qapi: Clean up modular built-in code generation a bit

We neglect to call .visit_module() for the special module we use for
built-ins. Harmless, but clean it up anyway. The
tests/qapi-schema/*.out now show the built-in module as 'module None'.

Subclasses of QAPISchemaModularCVisitor need to ._add_module() this
special module to enable code generation for built-ins. When this
hasn't been done, QAPISchemaModularCVisitor.visit_module() does
nothing for the special module. That looks like built-ins could
accidentally be generated into the wrong module when a subclass
neglects to call ._add_module(). Can't happen, because built-ins are
all visited before any other module. But that's non-obvious. Switch
off code generation explicitly.

Rename QAPISchemaModularCVisitor._begin_module() to
._begin_user_module().

New QAPISchemaModularCVisitor._is_builtin_module(), for clarity.

Backports commit dcac64711ea906e844ae60a5927e5580f7252c1e from qemu

qemu/scripts/qapi/common.py

@@ -2265,9 +2265,15 @@ class QAPISchemaModularCVisitor(QAPISchemaVisitor):
         self._what = what
         self._blurb = blurb
         self._pydoc = pydoc
+        self._genc = None
+        self._genh = None
         self._module = {}
         self._main_module = None
 
+    @staticmethod
+    def _is_builtin_module(name):
+        return not name
+
     def _module_basename(self, what, name):
         if name is None:
             return re.sub(r'-', '-builtin-', what)
@@ -2278,7 +2284,7 @@ class QAPISchemaModularCVisitor(QAPISchemaVisitor):
         return basename + '-' + os.path.splitext(os.path.basename(name))[0]
 
     def _add_module(self, name, blurb):
-        if self._main_module is None and name is not None:
+        if self._main_module is None and not self._is_builtin_module(name):
             self._main_module = name
         genc = QAPIGenC(blurb, self._pydoc)
         genh = QAPIGenH(blurb, self._pydoc)
@@ -2290,22 +2296,27 @@ class QAPISchemaModularCVisitor(QAPISchemaVisitor):
 
     def write(self, output_dir, opt_builtins=False):
         for name in self._module:
-            if name is None and not opt_builtins:
+            if self._is_builtin_module(name) and not opt_builtins:
                 continue
             basename = self._module_basename(self._what, name)
             (genc, genh) = self._module[name]
             genc.write(output_dir, basename + '.c')
             genh.write(output_dir, basename + '.h')
 
-    def _begin_module(self, name):
+    def _begin_user_module(self, name):
         pass
 
     def visit_module(self, name):
         if name in self._module:
             self._set_module(name)
-            return
+        elif self._is_builtin_module(name):
+            # The built-in module has not been created.  No code may
+            # be generated.
+            self._genc = None
+            self._genh = None
+        else:
             self._add_module(name, self._blurb)
-        self._begin_module(name)
+            self._begin_user_module(name)
 
     def visit_include(self, name, info):
         basename = self._module_basename(self._what, name)

qemu/scripts/qapi/events.py

@@ -146,7 +146,7 @@ class QAPISchemaGenEventVisitor(QAPISchemaModularCVisitor):
         self._enum_name = c_name(prefix + 'QAPIEvent', protect=False)
         self._event_names = []
 
-    def _begin_module(self, name):
+    def _begin_user_module(self, name):
         types = self._module_basename('qapi-types', name)
         visit = self._module_basename('qapi-visit', name)
         self._genc.add(mcgen('''

qemu/scripts/qapi/types.py

@@ -190,7 +190,7 @@ class QAPISchemaGenTypeVisitor(QAPISchemaModularCVisitor):
 /* #include "qapi/util.h" */
 '''))
 
-    def _begin_module(self, name):
+    def _begin_user_module(self, name):
         types = self._module_basename('qapi-types', name)
         visit = self._module_basename('qapi-visit', name)
         self._genc.preamble_add(mcgen('''

qemu/scripts/qapi/visit.py

@@ -292,7 +292,7 @@ class QAPISchemaGenVisitVisitor(QAPISchemaModularCVisitor):
 ''',
                                       prefix=prefix))
 
-    def _begin_module(self, name):
+    def _begin_user_module(self, name):
         types = self._module_basename('qapi-types', name)
         visit = self._module_basename('qapi-visit', name)
         self._genc.preamble_add(mcgen('''