core: don't use stale window images

Previously, when the backend doesn't implement root_change(), we will
deinit, and then init the backend again. However, we didn't free all the
images previously returned by the backend. There is no guarantee that
these images will be valid across deinit/init.

So, make sure they are freed.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/compton.c

@@ -707,16 +707,78 @@ static void restack_win(session_t *ps, win *w, xcb_window_t new_above) {
 	}
 }
 
+/// Free up all the images and deinit the backend
+static void destroy_backend(session_t *ps) {
+	for (win *w = ps->list, *next; w; w = next) {
+		next = w->next;
+		// Wrapping up fading in progress
+		win_skip_fading(ps, &w);
+
+		// `w` might be freed by win_check_fade_finished
+		if (!w) {
+			continue;
+		}
+		if (ps->o.experimental_backends) {
+			if (w->state == WSTATE_MAPPED) {
+				win_release_image(ps->backend_data, w);
+			} else {
+				assert(!w->win_image);
+				assert(!w->shadow_image);
+			}
+			if (ps->root_image) {
+				ps->backend_data->ops->release_image(ps->backend_data,
+				                                     ps->root_image);
+				ps->root_image = NULL;
+			}
+		} else {
+			free_paint(ps, &w->paint);
+		}
+	}
+
+	if (ps->o.experimental_backends) {
+		// deinit backend
+		ps->backend_data->ops->deinit(ps->backend_data);
+		ps->backend_data = NULL;
+	}
+}
+
+/// Init the backend and bind all the window pixmap to backend images
+static bool initialize_backend(session_t *ps) {
+	if (ps->o.experimental_backends) {
+		// Reinitialize win_data
+		ps->backend_data = backend_list[ps->o.backend]->init(ps);
+		ps->backend_data->ops = backend_list[ps->o.backend];
+		if (!ps->backend_data) {
+			log_fatal("Failed to initialize backend, aborting...");
+			ps->quit = true;
+			ev_break(ps->loop, EVBREAK_ALL);
+			return false;
+		}
+
+		for (win *w = ps->list; w; w = w->next) {
+			if (w->a.map_state == XCB_MAP_STATE_VIEWABLE) {
+				if (!win_bind_image(ps, w)) {
+					w->flags |= WIN_FLAGS_IMAGE_ERROR;
+				}
+			}
+		}
+	}
+
+	// The old backends binds pixmap lazily, nothing to do here
+	return true;
+}
+
 /// Handle configure event of a root window
 void configure_root(session_t *ps, int width, int height) {
+	log_info("Root configuration changed, new geometry: %dx%d", width, height);
 	// On root window changes
 	bool has_root_change = false;
 	if (ps->o.experimental_backends) {
 		has_root_change = ps->backend_data->ops->root_change != NULL;
 		if (!has_root_change) {
-			// deinit/reinit backend if the backend cannot handle root change
+			// deinit/reinit backend and free up resources if the backend
-			ps->backend_data->ops->deinit(ps->backend_data);
+			// cannot handle root change
-			ps->backend_data = NULL;
+			destroy_backend(ps);
 		}
 	} else {
 		free_paint(ps, &ps->tgt_buffer);
@@ -746,9 +808,7 @@ void configure_root(session_t *ps, int width, int height) {
 		if (has_root_change) {
 			ps->backend_data->ops->root_change(ps->backend_data, ps);
 		} else {
-			ps->backend_data = backend_list[ps->o.backend]->init(ps);
+			if (!initialize_backend(ps)) {
-			ps->backend_data->ops = backend_list[ps->o.backend];
-			if (!ps->backend_data) {
 				log_fatal("Failed to re-initialize backend after root "
 				          "change, aborting...");
 				ps->quit = true;
@@ -1268,24 +1328,8 @@ static bool redir_start(session_t *ps) {
 
 		x_sync(ps->c);
 
-		if (ps->o.experimental_backends) {
+		if (!initialize_backend(ps)) {
-			// Reinitialize win_data
+			return false;
-			ps->backend_data = backend_list[ps->o.backend]->init(ps);
-			ps->backend_data->ops = backend_list[ps->o.backend];
-			if (!ps->backend_data) {
-				log_fatal("Failed to initialize backend, aborting...");
-				ps->quit = true;
-				ev_break(ps->loop, EVBREAK_ALL);
-				return false;
-			}
-
-			for (win *w = ps->list; w; w = w->next) {
-				if (w->a.map_state == XCB_MAP_STATE_VIEWABLE) {
-					if (!win_bind_image(ps, w)) {
-						w->flags |= WIN_FLAGS_IMAGE_ERROR;
-					}
-				}
-			}
 		}
 
 		if (ps->o.experimental_backends) {
@@ -1319,34 +1363,8 @@ static bool redir_start(session_t *ps) {
 static void redir_stop(session_t *ps) {
 	if (ps->redirected) {
 		log_debug("Screen unredirected.");
-		// Destroy all Pictures as they expire once windows are unredirected
-		// If we don't destroy them here, looks like the resources are just
-		// kept inaccessible somehow
-		for (win *w = ps->list, *next; w; w = next) {
-			next = w->next;
-			// Wrapping up fading in progress
-			win_skip_fading(ps, &w);
 
-			// `w` might be freed by win_check_fade_finished
+		destroy_backend(ps);
-			if (!w) {
-				continue;
-			}
-			if (ps->o.experimental_backends) {
-				if (w->state == WSTATE_MAPPED) {
-					win_release_image(ps->backend_data, w);
-				} else {
-					assert(!w->win_image);
-					assert(!w->shadow_image);
-				}
-				if (ps->root_image) {
-					ps->backend_data->ops->release_image(
-					    ps->backend_data, ps->root_image);
-					ps->root_image = NULL;
-				}
-			} else {
-				free_paint(ps, &w->paint);
-			}
-		}
 
 		xcb_composite_unredirect_subwindows(ps->c, ps->root,
 		                                    XCB_COMPOSITE_REDIRECT_MANUAL);
@@ -1354,12 +1372,6 @@ static void redir_stop(session_t *ps) {
 		if (ps->overlay)
 			xcb_unmap_window(ps->c, ps->overlay);
 
-		if (ps->o.experimental_backends) {
-			// deinit backend
-			ps->backend_data->ops->deinit(ps->backend_data);
-			ps->backend_data = NULL;
-		}
-
 		// Free the damage ring
 		for (int i = 0; i < ps->ndamage; ++i) {
 			pixman_region32_fini(&ps->damage_ring[i]);