polkit: when authorizing via PK let's re-resolve callback/userdata instead of caching it

Previously, when doing an async PK query we'd store the original
callback/userdata pair and call it again after the PK request is
complete. This is problematic, since PK queries might be slow and in the
meantime the userdata might be released and re-acquired. Let's avoid
this by always traversing through the message handlers so that we always
re-resolve the callback and userdata pair and thus can be sure it's
up-to-date and properly valid.

src/shared/bus-polkit.c

@@ -159,14 +159,13 @@ typedef struct AsyncPolkitQuery {
         char **details;
 
         sd_bus_message *request, *reply;
-        sd_bus_message_handler_t callback;
-        void *userdata;
         sd_bus_slot *slot;
+
         Hashmap *registry;
+        sd_event_source *defer_event_source;
 } AsyncPolkitQuery;
 
 static void async_polkit_query_free(AsyncPolkitQuery *q) {
-
         if (!q)
                 return;
 
@@ -181,9 +180,22 @@ static void async_polkit_query_free(AsyncPolkitQuery *q) {
         free(q->action);
         strv_free(q->details);
 
+        sd_event_source_disable_unref(q->defer_event_source);
         free(q);
 }
 
+static int async_polkit_defer(sd_event_source *s, void *userdata) {
+        AsyncPolkitQuery *q = userdata;
+
+        assert(s);
+
+        /* This is called as idle event source after we processed the async polkit reply, hopefully after the
+         * method call we re-enqueued has been properly processed. */
+
+        async_polkit_query_free(q);
+        return 0;
+}
+
 static int async_polkit_callback(sd_bus_message *reply, void *userdata, sd_bus_error *error) {
         _cleanup_(sd_bus_error_free) sd_bus_error error_buffer = SD_BUS_ERROR_NULL;
         AsyncPolkitQuery *q = userdata;
@@ -192,21 +204,46 @@ static int async_polkit_callback(sd_bus_message *reply, void *userdata, sd_bus_e
         assert(reply);
         assert(q);
 
+        assert(q->slot);
         q->slot = sd_bus_slot_unref(q->slot);
+
+        assert(!q->reply);
         q->reply = sd_bus_message_ref(reply);
 
+        /* Now, let's dispatch the original message a second time be re-enqueing. This will then traverse the
+         * whole message processing again, and thus re-validating and re-retrieving the "userdata" field
+         * again.
+         *
+         * We install an idle event loop event to clean-up the PolicyKit request data when we are idle again,
+         * i.e. after the second time the message is processed is complete. */
+
+        assert(!q->defer_event_source);
+        r = sd_event_add_defer(sd_bus_get_event(sd_bus_message_get_bus(reply)), &q->defer_event_source, async_polkit_defer, q);
+        if (r < 0)
+                goto fail;
+
+        r = sd_event_source_set_priority(q->defer_event_source, SD_EVENT_PRIORITY_IDLE);
+        if (r < 0)
+                goto fail;
+
+        r = sd_event_source_set_enabled(q->defer_event_source, SD_EVENT_ONESHOT);
+        if (r < 0)
+                goto fail;
+
         r = sd_bus_message_rewind(q->request, true);
-        if (r < 0) {
-                r = sd_bus_reply_method_errno(q->request, r, NULL);
-                goto finish;
-        }
+        if (r < 0)
+                goto fail;
 
-        r = q->callback(q->request, q->userdata, &error_buffer);
-        r = bus_maybe_reply_error(q->request, r, &error_buffer);
+        r = sd_bus_enqeue_for_read(sd_bus_message_get_bus(q->request), q->request);
+        if (r < 0)
+                goto fail;
 
-finish:
+        return 1;
+
+fail:
+        log_debug_errno(r, "Processing asynchronous PolicyKit reply failed, ignoring: %m");
+        (void) sd_bus_reply_method_errno(q->request, r, NULL);
         async_polkit_query_free(q);
-
         return r;
 }
 
@@ -225,11 +262,9 @@ int bus_verify_polkit_async(
 #if ENABLE_POLKIT
         _cleanup_(sd_bus_message_unrefp) sd_bus_message *pk = NULL;
         AsyncPolkitQuery *q;
-        const char *sender;
-        sd_bus_message_handler_t callback;
-        void *userdata;
         int c;
 #endif
+        const char *sender;
         int r;
 
         assert(call);
@@ -293,20 +328,11 @@ int bus_verify_polkit_async(
         else if (r > 0)
                 return 1;
 
-#if ENABLE_POLKIT
-        if (sd_bus_get_current_message(call->bus) != call)
-                return -EINVAL;
-
-        callback = sd_bus_get_current_handler(call->bus);
-        if (!callback)
-                return -EINVAL;
-
-        userdata = sd_bus_get_current_userdata(call->bus);
-
         sender = sd_bus_message_get_sender(call);
         if (!sender)
                 return -EBADMSG;
 
+#if ENABLE_POLKIT
         c = sd_bus_message_get_allow_interactive_authorization(call);
         if (c < 0)
                 return c;
@@ -349,8 +375,6 @@ int bus_verify_polkit_async(
 
         *q = (AsyncPolkitQuery) {
                 .request = sd_bus_message_ref(call),
-                .callback = callback,
-                .userdata = userdata,
         };
 
         q->action = strdup(action);