scaled_scene_buffer: make dropping the buffer optional

In preparation to also use the scaled_scene_buffer for
theme components like rounded corner images and button
icons.

No functional change intended.

include/common/scaled_font_buffer.h

@@ -6,7 +6,7 @@
 
 struct wlr_scene_tree;
 struct wlr_scene_buffer;
-struct scaled_scene_buffere;
+struct scaled_scene_buffer;
 
 struct scaled_font_buffer {
 	struct wlr_scene_buffer *scene_buffer;

include/common/scaled_scene_buffer.h

@@ -26,6 +26,7 @@ struct scaled_scene_buffer {
 	void *data;  /* opaque user data */
 
 	/* Private */
+	bool drop_buffer;
 	double active_scale;
 	struct wl_list cache;  /* struct scaled_buffer_cache_entry.link */
 	struct wl_listener destroy;
@@ -51,10 +52,34 @@ struct scaled_scene_buffer {
  * wlr_scene_buffer is being destroyed. If implementation->destroy is set
  * it will also get called so a consumer of this API may clean up its own
  * allocations.
+ *
+ * All requested lab_data_buffers via impl->create_buffer() will be locked
+ * during the lifetime of the buffer in the internal cache and unlocked
+ * when being evacuated from the cache (due to LAB_SCALED_BUFFER_MAX_CACHE
+ * or the internal wlr_scene_buffer being destroyed).
+ *
+ * If drop_buffer was set during creation of the scaled_scene_buffer, the
+ * backing wlr_buffer behind a lab_data_buffer will also get dropped
+ * (via wlr_buffer_drop). If there are no more locks (consumers) of the
+ * respective buffer this will then cause the lab_data_buffer to be free'd.
+ *
+ * In the case of the buffer provider dropping the buffer itself (due to
+ * for example a Reconfigure event) the lock prevents the buffer from being
+ * destroyed until the buffer is evacuated from the internal cache and thus
+ * unlocked.
+ *
+ * This allows using scaled_scene_buffer for an autoscaling font_buffer
+ * (which gets free'd automatically) and also for theme components like
+ * rounded corner images or button icons whose buffers only exist once but
+ * are references by multiple windows with their own scaled_scene_buffers.
+ *
+ * The rough idea is: use drop_buffer = true for one-shot buffers and false
+ * for buffers that should outlive the scaled_scene_buffer instance itself.
  */
 struct scaled_scene_buffer *scaled_scene_buffer_create(
 	struct wlr_scene_tree *parent,
-	const struct scaled_scene_buffer_impl *implementation);
+	const struct scaled_scene_buffer_impl *implementation,
+	bool drop_buffer);
 
 /* Clear the cache of existing buffers, useful in case the content changes */
 void scaled_scene_buffer_invalidate_cache(struct scaled_scene_buffer *self);

src/common/scaled_font_buffer.c

@@ -30,10 +30,12 @@ static void
 _destroy(struct scaled_scene_buffer *scaled_buffer)
 {
 	struct scaled_font_buffer *self = scaled_buffer->data;
+	scaled_buffer->data = NULL;
+
 	zfree(self->text);
 	zfree(self->font.name);
 	zfree(self->arrow);
-	zfree(scaled_buffer->data);
+	free(self);
 }
 
 static const struct scaled_scene_buffer_impl impl = {
@@ -48,7 +50,7 @@ scaled_font_buffer_create(struct wlr_scene_tree *parent)
 	assert(parent);
 	struct scaled_font_buffer *self = znew(*self);
 	struct scaled_scene_buffer *scaled_buffer =
-		scaled_scene_buffer_create(parent, &impl);
+		scaled_scene_buffer_create(parent, &impl, /* drop_buffer */ true);
 	if (!scaled_buffer) {
 		free(self);
 		return NULL;

src/common/scaled_scene_buffer.c

@@ -36,11 +36,15 @@
 
 /* Internal API */
 static void
-_cache_entry_destroy(struct scaled_scene_buffer_cache_entry *cache_entry)
+_cache_entry_destroy(struct scaled_scene_buffer_cache_entry *cache_entry, bool drop_buffer)
 {
 	wl_list_remove(&cache_entry->link);
 	if (cache_entry->buffer) {
-		wlr_buffer_drop(cache_entry->buffer);
+		/* Allow the buffer to get dropped if there are no further consumers */
+		wlr_buffer_unlock(cache_entry->buffer);
+		if (drop_buffer) {
+			wlr_buffer_drop(cache_entry->buffer);
+		}
 	}
 	free(cache_entry);
 }
@@ -64,6 +68,10 @@ _update_buffer(struct scaled_scene_buffer *self, double scale)
 
 	/* Create new buffer, will get destroyed along the backing wlr_buffer */
 	struct lab_data_buffer *buffer = self->impl->create_buffer(self, scale);
+	if (buffer) {
+		/* Ensure the buffer doesn't get deleted behind our back */
+		wlr_buffer_lock(&buffer->base);
+	}
 	self->width = buffer ? buffer->unscaled_width : 0;
 	self->height = buffer ? buffer->unscaled_height : 0;
 
@@ -73,7 +81,11 @@ _update_buffer(struct scaled_scene_buffer *self, double scale)
 	} else {
 		cache_entry = wl_container_of(self->cache.prev, cache_entry, link);
 		if (cache_entry->buffer) {
-			wlr_buffer_drop(cache_entry->buffer);
+			/* Allow the old buffer to get dropped if there are no further consumers */
+			wlr_buffer_unlock(cache_entry->buffer);
+			if (self->drop_buffer) {
+				wlr_buffer_drop(cache_entry->buffer);
+			}
 		}
 		wl_list_remove(&cache_entry->link);
 	}
@@ -100,7 +112,7 @@ _handle_node_destroy(struct wl_listener *listener, void *data)
 	wl_list_remove(&self->output_leave.link);
 
 	wl_list_for_each_safe(cache_entry, cache_entry_tmp, &self->cache, link) {
-		_cache_entry_destroy(cache_entry);
+		_cache_entry_destroy(cache_entry, self->drop_buffer);
 	}
 	assert(wl_list_empty(&self->cache));
 
@@ -146,7 +158,8 @@ _handle_output_leave(struct wl_listener *listener, void *data)
 /* Public API */
 struct scaled_scene_buffer *
 scaled_scene_buffer_create(struct wlr_scene_tree *parent,
-		const struct scaled_scene_buffer_impl *impl)
+		const struct scaled_scene_buffer_impl *impl,
+		bool drop_buffer)
 {
 	assert(parent);
 	assert(impl);
@@ -162,6 +175,7 @@ scaled_scene_buffer_create(struct wlr_scene_tree *parent,
 
 	self->impl = impl;
 	self->active_scale = 1;
+	self->drop_buffer = drop_buffer;
 	wl_list_init(&self->cache);
 
 	/* Listen to output enter/leave so we get notified about scale changes */
@@ -183,7 +197,7 @@ scaled_scene_buffer_invalidate_cache(struct scaled_scene_buffer *self)
 	assert(self);
 	struct scaled_scene_buffer_cache_entry *cache_entry, *cache_entry_tmp;
 	wl_list_for_each_safe(cache_entry, cache_entry_tmp, &self->cache, link) {
-		_cache_entry_destroy(cache_entry);
+		_cache_entry_destroy(cache_entry, self->drop_buffer);
 	}
 	assert(wl_list_empty(&self->cache));
 	_update_buffer(self, self->active_scale);