restart: :transient does not affect Task.Supervisor.async/async_nolink

[sikanhe]

For task supervisors, the processes can only be restarted when using start_child method. We cannot currently await on a task that we want to retry.

{:ok, tas} = Task.Supervisor.start_link(restart: :transient)

Task.Supervisor.async_nolink(tas, fn -> 1 = 2 end) 
// does not restart

I understand there is a limitation that a Task is tied to a process, so it cannot possibly know about the new process that could get started. But this was unclear in the documentation.

Do you think there is a way to await on a supervised task that be restarted on crash? If so we should implement it because there is many cases where this could be helpful. If not I think we should clarify this in the docs more.

[josevalim]

Transient works. If you use Task.Supervisor.children, you will see new PIDs, showing the task is being restarted. Your conclusion is correct though, transient won't work for async because it will never receive the initial request.

@fishcakez is there something we can do here? Maybe if the supervised task timeouts and the parent process is dead, maybe there is a better error message we can use?

[fishcakez]

eba153d prevents the Task.Supervisor from reaching max restarts if the caller is dead and restarts is :transient. That's about all we can do in this situation.

It is not possible to await on a restarted task because the monitor reference can not be reused or new pid detected by the caller. An improvement to the docs is very welcome.

[josevalim]

Thank you @fishcakez ❤️

@sikanhe you mentioned the docs were unclear, if so, please submit a pull request to improve them, we would really appreciate it. Thank you!