buffer: add a release event

Consumers call wlr_buffer_lock. Once all consumers are done with the
buffer, only the producer should have a reference to the buffer. In this
case, we can release the buffer (and let the producer re-use it).

types/wlr_buffer.c

@@ -10,23 +10,12 @@ void wlr_buffer_init(struct wlr_buffer *buffer,
 		const struct wlr_buffer_impl *impl) {
 	assert(impl->destroy);
 	buffer->impl = impl;
-	buffer->n_refs = 1;
 	wl_signal_init(&buffer->events.destroy);
+	wl_signal_init(&buffer->events.release);
 }
 
-struct wlr_buffer *wlr_buffer_ref(struct wlr_buffer *buffer) {
-	buffer->n_refs++;
-	return buffer;
-}
-
-void wlr_buffer_unref(struct wlr_buffer *buffer) {
-	if (buffer == NULL) {
-		return;
-	}
-
-	assert(buffer->n_refs > 0);
-	buffer->n_refs--;
-	if (buffer->n_refs > 0) {
+static void buffer_consider_destroy(struct wlr_buffer *buffer) {
+	if (!buffer->dropped || buffer->n_locks > 0) {
 		return;
 	}
 
@@ -35,6 +24,36 @@ void wlr_buffer_unref(struct wlr_buffer *buffer) {
 	buffer->impl->destroy(buffer);
 }
 
+void wlr_buffer_drop(struct wlr_buffer *buffer) {
+	if (buffer == NULL) {
+		return;
+	}
+
+	assert(!buffer->dropped);
+	buffer->dropped = true;
+	buffer_consider_destroy(buffer);
+}
+
+struct wlr_buffer *wlr_buffer_lock(struct wlr_buffer *buffer) {
+	buffer->n_locks++;
+	return buffer;
+}
+
+void wlr_buffer_unlock(struct wlr_buffer *buffer) {
+	if (buffer == NULL) {
+		return;
+	}
+
+	assert(buffer->n_locks > 0);
+	buffer->n_locks--;
+
+	if (buffer->n_locks == 0) {
+		wl_signal_emit(&buffer->events.release, NULL);
+	}
+
+	buffer_consider_destroy(buffer);
+}
+
 bool wlr_buffer_get_dmabuf(struct wlr_buffer *buffer,
 		struct wlr_dmabuf_attributes *attribs) {
 	if (!buffer->impl->get_dmabuf) {
@@ -134,7 +153,17 @@ static void client_buffer_resource_handle_destroy(struct wl_listener *listener,
 	// which case we'll read garbage. We decide to accept this risk.
 }
 
-struct wlr_client_buffer *wlr_client_buffer_create(
+static void client_buffer_handle_release(struct wl_listener *listener,
+		void *data) {
+	struct wlr_client_buffer *buffer =
+		wl_container_of(listener, buffer, release);
+	if (!buffer->resource_released && buffer->resource != NULL) {
+		wl_buffer_send_release(buffer->resource);
+		buffer->resource_released = true;
+	}
+}
+
+struct wlr_client_buffer *wlr_client_buffer_import(
 		struct wlr_renderer *renderer, struct wl_resource *resource) {
 	assert(wlr_resource_is_buffer(resource));
 
@@ -198,6 +227,13 @@ struct wlr_client_buffer *wlr_client_buffer_create(
 	wl_resource_add_destroy_listener(resource, &buffer->resource_destroy);
 	buffer->resource_destroy.notify = client_buffer_resource_handle_destroy;
 
+	buffer->release.notify = client_buffer_handle_release;
+	wl_signal_add(&buffer->base.events.release, &buffer->release);
+
+	// Ensure the buffer will be released before being destroyed
+	wlr_buffer_lock(&buffer->base);
+	wlr_buffer_drop(&buffer->base);
+
 	return buffer;
 }
 
@@ -206,7 +242,7 @@ struct wlr_client_buffer *wlr_client_buffer_apply_damage(
 		pixman_region32_t *damage) {
 	assert(wlr_resource_is_buffer(resource));
 
-	if (buffer->base.n_refs > 1) {
+	if (buffer->base.n_locks > 1) {
 		// Someone else still has a reference to the buffer
 		return NULL;
 	}

types/wlr_output.c

@@ -429,7 +429,7 @@ static void output_state_clear_buffer(struct wlr_output_state *state) {
 		return;
 	}
 
-	wlr_buffer_unref(state->buffer);
+	wlr_buffer_unlock(state->buffer);
 	state->buffer = NULL;
 
 	state->committed &= ~WLR_OUTPUT_STATE_BUFFER;
@@ -601,7 +601,7 @@ bool wlr_output_attach_buffer(struct wlr_output *output,
 	output_state_clear_buffer(&output->pending);
 	output->pending.committed |= WLR_OUTPUT_STATE_BUFFER;
 	output->pending.buffer_type = WLR_OUTPUT_STATE_BUFFER_SCANOUT;
-	output->pending.buffer = wlr_buffer_ref(buffer);
+	output->pending.buffer = wlr_buffer_lock(buffer);
 	return true;
 }
 

types/wlr_surface.c

@@ -283,7 +283,7 @@ static void surface_apply_damage(struct wlr_surface *surface) {
 	if (resource == NULL) {
 		// NULL commit
 		if (surface->buffer != NULL) {
-			wlr_buffer_unref(&surface->buffer->base);
+			wlr_buffer_unlock(&surface->buffer->base);
 		}
 		surface->buffer = NULL;
 		return;
@@ -300,14 +300,14 @@ static void surface_apply_damage(struct wlr_surface *surface) {
 	}
 
 	struct wlr_client_buffer *buffer =
-		wlr_client_buffer_create(surface->renderer, resource);
+		wlr_client_buffer_import(surface->renderer, resource);
 	if (buffer == NULL) {
 		wlr_log(WLR_ERROR, "Failed to upload buffer");
 		return;
 	}
 
 	if (surface->buffer != NULL) {
-		wlr_buffer_unref(&surface->buffer->base);
+		wlr_buffer_unlock(&surface->buffer->base);
 	}
 	surface->buffer = buffer;
 }
@@ -580,7 +580,7 @@ static void surface_handle_resource_destroy(struct wl_resource *resource) {
 	pixman_region32_fini(&surface->opaque_region);
 	pixman_region32_fini(&surface->input_region);
 	if (surface->buffer != NULL) {
-		wlr_buffer_unref(&surface->buffer->base);
+		wlr_buffer_unlock(&surface->buffer->base);
 	}
 	free(surface);
 }