ts/Task: don't drain sub tasks after state transition and iteration

Subtasks are used when current async processing needs to execute
a `Future` via a sync function (eg. a call to a C function).
In this case `Context::block_on` would block the whole `Context`,
leading to a deadlock.

The main use case for this is the `Pad{Src,Sink}` functions:
when we `PadSrc::push` and the peer pad is a `PadSink`, we want
`PadSrc::push` to complete after the async function on the
`PadSink` completes. In this case the `PadSink` async function
is added as a subtask of current scheduler task and
`PadSrc::push` only returns when the subtask is executed.

In `runtime::Task` (`Task` here is the execution Task with a
state machine, not a scheduler task), we used to spawn state
transition actions and iteration loop (leading to a new
scheduler Task). At the time, it seemed convenient for the user
to automatically drain sub tasks after a state transition action
or an iteration. User wouldn't have to worry about this, similarly
to the `Pad{Src,Sink}` case.

In current implementation, the `Task` state machine now operates
directly on the target `Context`. State transtions actions and
the iteration loop are no longer spawned. It seems now useless to
abstract the subtasks draining from the user. Either they
transitively use a mechanism such as `Pad{Src,Sink}` which already
handles this automatically, or they add substasks on purpose, in
which case they know better when subtasks must be drained.

generic/threadshare/src/runtime/task.rs

@@ -28,7 +28,6 @@ use futures::prelude::*;
 use std::fmt;
 use std::ops::Deref;
 use std::pin::Pin;
-use std::stringify;
 use std::sync::{Arc, Mutex, MutexGuard};
 use std::task::Poll;
 
@@ -376,8 +375,7 @@ pub trait TaskImpl: Send + 'static {
 
     /// Handles an error occuring during the execution of the `Task` loop.
     ///
-    /// This include errors returned by [`Self::try_next`], [`Self::handle_item`]
-    /// as well as errors returned by subtasks drain at the end of an iteration.
+    /// This include errors returned by [`Self::try_next`] & [`Self::handle_item`].
     ///
     /// If the error is unrecoverable, implementations might use
     /// `gst::Element::post_error_message` and return `Trigger::Error`.
@@ -817,27 +815,10 @@ struct StateMachine<Item: Send + 'static> {
     pending_triggering_evt: Option<TriggeringEvent>,
 }
 
-// Make sure the Context doesn't throttle otherwise we end up  with long delays executing
-// transition actions in a pipeline with many elements. This is because pipeline serializes
-// the transition actions and the Context's scheduler gets a chance to reach its throttling
-// state between 2 elements.
-
 macro_rules! exec_action {
     ($self:ident, $action:ident, $triggering_evt:expr, $origin:expr, $task_inner:expr) => {{
         match $self.task_impl.$action().await {
-            Ok(()) => {
-                gst::trace!(RUNTIME_CAT, "Draining subtasks for {}", stringify!($action));
-                if let Err(err) = Context::drain_sub_tasks().await {
-                    gst::log!(
-                        RUNTIME_CAT,
-                        "{} subtask returned {:?}",
-                        stringify!($action),
-                        err
-                    );
-                }
-
-                Ok($triggering_evt)
-            }
+            Ok(()) => Ok($triggering_evt),
             Err(err) => {
                 // FIXME problem is that we loose the origin trigger in the
                 //       final TransitionStatus.
@@ -1093,11 +1074,6 @@ impl<Item: Send + 'static> StateMachine<Item> {
                     // Unprepare is not joined by an ack_rx but by joining the state machine handle
                     self.task_impl.unprepare().await;
 
-                    gst::trace!(RUNTIME_CAT, "Draining subtasks for unprepare");
-                    if let Err(err) = Context::drain_sub_tasks().await {
-                        gst::log!(RUNTIME_CAT, "unprepare subtask returned {:?}", err);
-                    }
-
                     task_inner
                         .lock()
                         .unwrap()
@@ -1174,11 +1150,6 @@ impl<Item: Send + 'static> StateMachine<Item> {
                 gst::debug!(RUNTIME_CAT, "TaskImpl::handle_item returned {:?}", err);
                 err
             })?;
-
-            gst::trace!(RUNTIME_CAT, "Draining subtasks after Task loop iteration");
-            if let Err(err) = Context::drain_sub_tasks().await {
-                gst::debug!(RUNTIME_CAT, "Task iteration subtask returned {:?}", err);
-            }
         }
     }
 }
@@ -1187,10 +1158,19 @@ impl<Item: Send + 'static> StateMachine<Item> {
 mod tests {
     use futures::channel::{mpsc, oneshot};
     use futures::executor::block_on;
+    use futures::future::BoxFuture;
+    use futures::prelude::*;
     use std::time::Duration;
 
-    use super::{TaskState::*, TransitionOk::*, TransitionStatus::*, Trigger::*, *};
-    use crate::runtime::Context;
+    use super::{
+        Task, TaskImpl,
+        TaskState::{self, *},
+        TransitionError,
+        TransitionOk::*,
+        TransitionStatus::*,
+        Trigger::{self, *},
+    };
+    use crate::runtime::{Context, RUNTIME_CAT};
 
     #[track_caller]
     fn stop_then_unprepare(task: Task) {
@@ -1534,7 +1514,7 @@ mod tests {
                         err
                     );
                     match (trigger, state) {
-                        (Trigger::Prepare, TaskState::Unprepared) => {
+                        (Prepare, Unprepared) => {
                             self.prepare_error_sender.send(()).await.unwrap();
                         }
                         other => unreachable!("{:?}", other),
@@ -1772,7 +1752,7 @@ mod tests {
                         err
                     );
                     match (trigger, state) {
-                        (Trigger::Prepare, TaskState::Unprepared) => {
+                        (Prepare, Unprepared) => {
                             self.prepare_error_sender.send(()).await.unwrap();
                         }
                         other => panic!("action error for {:?}", other),
@@ -2789,11 +2769,11 @@ mod tests {
 
         gst::debug!(RUNTIME_CAT, "flush_concurrent_start: // start");
         match block_on(task.start()) {
-            Ok(TransitionOk::Complete {
+            Ok(Complete {
                 origin: Paused,
                 target: Started,
             }) => (),
-            Ok(TransitionOk::Complete {
+            Ok(Complete {
                 origin: PausedFlushing,
                 target: Flushing,
             }) => (),