buffer: introduce wlr_buffer_acquire()

A footgun in the wlr_buffer API is that there's no difference
between acquiring a buffer and increasing the lock count. In other
words, transitioning a buffer from the released state to the
acquired state is not explicit. This may result in hard-to-debug
failures if there is a dangling released wlr_buffer somewhere
(e.g. wlr_client_buffer.source [1]) and some piece of code calls
wlr_buffer_lock(). In that case, the buffer will be acquired and
released again. In the context of a wlr_buffer issued from a
Wayland protocol wl_buffer object, this can cause the underlying
memory to be used after wl_buffer.release has been sent to the
client, and a double wl_buffer.release event to be sent.

Make it so acquiring a buffer is an explicit operation to make sure
the caller means the state transition and is prepared for a new
release event. wlr_buffer_acquire() forbids calls on already-acquired
buffers, and wlr_buffer_lock() now forbids calls on released buffers.

[1]: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4904

include/wlr/types/wlr_buffer.h

@@ -44,9 +44,11 @@ enum wlr_buffer_cap {
  * A buffer containing pixel data.
  *
  * A buffer has a single producer (the party who created the buffer) and
- * multiple consumers (parties reading the buffer). When all consumers are done
+ * multiple consumers (parties reading the buffer). Initially, a buffer is
- * with the buffer, it gets released and can be re-used by the producer. When
+ * released. When the consumer passes the buffer to a consumer, the buffer is
- * the producer and all consumers are done with the buffer, it gets destroyed.
+ * acquired. When all consumers are done with the buffer, it gets released and
+ * can be re-used by the producer. When the producer and all consumers are done
+ * with the buffer, it gets destroyed.
  */
 struct wlr_buffer {
 	const struct wlr_buffer_impl *impl;
@@ -70,10 +72,20 @@ struct wlr_buffer {
  * they are done with the buffer.
  */
 void wlr_buffer_drop(struct wlr_buffer *buffer);
+/**
+ * Acquire the buffer. This function should be called by producers when they
+ * pass a released buffer to a consumer. The consumer is responsible for
+ * calling wlr_buffer_unlock() once they are done with the buffer.
+ *
+ * This function aborts if the buffer has already been acquired.
+ */
+struct wlr_buffer *wlr_buffer_acquire(struct wlr_buffer *buffer);
 /**
  * Lock the buffer. This function should be called by consumers to make
  * sure the buffer can be safely read from. Once the consumer is done with the
  * buffer, they should call wlr_buffer_unlock().
+ *
+ * This function aborts if the buffer hasn't been acquired.
  */
 struct wlr_buffer *wlr_buffer_lock(struct wlr_buffer *buffer);
 /**

render/swapchain.c

@@ -74,7 +74,7 @@ static struct wlr_buffer *slot_acquire(struct wlr_swapchain *swapchain,
 	slot->release.notify = slot_handle_release;
 	wl_signal_add(&slot->buffer->events.release, &slot->release);
 
-	return wlr_buffer_lock(slot->buffer);
+	return wlr_buffer_acquire(slot->buffer);
 }
 
 struct wlr_buffer *wlr_swapchain_acquire(struct wlr_swapchain *swapchain) {

types/buffer/buffer.c

@@ -45,7 +45,14 @@ void wlr_buffer_drop(struct wlr_buffer *buffer) {
 	buffer_consider_destroy(buffer);
 }
 
+struct wlr_buffer *wlr_buffer_acquire(struct wlr_buffer *buffer) {
+	assert(buffer->n_locks == 0);
+	buffer->n_locks++;
+	return buffer;
+}
+
 struct wlr_buffer *wlr_buffer_lock(struct wlr_buffer *buffer) {
+	assert(buffer->n_locks > 0);
 	buffer->n_locks++;
 	return buffer;
 }

types/buffer/client.c

@@ -89,7 +89,7 @@ struct wlr_client_buffer *wlr_client_buffer_create(struct wlr_buffer *buffer,
 	client_buffer->renderer_destroy.notify = client_buffer_handle_renderer_destroy;
 
 	// Ensure the buffer will be released before being destroyed
-	wlr_buffer_lock(&client_buffer->base);
+	wlr_buffer_acquire(&client_buffer->base);
 	wlr_buffer_drop(&client_buffer->base);
 
 	return client_buffer;

types/buffer/resource.c

@@ -57,5 +57,12 @@ struct wlr_buffer *wlr_buffer_try_from_resource(struct wl_resource *resource) {
 		return NULL;
 	}
 
-	return wlr_buffer_lock(buffer);
+	// If the buffer is released, we want to acquire it. If the buffer is
+	// already acquired (e.g. because it has been attached to another surface),
+	// we want to lock it.
+	if (buffer->n_locks > 0) {
+		return wlr_buffer_lock(buffer);
+	} else {
+		return wlr_buffer_acquire(buffer);
+	}
 }