Bugfix: executor doesn't propagate exception from task that awaited a future

[nadavelkabets]

Fixes #1642

Changes

A task now registers itself on the awaited future via new Future._add_waiting_task.
The executor then dispatches the same Task object and sees its exception through handler.exception() as intended.

• Future._callbacks now holds Union[Callable, Task]
• _schedule_or_invoke_done_callbacks handles Task entries through executor._call_task_in_next_spin.
• _wait_for_ready_callbacks skips tasks that were cancelled or completed between being queued and being popped.
• A warnings.warn fires if a waiting Task is dropped because the executor weakref could not be resolved.

[fujitatomoya]

i think that the approach of preserving task identity rather than wrapping in a new task is the right call. almost lgtm, with just one minor suggestion.

[fujitatomoya]

lgtm with green CI.

[fujitatomoya]

Pulls: #1643
Gist: https://gist.githubusercontent.com/fujitatomoya/304d03354c28f17e75b36dec7465ed9c/raw/364b136f9e5a7268dd0d7017890a230f6bb89273/ros2.repos
BUILD args: --packages-above-and-dependencies rclpy
TEST args: --packages-above rclpy
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/18897

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[nadavelkabets]

@fujitatomoya could you run windows again?

[fujitatomoya]

• Linux-aarch64
• Windows

[mjcarroll]

• Linux-aarch64
• Windows

[mjcarroll]

@Mergifyio backport kilted jazzy humble

[mergify]

backport kilted jazzy humble

✅ Backports have been created

Details

• #1651 Bugfix: executor doesn't propagate exception from task that awaited a future (backport #1643) has been created for branch kilted
• #1652 Bugfix: executor doesn't propagate exception from task that awaited a future (backport #1643) has been created for branch jazzy but encountered conflicts

Cherry-pick of aac0ebb has failed:

On branch mergify/bp/jazzy/pr-1643
Your branch is up to date with 'origin/jazzy'.

You are currently cherry-picking commit aac0ebb.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   rclpy/rclpy/executors.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rclpy/rclpy/task.py
	both modified:   rclpy/test/test_executor.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

• #1653 Bugfix: executor doesn't propagate exception from task that awaited a future (backport #1643) has been created for branch humble but encountered conflicts

Cherry-pick of aac0ebb has failed:

On branch mergify/bp/humble/pr-1643
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit aac0ebb.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   rclpy/rclpy/executors.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rclpy/rclpy/task.py
	both modified:   rclpy/test/test_executor.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally