Add wlr_output_impl.rollback

Most of the pending output state is not forwarded to the backend prior
to an output commit. For instance, wlr_output_set_mode just stashes the
mode without calling any wlr_output_impl function.
wlr_output_impl.commit is responsible for applying the pending mode.

However, there are exceptions to this rule. The first one is
wlr_output_attach_render. It won't go away before renderer v6 is
complete, because it needs to set the current EGL surface.

The second one is wlr_output_attach_buffer.
wlr_output_impl.attach_buffer is removed in [1].

When wlr_output_rollback is called, all pending state is supposed to be
cleared. This works for all the state except the two exceptions
mentionned above. To fix this, introduce wlr_output_impl.rollback.

Right now, the backend resets the current EGL surface. This prevents GL
commands from affecting the output after wlr_output_rollback.

This patch is required for FBO-based outputs to work properly. The
compositor might be using FBOs for its own purposes [2], having leftover
FBO state can have bad consequences.

[1]: https://github.com/swaywm/wlroots/pull/2097
[2]: https://github.com/swaywm/wlroots/pull/2063#issuecomment-597614312

backend/drm/drm.c

@@ -574,6 +574,11 @@ static bool drm_connector_commit(struct wlr_output *output) {
 	return true;
 }
 
+static void drm_connector_rollback(struct wlr_output *output) {
+	struct wlr_drm_backend *drm = get_drm_backend_from_backend(output->backend);
+	wlr_egl_make_current(&drm->renderer.egl, EGL_NO_SURFACE, NULL);
+}
+
 static void fill_empty_gamma_table(size_t size,
 		uint16_t *r, uint16_t *g, uint16_t *b) {
 	assert(0xFFFF < UINT64_MAX / (size - 1));
@@ -1094,6 +1099,7 @@ static const struct wlr_output_impl output_impl = {
 	.attach_render = drm_connector_attach_render,
 	.test = drm_connector_test,
 	.commit = drm_connector_commit,
+	.rollback = drm_connector_rollback,
 	.set_gamma = set_drm_connector_gamma,
 	.get_gamma_size = drm_connector_get_gamma_size,
 	.export_dmabuf = drm_connector_export_dmabuf,

backend/headless/output.c

@@ -97,6 +97,12 @@ static bool output_commit(struct wlr_output *wlr_output) {
 	return true;
 }
 
+static void output_rollback(struct wlr_output *wlr_output) {
+	struct wlr_headless_output *output =
+		headless_output_from_output(wlr_output);
+	wlr_egl_make_current(&output->backend->egl, EGL_NO_SURFACE, NULL);
+}
+
 static void output_destroy(struct wlr_output *wlr_output) {
 	struct wlr_headless_output *output =
 		headless_output_from_output(wlr_output);
@@ -113,6 +119,7 @@ static const struct wlr_output_impl output_impl = {
 	.destroy = output_destroy,
 	.attach_render = output_attach_render,
 	.commit = output_commit,
+	.rollback = output_rollback,
 };
 
 bool wlr_output_is_headless(struct wlr_output *wlr_output) {

backend/wayland/output.c

@@ -302,6 +302,12 @@ static bool output_commit(struct wlr_output *wlr_output) {
 	return true;
 }
 
+static void output_rollback(struct wlr_output *wlr_output) {
+	struct wlr_wl_output *output =
+		get_wl_output_from_output(wlr_output);
+	wlr_egl_make_current(&output->backend->egl, EGL_NO_SURFACE, NULL);
+}
+
 static bool output_set_cursor(struct wlr_output *wlr_output,
 		struct wlr_texture *texture, int32_t scale,
 		enum wl_output_transform transform,
@@ -431,6 +437,7 @@ static const struct wlr_output_impl output_impl = {
 	.attach_render = output_attach_render,
 	.test = output_test,
 	.commit = output_commit,
+	.rollback = output_rollback,
 	.set_cursor = output_set_cursor,
 	.move_cursor = output_move_cursor,
 };

backend/x11/output.c

@@ -162,11 +162,17 @@ static bool output_commit(struct wlr_output *wlr_output) {
 	return true;
 }
 
+static void output_rollback(struct wlr_output *wlr_output) {
+	struct wlr_x11_output *output = get_x11_output_from_output(wlr_output);
+	wlr_egl_make_current(&output->x11->egl, EGL_NO_SURFACE, NULL);
+}
+
 static const struct wlr_output_impl output_impl = {
 	.destroy = output_destroy,
 	.attach_render = output_attach_render,
 	.test = output_test,
 	.commit = output_commit,
+	.rollback = output_rollback,
 };
 
 struct wlr_output *wlr_x11_output_create(struct wlr_backend *backend) {

include/wlr/interfaces/wlr_output.h

@@ -23,6 +23,7 @@ struct wlr_output_impl {
 	bool (*attach_render)(struct wlr_output *output, int *buffer_age);
 	bool (*test)(struct wlr_output *output);
 	bool (*commit)(struct wlr_output *output);
+	void (*rollback)(struct wlr_output *output);
 	bool (*set_gamma)(struct wlr_output *output, size_t size,
 		const uint16_t *r, const uint16_t *g, const uint16_t *b);
 	size_t (*get_gamma_size)(struct wlr_output *output);

types/wlr_output.c

@@ -606,6 +606,10 @@ bool wlr_output_commit(struct wlr_output *output) {
 
 void wlr_output_rollback(struct wlr_output *output) {
 	output_state_clear(&output->pending);
+
+	if (output->impl->rollback) {
+		output->impl->rollback(output);
+	}
 }
 
 void wlr_output_attach_buffer(struct wlr_output *output,