Created attachment 251205 [details] [review]
dataqueue: add gst_data_queue_push_force

Adds a variant of the _push function that doesn't check the queue limits
before adding the new item. It is useful when pushing an element to the
queue shouldn't lock the thread.

One particular scenario is when the queue is used to serialize buffers
and events that are going to be pushed from another thread. The
dataqueue should have a limit on the amount of buffers to be stored to
avoid large memory consumption, but events can be considered to have
negligible impact on memory compared to buffers. So it is useful to be
used to push items into the queue that contain events, even though the
queue is already full, it shouldn't matter inserting an item that has
no significative size.

This scenario happens on adaptive elements (dashdemux / mssdemux) as
there is a single download thread fetching buffers and putting into the
dataqueues for the streams. This same download thread can als generate
events in some situations as caps changes, eos or a internal control
events. There can be a deadlock at preroll if the first buffer fetched
is large enough to fill the dataqueue and the download thread and the
next iteration of the download thread decides to push an event to this
same dataqueue before fetching buffers to other streams, if this push
locks, the pipeline will be stuck in preroll as no more buffers will be
downloaded.
There is a somewhat common practice in dash streams to have a single
very large buffer for audio and one for video, so this will always
happen as the download thread will have to push an EOS right after
fetching the first buffer for any stream.

API: gst_data_queue_push_force

Review of attachment 251205 [details] [review]:

Can't you just make the events "invisible" on the queue then? Apart from that, a pop_force() would be useful too :)

(In reply to comment #2)
> Review of attachment 251205 [details] [review]:
> 
> Can't you just make the events "invisible" on the queue then? Apart from that,

I thought on this, but it would be a behaviour break. The 'visible' attribute for items is only used for counting the total of items on the queue. An item can be 'invisible' and have a duration and size. I thought on considering the item 'force pushable' if all item parameters were 0, meaning it has no size, no duration and is invisible. But this would still be a behaviour break and having a separate 'push_force()' function seems more explicit about the action.

> a pop_force() would be useful too :)
I don't get how a 'pop_force()' would work? The dataqueue has no lower limits. Maybe you meant 'try_pop()'?

(In reply to comment #3)
> (In reply to comment #2)
> > Review of attachment 251205 [details] [review] [details]:
> > 
> > Can't you just make the events "invisible" on the queue then? Apart from that,
> 
> I thought on this, but it would be a behaviour break. The 'visible' attribute
> for items is only used for counting the total of items on the queue. An item
> can be 'invisible' and have a duration and size. I thought on considering the
> item 'force pushable' if all item parameters were 0, meaning it has no size, no
> duration and is invisible. But this would still be a behaviour break and having
> a separate 'push_force()' function seems more explicit about the action.

Makes sense, please push :)

> > a pop_force() would be useful too :)
> I don't get how a 'pop_force()' would work? The dataqueue has no lower limits.
> Maybe you meant 'try_pop()'?

Oh I was just confused :) I needed something like a force_pop() in queue at some point to be able to pop elements although the queue was flushing.

Comment on attachment 251205 [details] [review]
dataqueue: add gst_data_queue_push_force

Please add Since: 1.2 markers to the docs, add the new function to the docs and the symbol to the win32 .def file

Pushed as commit 581c4297d0953fe492b34889e95d0353f4b8a509

Comment on attachment 251205 [details] [review]
dataqueue: add gst_data_queue_push_force

Added requested changes and pushed.