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]

docs/devel/qapi-code-gen.txt

@@ -1385,16 +1385,10 @@ Example:
     void qapi_event_send_my_event(void)
     {
         QDict *qmp;
-        QMPEventFuncEmit emit;
-
-        emit = qmp_event_get_func_emit();
-        if (!emit) {
-            return;
-        }
 
         qmp = qmp_event_build_dict("MY_EVENT");
 
-        emit(EXAMPLE_QAPI_EVENT_MY_EVENT, qmp);
+        example_qapi_event_emit(EXAMPLE_QAPI_EVENT_MY_EVENT, qmp);
 
         qobject_unref(qmp);
     }

include/qapi/qmp-event.h

@@ -14,11 +14,5 @@
 #ifndef QMP_EVENT_H
 #define QMP_EVENT_H
 
-typedef void (*QMPEventFuncEmit)(unsigned event, QDict *dict);
-
-void qmp_event_set_func_emit(QMPEventFuncEmit emit);
-
-QMPEventFuncEmit qmp_event_get_func_emit(void);
-
 QDict *qmp_event_build_dict(const char *event_name);
 #endif

monitor.c

@@ -590,8 +590,7 @@ monitor_qapi_event_queue_no_reenter(QAPIEvent event, QDict *qdict)
     qemu_mutex_unlock(&monitor_lock);
 }
 
-static void
-monitor_qapi_event_queue(QAPIEvent event, QDict *qdict)
+void qapi_event_emit(QAPIEvent event, QDict *qdict)
 {
     /*
      * monitor_qapi_event_queue_no_reenter() is not reentrant: it
@@ -704,7 +703,6 @@ static void monitor_qapi_event_init(void)
 {
     monitor_qapi_event_state = g_hash_table_new(qapi_event_throttle_hash,
                                                 qapi_event_throttle_equal);
-    qmp_event_set_func_emit(monitor_qapi_event_queue);
 }
 
 static void handle_hmp_command(Monitor *mon, const char *cmdline);

qapi/qmp-event.c

@@ -19,18 +19,6 @@
 #include "qapi/qmp/qdict.h"
 #include "qapi/qmp/qjson.h"
 
-static QMPEventFuncEmit qmp_emit;
-
-void qmp_event_set_func_emit(QMPEventFuncEmit emit)
-{
-    qmp_emit = emit;
-}
-
-QMPEventFuncEmit qmp_event_get_func_emit(void)
-{
-    return qmp_emit;
-}
-
 static void timestamp_put(QDict *qdict)
 {
     int err;

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))
 
 

stubs/monitor.c

@@ -1,5 +1,6 @@
 #include "qemu/osdep.h"
 #include "qapi/error.h"
+#include "qapi/qapi-events.h"
 #include "qemu-common.h"
 #include "monitor/monitor.h"
 
@@ -14,3 +15,7 @@ int monitor_get_fd(Monitor *mon, const char *name, Error **errp)
 void monitor_init(Chardev *chr, int flags)
 {
 }
+
+void qapi_event_emit(QAPIEvent event, QDict *qdict)
+{
+}

tests/Makefile.include

@@ -528,7 +528,7 @@ QEMU_CFLAGS += -I$(SRC_PATH)/tests
 test-util-obj-y = libqemuutil.a
 test-qom-obj-y = $(qom-obj-y) $(test-util-obj-y)
 test-qapi-obj-y = tests/test-qapi-visit.o tests/test-qapi-types.o \
-	tests/test-qapi-events.o tests/test-qapi-introspect.o \
+	tests/test-qapi-introspect.o \
 	$(test-qom-obj-y)
 benchmark-crypto-obj-y = $(crypto-obj-y) $(test-qom-obj-y)
 test-crypto-obj-y = $(crypto-obj-y) $(test-qom-obj-y)
@@ -620,7 +620,7 @@ tests/qapi-schema/doc-good.test.texi: $(SRC_PATH)/tests/qapi-schema/doc-good.jso
 
 tests/test-string-output-visitor$(EXESUF): tests/test-string-output-visitor.o $(test-qapi-obj-y)
 tests/test-string-input-visitor$(EXESUF): tests/test-string-input-visitor.o $(test-qapi-obj-y)
-tests/test-qmp-event$(EXESUF): tests/test-qmp-event.o $(test-qapi-obj-y)
+tests/test-qmp-event$(EXESUF): tests/test-qmp-event.o $(test-qapi-obj-y) tests/test-qapi-events.o
 tests/test-qobject-output-visitor$(EXESUF): tests/test-qobject-output-visitor.o $(test-qapi-obj-y)
 tests/test-clone-visitor$(EXESUF): tests/test-clone-visitor.o $(test-qapi-obj-y)
 tests/test-qobject-input-visitor$(EXESUF): tests/test-qobject-input-visitor.o $(test-qapi-obj-y)

tests/test-qmp-event.c

@@ -93,9 +93,7 @@ static bool qdict_cmp_simple(QDict *a, QDict *b)
     return d.result;
 }
 
-/* This function is hooked as final emit function, which can verify the
-   correctness. */
-static void event_test_emit(test_QAPIEvent event, QDict *d)
+void test_qapi_event_emit(test_QAPIEvent event, QDict *d)
 {
     QDict *t;
     int64_t s, ms;
@@ -241,8 +239,6 @@ static void test_event_d(TestEventData *data,
 
 int main(int argc, char **argv)
 {
-    qmp_event_set_func_emit(event_test_emit);
-
     g_test_init(&argc, &argv, NULL);
 
     event_test_add("/event/event_a", test_event_a);