This commit fixes several issues with the `Ts*Src` elements.
The pause functions used cancel_task which breaks the Task loop at await
points. For some elements, this implies making sure no item is being lost.
Moreover, cancelling the Task also cancels downstream processing, which
makes it difficult to ensure elements can handle all cases.
This commit reimplements Task::pause which allows completing the running
loop iteration before pausing the loop.
See https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/277#note_439529
In the Paused state, incoming items were rejected by TsAppSrc and DataQueue.
See https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/277#note_438455
- FlushStart must engage items rejection and cancel the Task.
- FlushStop must purge the internal stream & accept items again.
If the task was cancelled, `push_prelude` could set `need_initial_events`
to `true` when the events weren't actually pushed yet.
TsAppSrc used to renew its internal channel which could cause Buffer loss
when transitionning Playing -> Paused -> Playing.
See https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/issues/98
The biggest changes are
- Many functions are not asynchronous anymore as it would be difficult
to run them correctly with our mix of synchronous C code and Rust
code.
- The pad context and its corresponding custom event are gone and
instead thread local storage and task local storage are used. This
makes it easier to correctly pass it through the different layers
of Rust and C code and back.
- Sink events have a different function for serialized and oob events,
src events are handled correctly by default now by simply forwarding
them.
- Task::prepare() has a separate variant that takes a preparation
function as this is a very common task.
- The task loop function can signal via its return value if it wants to
be called again or not.
