Created attachment 324956 [details] [review]
Proposed patch

The videorate element has a reference to the last buffer in videorate->prevbuf.
In gst_video_rate_flush_prev, before push the buffer, videorate has to change the timestamp of the buffer so it make it writable with gst_buffer_make_writable. Before that, videorate take another reference on the buffer because one of these ref will be transfered at the push pad and the other one is for the videorate->prevbuf.

So when gst_buffer_make_writable is call, the ref count of the buffer is at least 2 and the buffer will be copied. If the memory is not sharable, there will be a deep copy.

This is the expected behavior but it's not optimal in the drop-only mode.
In drop only mode, videorate will never duplicate a buffer so it don't have to keep a reference on the previous buffer. In fact, video rate don't keep the previous buffer : just after calling gst_video_rate_flush_prev, it release its ref to the buffer :

/* No need to keep the buffer around for longer */
gst_buffer_replace (&videorate->prevbuf, NULL);

However this is done after taking the ref for the push pad and after the gst_buffer_make_writable, so the copy is already done, and the videorate never use this copy.

The proposed patch release the useless reference to prevbuf before make the buffer writable, in the case we are in drop-only mode.