Fix disconnecting not connected signals (#950)

When a function is disconnected from a signal ("disconnect_signal") that is not
actually connected to the function, two things happened:

1. The attempt to remove the function from the signal array didn't do anything
2. Unreferencing the function noticed that the function wasn't referenced

The second step printed a big, fat scary warning.

Actually, this has the possibility of causing errors. For example, in the
following code, awesome would wrongly unreference the function at the
disconnect_signal() call and might later still try to call it when the
"refresh" signal is emitted:

do
    local function f() end
    awesome.connect_signal("refresh", f)
    awesome.disconnect_signal("debug::error", f)
end

Fix this by making signal_disconnect() return a boolean value indicating if it
actually did something. All callers are fixed to use this value and only update
the reference counts if something was actually disconnected.

Fixes: https://github.com/awesomeWM/awesome/issues/814
Signed-off-by: Uli Schlachter <psychon@znc.in>

common/luaclass.c

@@ -278,7 +278,7 @@ luaA_class_disconnect_signal_from_stack(lua_State *L, lua_class_t *lua_class,
 {
     luaA_checkfunction(L, ud);
     void *ref = (void *) lua_topointer(L, ud);
-    signal_disconnect(&lua_class->signals, name, ref);
+    if (signal_disconnect(&lua_class->signals, name, ref))
         luaA_object_unref(L, (void *) ref);
     lua_remove(L, ud);
 }

common/luaobject.c

@@ -222,7 +222,7 @@ luaA_object_disconnect_signal_from_stack(lua_State *L, int oud,
     luaA_checkfunction(L, ud);
     lua_object_t *obj = lua_touserdata(L, oud);
     void *ref = (void *) lua_topointer(L, ud);
-    signal_disconnect(&obj->signals, name, ref);
+    if (signal_disconnect(&obj->signals, name, ref))
         luaA_object_unref_item(L, oud, ref);
     lua_remove(L, ud);
 }

common/signal.h

@@ -81,7 +81,7 @@ signal_connect(signal_array_t *arr, const char *name, const void *ref)
  * \param name The signal name.
  * \param ref The reference to remove.
  */
-static inline void
+static inline bool
 signal_disconnect(signal_array_t *arr, const char *name, const void *ref)
 {
     signal_t *sigfound = signal_array_getbyid(arr,
@@ -92,9 +92,10 @@ signal_disconnect(signal_array_t *arr, const char *name, const void *ref)
             if(ref == *func)
             {
                 cptr_array_remove(&sigfound->sigfuncs, func);
-                break;
+                return true;
             }
     }
+    return false;
 }
 
 #endif

dbus.c

@@ -789,7 +789,7 @@ luaA_dbus_disconnect_signal(lua_State *L)
     const char *name = luaL_checkstring(L, 1);
     luaA_checkfunction(L, 2);
     const void *func = lua_topointer(L, 2);
-    signal_disconnect(&dbus_signals, name, func);
+    if (signal_disconnect(&dbus_signals, name, func))
         luaA_object_unref(L, func);
     return 0;
 }

luaa.c

@@ -395,7 +395,7 @@ luaA_awesome_disconnect_signal(lua_State *L)
     const char *name = luaL_checkstring(L, 1);
     luaA_checkfunction(L, 2);
     const void *func = lua_topointer(L, 2);
-    signal_disconnect(&global_signals, name, func);
+    if (signal_disconnect(&global_signals, name, func))
         luaA_object_unref(L, (void *) func);
     return 0;
 }