tests/kms-device: Add test for scheduled no-ops

We can schedule an update from the cursor manager, but that doesn't mean
there will be an actual plane assignment changed at the time of the
update processing, since for example we might have "touched" a CRTC, but
already left it before the processing started, meaning we have nothing
to change after all.

Add a test case that checks that this works properly.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2777>

src/backends/native/meta-kms-impl-device.h

@@ -184,6 +184,7 @@ void meta_kms_impl_device_handle_update (MetaKmsImplDevice *impl_device,
 void meta_kms_impl_device_await_flush (MetaKmsImplDevice *impl_device,
                                        MetaKmsCrtc       *crtc);
 
+META_EXPORT_TEST
 void meta_kms_impl_device_schedule_process (MetaKmsImplDevice *impl_device,
                                             MetaKmsCrtc       *crtc);
 

src/tests/native-kms-device.c

@@ -34,6 +34,7 @@
 #include "backends/native/meta-kms-private.h"
 #include "backends/native/meta-kms-update.h"
 #include "backends/native/meta-seat-native.h"
+#include "backends/native/meta-thread-impl.h"
 #include "meta-test/meta-context-test.h"
 #include "tests/meta-kms-test-utils.h"
 #include "tests/meta-test-utils.h"
@@ -595,6 +596,122 @@ meta_test_kms_device_discard_disabled (void)
   meta_device_file_release (device_file);
 }
 
+static gpointer
+schedule_process_in_impl (MetaThreadImpl  *thread_impl,
+                          gpointer         user_data,
+                          GError         **error)
+{
+  MetaKmsCrtc *crtc = META_KMS_CRTC (user_data);
+  MetaKmsDevice *device = meta_kms_crtc_get_device (crtc);
+  MetaKmsImplDevice *impl_device = meta_kms_device_get_impl_device (device);
+
+  meta_kms_impl_device_schedule_process (impl_device, crtc);
+
+  return NULL;
+}
+
+static gboolean
+quit_loop (gpointer user_data)
+{
+  GMainLoop *loop = user_data;
+
+  g_main_loop_quit (loop);
+
+  return G_SOURCE_REMOVE;
+}
+
+static gpointer
+quit_loop_timeout_in_impl (MetaThreadImpl  *thread_impl,
+                           gpointer         user_data,
+                           GError         **error)
+{
+  GMainLoop *loop = user_data;
+  g_autoptr (GSource) timeout_source = NULL;
+
+  timeout_source = meta_thread_impl_add_source (thread_impl,
+                                                quit_loop,
+                                                loop,
+                                                NULL);
+  g_source_set_ready_time (timeout_source,
+                           g_get_monotonic_time () + s2us (2));
+
+  return NULL;
+}
+
+static void
+meta_test_kms_device_empty_update (void)
+{
+  MetaBackend *backend = meta_context_get_backend (test_context);
+  MetaBackendNative *backend_native = META_BACKEND_NATIVE (backend);
+  MetaKms *kms = meta_backend_native_get_kms (backend_native);
+  MetaKmsCursorManager *cursor_manager = meta_kms_get_cursor_manager (kms);
+  g_autoptr (GArray) layout_array = NULL;
+  MetaKmsCrtcLayout layout;
+  MetaKmsDevice *device;
+  MetaKmsUpdate *update;
+  MetaKmsCrtc *crtc;
+  MetaKmsConnector *connector;
+  MetaKmsMode *mode;
+  MetaKmsPlane *primary_plane;
+  g_autoptr (MetaDrmBuffer) primary_buffer = NULL;
+  g_autoptr (MetaDrmBuffer) cursor_buffer = NULL;
+  MetaKmsFeedback *feedback;
+
+  device = meta_get_test_kms_device (test_context);
+  crtc = meta_get_test_kms_crtc (device);
+  connector = meta_get_test_kms_connector (device);
+  mode = meta_kms_connector_get_preferred_mode (connector);
+  primary_plane = meta_kms_device_get_primary_plane_for (device, crtc);
+  primary_buffer = meta_create_test_mode_dumb_buffer (device, mode);
+
+  /*
+   * Setup base state, mode + primary plane.
+   */
+
+  layout_array = g_array_new (FALSE, TRUE, sizeof (MetaKmsCrtcLayout));
+  layout = (MetaKmsCrtcLayout) {
+    .crtc = crtc,
+    .layout = {
+      .size = {
+        .width = meta_kms_mode_get_width (mode),
+        .height = meta_kms_mode_get_height (mode),
+      },
+    },
+    .scale = 1.0,
+  };
+  g_array_append_val (layout_array, layout);
+  meta_kms_cursor_manager_update_crtc_layout (cursor_manager, layout_array);
+
+  update = meta_kms_update_new (device);
+  meta_kms_update_mode_set (update, crtc,
+                            g_list_append (NULL, connector),
+                            mode);
+  meta_kms_update_assign_plane (update,
+                                crtc,
+                                primary_plane,
+                                primary_buffer,
+                                meta_get_mode_fixed_rect_16 (mode),
+                                meta_get_mode_rect (mode),
+                                META_KMS_ASSIGN_PLANE_FLAG_NONE);
+
+  feedback = meta_kms_device_process_update_sync (device, update,
+                                                  META_KMS_UPDATE_FLAG_MODE_SET);
+  meta_kms_feedback_unref (feedback);
+
+  meta_thread_post_impl_task (META_THREAD (kms),
+                              schedule_process_in_impl,
+                              crtc, NULL,
+                              NULL, NULL);
+  g_autoptr (GMainLoop) loop = NULL;
+  loop = g_main_loop_new (NULL, FALSE);
+  meta_thread_post_impl_task (META_THREAD (kms),
+                              quit_loop_timeout_in_impl,
+                              loop, NULL,
+                              NULL, NULL);
+
+  g_main_loop_run (loop);
+}
+
 static void
 init_tests (void)
 {
@@ -606,6 +723,8 @@ init_tests (void)
                    meta_test_kms_device_power_save);
   g_test_add_func ("/backends/native/kms/device/discard-disabled",
                    meta_test_kms_device_discard_disabled);
+  g_test_add_func ("/backends/native/kms/device/empty-update",
+                   meta_test_kms_device_empty_update);
 }
 
 int