multiprocessing: spawn and forkserver leak inheritable fds to further sub-processes #161408

Bug report

When using multiprocessing with spawn or forkserver start methods, multiprocessing passes file descriptors to the child that are marked inheritable (including the sentinel pipe, the resource tracker and forkserver pipes, and the fds of pickled Connections and Queues). They leak across exec¹ into further sub-processes of the child process, which is problematic for two reasons:

1. It is inconsistent with PEP 446. These pipes and sockets are meaningful between the parent and the child, but not to further sub-processes of the child. Exposing them is bad practice for all the reasons detailed in PEP 446.
2. It can cause hangs. If the child leaves a background sub-process behind, that process keeps the fds open, and as a result:
   • Under spawn, p.join(timeout=...) blocks: it doesn't use waitpid but waits for EOF on the sentinel pipe, which the background process keeps open.
   • Under both spawn and forkserver, the resource tracker outlives the main process: it waits for EOF on its pipe, which the background process keeps open, and meanwhile keeps the main process's stdout and stderr open. This blocks pipes like python3 example.py | tee.

A simple reproducer for the hanging issue is this:

import multiprocessing, os, sys, time

def child():
    # Start a background process (note the &) with stdio detached
    os.system("sleep 5 < /dev/null > /dev/null 2>&1 &")

if __name__ == "__main__":
    method = sys.argv[1]
    multiprocessing.set_start_method(method)
    p = multiprocessing.Process(target=child)
    p.start()
    start = time.monotonic()
    p.join(timeout=60)
    print(f"{method=}\njoin: {time.monotonic() - start:.1f}s")

Which can be run like so:

$ for method in fork spawn forkserver; do time sh -c "python3 example.py $method | tee"; done
method='fork'
join: 0.0s
real 0m0.036s

method='spawn'
join: 5.0s      # join blocks
real 0m5.072s

method='forkserver'
join: 0.0s      # join does not block, fine
real 0m5.088s   # but the resource tracker keeps stdout hostage, so tee is blocked

The fix would be to call os.set_inheritable(fd, False) on these fds in the child.

CPython versions tested on:

CPython main branch, 3.14

Operating systems tested on:

Linux, macOS