I want to implement retry logic with Python's concurrent.futures.ThreadPoolExecutor. I would like the following properties:

1. A new future is added to the work queue as soon as it fails.
2. A retried future can be retried again, either indefinitely or up to a maximum retry count.

A lot of existing code I found online basically operates in "rounds", where they call as_completed on an initial list of futures, resubmits failed futures, gathers those futures in a new list, and goes back to calling as_completed on the new list if it's not empty. Basically something like this:

with concurrent.futures.ThreadPoolExecutor(...) as executor:
    futures = {executor.submit(fn, job): job for job in jobs}
    while len(futures) > 0:
        new_futures = {}
        for fut in concurrent.futures.as_completed(futures):
            if fut.exception():
                job = futures[fut]
                new_futures[executor.submit(fn, job)] = job
            else:
                ...  # logic to handle successful job
        futures = new_futures

However, I think that doesn't satisfy the first property, since it's possible that a retried future completes before the initial futures, but we won't process it until all the initial futures complete.

Here's a hypothetical pathological case. Let's say we have two jobs, the first runs for 1 second but has a 90% chance of failure, while the second runs for 100 seconds. If our executor has 2 workers, and the first job fails after 1 second, we'll retry it immediately. But if it failed again, we won't be able to retry until the second job completes.

---

So my question is, is it possible to implement retry logic with these desired properties, without using external libraries or rewriting low-level executor logic? One thing I tried is putting the retry logic in the code sent to the worker:

def worker_job(fn):
    try:
        return fn()
    except Exception:
        executor.submit(fn)

with concurrent.futures.ThreadPoolExecutor(...) as executor:
    jobs = [functools.partial(fn, arg) for arg in args]
    executor.map(worker_job, jobs)

But it seems like submitting new jobs from within a job doesn't work.

Retry using as_completed

Simple way

Loop with wait(..., return_when=FIRST_COMPLETED) instead of as_completed(...).

Trade-offs:

1. Overhead of pending futures (re-adding waiter, building new_futures).
2. Troublesome if want to specify overall timeout.

with concurrent.futures.ThreadPoolExecutor() as executor:
    futures = {executor.submit(fn, job): job for job in jobs}
    while len(futures) > 0:
        new_futures = {}
        done, pending = concurrent.futures.wait(futures, return_when=FIRST_COMPLETED)
        for fut in done:
            if fut.exception():
                job = futures[fut]
                new_futures[executor.submit(fn, job)] = job
            else:
                ...  # logic to handle successful job
        for fut in pending:
            job = futures[fut]
            new_futures[fut] = job
        futures = new_futures

Efficient way

Tweak as_completed(...) to add to fs and pending, and use waiter.

Trade-off: Maintenance.

Advantage: Ability to specify overall timeout if wanted.

class AsCompletedWaiterWrapper:
    def __init__(self):
        self.fs = None
        self.pending = None
        self.waiter = None

    def listen(self, fut):
        with self.waiter.lock:
            self.fs.add(fut)
            self.pending.add(fut)
            fut._waiters.append(self.waiter)

    def as_completed(self, fs, timeout=None):
        """
        concurrent.futures.as_completed plus the 3 lines marked with +.
        """
        if timeout is not None:
            end_time = timeout + time.monotonic()

        fs = set(fs)
        total_futures = len(fs)
        with _AcquireFutures(fs):
            finished = set(
                    f for f in fs
                    if f._state in [CANCELLED_AND_NOTIFIED, FINISHED])
            pending = fs - finished
            waiter = _create_and_install_waiters(fs, _AS_COMPLETED)
        self.fs = fs            # +
        self.pending = pending  # +
        self.waiter = waiter    # +
        finished = list(finished)
        try:
            yield from _yield_finished_futures(finished, waiter,
                                               ref_collect=(fs,))

            while pending:
                if timeout is None:
                    wait_timeout = None
                else:
                    wait_timeout = end_time - time.monotonic()
                    if wait_timeout < 0:
                        raise TimeoutError(
                                '%d (of %d) futures unfinished' % (
                                len(pending), total_futures))

                waiter.event.wait(wait_timeout)

                with waiter.lock:
                    finished = waiter.finished_futures
                    waiter.finished_futures = []
                    waiter.event.clear()

                # reverse to keep finishing order
                finished.reverse()
                yield from _yield_finished_futures(finished, waiter,
                                                   ref_collect=(fs, pending))

        finally:
            # Remove waiter from unfinished futures
            for f in fs:
                with f._condition:
                    f._waiters.remove(waiter)

Usage:

with concurrent.futures.ThreadPoolExecutor() as executor:
    futures = {executor.submit(fn, job): job for job in jobs}
    w = AsCompletedWaiterWrapper()
    for fut in w.as_completed(futures):
        if fut.exception():
            job = futures[fut]
            new_fut = executor.submit(fn, job)
            futures[new_fut] = job
            w.listen(new_fut)
        else:
            ...  # logic to handle successful job

Retry from job helper

Wait for events in with ... executor: as ThreadPoolExecutor.__exit__ shuts down executor so it cannot schedule new futures.

Trade-offs:

1. Would not work with ProcessPoolExecutor due to executor reference in main process.
2. Troublesome if want to specify overall timeout.

def worker_job(fn, event):
    try:
        rv = fn()
        event.set()
        return rv
    except Exception:
        executor.submit(worker_job, fn, event)

with concurrent.futures.ThreadPoolExecutor() as executor:
    jobs = [functools.partial(fn, arg) for arg in args]
    events = [threading.Event() for _ in range(len(jobs))]
    executor.map(worker_job, jobs, events)
    for e in events:
        e.wait()