With Python 3.4.1 compiled from source, I'm having an issue in which every now and then some Futures are not marked as completed even though the underlying workload is done.

My workload is launching two subprocess in parallel, and whenever one is ready, launches another one. In one of the runs, the whole process got stuck after launching about 3K subprocess, and the underlying processes had in fact finished.

To wait for the finished subprocesses, I'm using FIRST_COMPLETED. Below is the core of my workload:

for element in element_generator:
    while len(running) >= max_tasks:
        done, pending = concurrent.futures.wait(running, timeout=15.0, return_when=concurrent.futures.FIRST_COMPLETED)
        process_results(done)
        running = pending

    running.add(executor.submit(exe_subprocess, element)) 
 

Replicating the issue takes time, but I've been able to successfully reproduce it with 2 and 3 processes in parallel.

Note: this was already posted in comments to http://bugs.python.org/issue20319, however it did not receive the proper atention as that issue is already closed.

As suggested there I printed the status of the never finished futures and this is the result:

State: RUNNING, Result: None, Exception: None, Waiters: 0, Cancelled: False, Running: True, Done: False

The information does not seem very relevant. However, I can attach a console and debug from there.