libcamera: v4l2_videodevice: Prevent queueing buffers without a cache

The V4l2 buffer cache allows us to map incoming buffers to an instance
of the V4L2 buffer required to actually queue.

If the cache_ is not available, then the buffers required to allow
queuing to a device have been released, and this indicates an issue at
the pipeline handler.

This could be a common mistake, as it could happen if a pipeline handler
always requeues buffers to the device after they complete, without
checking if they are cancelled.

Catch any invalid queueing of buffers to the V4L2 video device when
resources have been released by adding a Fatal log message to highlight
the error during development.

Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/v4l2_videodevice.cpp

@@ -1391,6 +1391,16 @@ int V4L2VideoDevice::queueBuffer(FrameBuffer *buffer)
 	struct v4l2_buffer buf = {};
 	int ret;
 
+	/*
+	 * Pipeline handlers should not requeue buffers after releasing the
+	 * buffers on the device. Any occurence of this error should be fixed
+	 * in the pipeline handler directly.
+	 */
+	if (!cache_) {
+		LOG(V4L2, Fatal) << "No BufferCache available to queue.";
+		return -ENOENT;
+	}
+
 	ret = cache_->get(*buffer);
 	if (ret < 0)
 		return ret;