.items()):
+            if visitor.visit_needed(entity):
+                entity.visit(visitor)
         visitor.visit_end()
 
 #