Previous version used the Context::block_on_or_add_sub_task which
spawns a full-fledged executor with timer and io Reactor for no
reason when we just need to wait for a Receiver or JoinHandle.
When the iteration loop is throttling, the call to `abort` on the
`loop_abort_handle` returns immediately, but the actual `Future`
for the iteration loop is aborted only when the scheduler throttling
completes. State transitions which requires the loop to be aborted &
which are serialized at the pipeline level can incur long delays.
This commit makes sure the Task Context's scheduler is awaken as soon
as the task loop is aborted.
The threadshare executor was based on a modified version of tokio
which implemented the throttling strategy in the BasicScheduler.
Upstream tokio codebase has significantly diverged from what it
was when the throttling strategy was implemented making it hard
to follow. This means that we can hardly get updates from the
upstream project and when we cherry pick fixes, we can't reflect
the state of the project on our fork's version. As a consequence,
tools such as cargo-deny can't check for RUSTSEC fixes in our fork.
The smol ecosystem makes it quite easy to implement and maintain
a custom async executor. This MR imports the smol parts that
need modifications to comply with the threadshare model and implements
a throttling executor in place of the tokio fork.
Networking tokio specific types are replaced with Async wrappers
in the spirit of [smol-rs/async-io]. Note however that the Async
wrappers needed modifications in order to use the per thread
Reactor model. This means that higher level upstream networking
crates such as [async-net] can not be used with our Async
implementation.
Based on the example benchmark with ts-udpsrc, performances seem on par
with what we achieved using the tokio fork.
Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/issues/118
Related to https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/604
StateMachines are spawned on a runtime::Context which uses a tokio
runtime. The StateMachine doesn't need all the features from tokio
such as the IO and timers drivers.
This commit makes use of a light-weight futures executor to spawn
the StateMachines.
