multiprocessing.popen_spawn_posix does not close child fds after spawn, can hang

Bug report

Bug description:

cpython/Lib/multiprocessing/popen_spawn_posix.py

Lines 51 to 72 in a705c1e

    
           parent_r = child_w = child_r = parent_w = None 
        
           try: 
        
               parent_r, child_w = os.pipe() 
        
               child_r, parent_w = os.pipe() 
        
               cmd = spawn.get_command_line(tracker_fd=tracker_fd, 
        
                                            pipe_handle=child_r) 
        
               self._fds.extend([child_r, child_w]) 
        
               self.pid = util.spawnv_passfds(spawn.get_executable(), 
        
                                              cmd, self._fds) 
        
               self.sentinel = parent_r 
        
               with open(parent_w, 'wb', closefd=False) as f: 
        
                   f.write(fp.getbuffer()) 
        
           finally: 
        
               fds_to_close = [] 
        
               for fd in (parent_r, parent_w): 
        
                   if fd is not None: 
        
                       fds_to_close.append(fd) 
        
               self.finalizer = util.Finalize(self, util.close_fds, fds_to_close) 
        
               for fd in (child_r, child_w): 
        
                   if fd is not None: 
        
                       os.close(fd)

What happens here:

Two pipes get opened.
The child proc gets spawned, the child fds are passed to it.
It is written into the parent fd.
The child fds are closed.

This order is wrong and can lead to hangs when the child proc crashes in between for whatever reason. Specifically, when the client crashes, then the parent will hang while trying to write into the parent fd, in this line:

cpython/Lib/multiprocessing/popen_spawn_posix.py

Line 62 in a705c1e

f.write(fp.getbuffer())

(I have this case because of some unpickling error happening in the child, but that's not really the point of the issue here.)

The fix should be easy: Add extra code to close the child fds, right after the spawn. This is also the standard pattern for this kind of code.

Also reported here: rwth-i6/returnn#1514

CPython versions tested on:

CPython main branch, CPython 3.11

Operating systems tested on:

Linux, Other

Linked PRs

fix potential hang

nice edge case find!

gh-118981: multiprocessing.popen_spawn_posix, fix potential hang (gh-…

pythongh-118981: multiprocessing.popen_spawn_posix, fix potential hang (

[3.13] gh-118981: multiprocessing.popen_spawn_posix, fix potential ha…

pythongh-118981: multiprocessing.popen_spawn_posix, fix potential hang (

[3.14] gh-118981: multiprocessing.popen_spawn_posix, fix potential ha…

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`multiprocessing.popen_spawn_posix` does not close child fds after spawn, can hang #118981

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CPython versions tested on:

Operating systems tested on:

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	parent_r = child_w = child_r = parent_w = None
	try:
	parent_r, child_w = os.pipe()
	child_r, parent_w = os.pipe()
	cmd = spawn.get_command_line(tracker_fd=tracker_fd,
	pipe_handle=child_r)
	self._fds.extend([child_r, child_w])
	self.pid = util.spawnv_passfds(spawn.get_executable(),
	cmd, self._fds)
	self.sentinel = parent_r
	with open(parent_w, 'wb', closefd=False) as f:
	f.write(fp.getbuffer())
	finally:
	fds_to_close = []
	for fd in (parent_r, parent_w):
	if fd is not None:
	fds_to_close.append(fd)
	self.finalizer = util.Finalize(self, util.close_fds, fds_to_close)

	for fd in (child_r, child_w):
	if fd is not None:
	os.close(fd)

multiprocessing.popen_spawn_posix does not close child fds after spawn, can hang #118981

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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gpshead commented on Sep 6, 2025

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`multiprocessing.popen_spawn_posix` does not close child fds after spawn, can hang #118981