qapi: Eliminate indirection through qmp_event_get_func_emit()

The qapi_event_send_FOO() functions emit events like this:

    QMPEventFuncEmit emit;

    emit = qmp_event_get_func_emit();
    if (!emit) {
        return;
    }

    qmp = qmp_event_build_dict("FOO");
    [put event arguments into @qmp...]

    emit(QAPI_EVENT_FOO, qmp);

The value of qmp_event_get_func_emit() depends only on the program:

* In qemu-system-FOO, it's always monitor_qapi_event_queue.

* In tests/test-qmp-event, it's always event_test_emit.

* In all other programs, it's always null.

This is exactly the kind of dependence the linker is supposed to
resolve; we don't actually need an indirection.

Note that things would fall apart if we linked more than one QAPI
schema into a single program: each set of qapi_event_send_FOO() uses
its own event enumeration, yet they share a single emit function.
Which takes the event enumeration as an argument.  Which one if
there's more than one?

More seriously: how does this work even now?  qemu-system-FOO wants
QAPIEvent, and passes a function taking that to
qmp_event_set_func_emit().  test-qmp-event wants test_QAPIEvent, and
passes a function taking that to qmp_event_set_func_emit().

It works by type trickery, of course:

    typedef void (*QMPEventFuncEmit)(unsigned event, QDict *dict);

    void qmp_event_set_func_emit(QMPEventFuncEmit emit);

    QMPEventFuncEmit qmp_event_get_func_emit(void);

We use unsigned instead of the enumeration type.  Relies on both
enumerations boiling down to unsigned, which happens to be true for
the compilers we use.

Clean this up as follows:

* Generate qapi_event_send_FOO() that call PREFIX_qapi_event_emit()
  instead of the value of qmp_event_set_func_emit().

* Generate a prototype for PREFIX_qapi_event_emit() into
  qapi-events.h.

* PREFIX_ is empty for qapi/qapi-schema.json, and test_ for
  tests/qapi-schema/qapi-schema-test.json.  It's qga_ for
  qga/qapi-schema.json, and doc-good- for
  tests/qapi-schema/doc-good.json, but those don't define any events.

* Rename monitor_qapi_event_queue() to qapi_event_emit() instead of
  passing it to qmp_event_set_func_emit().  This takes care of
  qemu-system-FOO.

* Rename event_test_emit() to test_qapi_event_emit() instead of
  passing it to qmp_event_set_func_emit().  This takes care of
  tests/test-qmp-event.

* Add a qapi_event_emit() that does nothing to stubs/monitor.c.  This
  takes care of all other programs that link code emitting QMP events.

* Drop qmp_event_set_func_emit(), qmp_event_get_func_emit().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20181218182234.28876-3-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[Commit message typos fixed]

scripts/qapi/events.py

@@ -58,7 +58,7 @@ def gen_param_var(typ):
     return ret
 
 
-def gen_event_send(name, arg_type, boxed, event_enum_name):
+def gen_event_send(name, arg_type, boxed, event_enum_name, event_emit):
     # FIXME: Our declaration of local variables (and of 'errp' in the
     # parameter list) can collide with exploded members of the event's
     # data type passed in as parameters.  If this collision ever hits in
@@ -70,7 +70,6 @@ def gen_event_send(name, arg_type, boxed, event_enum_name):
 %(proto)s
 {
     QDict *qmp;
-    QMPEventFuncEmit emit;
 ''',
                 proto=build_event_send_proto(name, arg_type, boxed))
 
@@ -86,11 +85,6 @@ def gen_event_send(name, arg_type, boxed, event_enum_name):
 
     ret += mcgen('''
 
-    emit = qmp_event_get_func_emit();
-    if (!emit) {
-        return;
-    }
-
     qmp = qmp_event_build_dict("%(name)s");
 
 ''',
@@ -121,9 +115,10 @@ def gen_event_send(name, arg_type, boxed, event_enum_name):
 ''')
 
     ret += mcgen('''
-    emit(%(c_enum)s, qmp);
+    %(event_emit)s(%(c_enum)s, qmp);
 
 ''',
+                 event_emit=event_emit,
                  c_enum=c_enum_const(event_enum_name, name))
 
     if arg_type and not arg_type.is_empty():
@@ -145,6 +140,7 @@ class QAPISchemaGenEventVisitor(QAPISchemaModularCVisitor):
             ' * Schema-defined QAPI/QMP events', __doc__)
         self._event_enum_name = c_name(prefix + 'QAPIEvent', protect=False)
         self._event_enum_members = []
+        self._event_emit_name = c_name(prefix + 'qapi_event_emit')
 
     def _begin_module(self, name):
         types = self._module_basename('qapi-types', name)
@@ -170,15 +166,23 @@ class QAPISchemaGenEventVisitor(QAPISchemaModularCVisitor):
 
     def visit_end(self):
         (genc, genh) = self._module[self._main_module]
-        genh.add(gen_enum(self._event_enum_name, self._event_enum_members))
+        genh.add(gen_enum(self._event_enum_name,
+                          self._event_enum_members))
         genc.add(gen_enum_lookup(self._event_enum_name,
                                  self._event_enum_members))
+        genh.add(mcgen('''
+
+void %(event_emit)s(%(event_enum)s event, QDict *qdict);
+''',
+                       event_emit=self._event_emit_name,
+                       event_enum=self._event_enum_name))
 
     def visit_event(self, name, info, ifcond, arg_type, boxed):
         with ifcontext(ifcond, self._genh, self._genc):
             self._genh.add(gen_event_send_decl(name, arg_type, boxed))
             self._genc.add(gen_event_send(name, arg_type, boxed,
-                                          self._event_enum_name))
+                                          self._event_enum_name,
+                                          self._event_emit_name))
         self._event_enum_members.append(QAPISchemaMember(name, ifcond))