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gh-137335: Fix unlikely name conflicts for named pipes in multiprocessing and asyncio on Windows #137389

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Merged
serhiy-storchaka merged 9 commits into from
Feb 24, 2026
+57 −26
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gh-137335: Fix unlikely name conflicts for named pipes in multiprocessing and asyncio on Windows #137389

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2a0a5e9
gh-137335: Fix unlikely nmae conflicts for named pipes in multiproces…
serhiy-storchaka Aug 4, 2025
5b91a56
Fix typo
serhiy-storchaka Aug 5, 2025
ed050a0
Do not use os.getpid() and tempfile._get_candidate_names().
serhiy-storchaka Aug 5, 2025
7363c99
Merge branch 'main' into temp-named-pipes
serhiy-storchaka Feb 5, 2026
5243b9e
Use os.urandom().
serhiy-storchaka Feb 5, 2026
81cb66e
Merge branch 'main' into temp-named-pipes
serhiy-storchaka Feb 10, 2026
5207bf6
Restore the PID and sequential number for debugging and reducing chan…
serhiy-storchaka Feb 10, 2026
cae80b9
Limit the number of attempts.
serhiy-storchaka Feb 10, 2026
de04089
Apply suggestion from @serhiy-storchaka
serhiy-storchaka Feb 10, 2026
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gh-137335: Fix unlikely nmae conflicts for named pipes in multiproces… 
…sing and asyncio on Windows

Since os.stat() raises an OSError for existing named pipe "\\.\pipe\...",
os.path.exists() always returns False for it, and tempfile.mktemp() can
return a name that matches an existing named pipe.

So, tempfile.mktemp() cannot be used to generate unique names for named
pipes. Instead, CreateNamedPipe() should be called in a loop with
different names until it completes successfully.
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@serhiy-storchaka
serhiy-storchaka committed Aug 4, 2025
commit 2a0a5e981535d71f2eb76eb5ea2daa759e92b089
22 changes: 13 additions & 9 deletions Lib/asyncio/windows_utils.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,6 @@
raise ImportError('win32 only')

import _winapi
import itertools
import msvcrt
import os
import subprocess
Expand All @@ -23,18 +22,13 @@
BUFSIZE = 8192
PIPE = subprocess.PIPE
STDOUT = subprocess.STDOUT
_mmap_counter = itertools.count()


# Replacement for os.pipe() using handles instead of fds


def pipe(*, duplex=False, overlapped=(True, True), bufsize=BUFSIZE):
"""Like os.pipe() but with overlapped support and using handles not fds."""
address = tempfile.mktemp(
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kumaraditya303 marked this conversation as resolved.
prefix=r'\\.\pipe\python-pipe-{:d}-{:d}-'.format(
os.getpid(), next(_mmap_counter)))

if duplex:
openmode = _winapi.PIPE_ACCESS_DUPLEX
access = _winapi.GENERIC_READ | _winapi.GENERIC_WRITE
Expand All @@ -54,11 +48,21 @@ def pipe(*, duplex=False, overlapped=(True, True), bufsize=BUFSIZE):
else:
flags_and_attribs = 0

prefix = fr'\\.\pipe\python-pipe-{os.getpid()}-'
names = tempfile._get_candidate_names()
h1 = h2 = None
try:
h1 = _winapi.CreateNamedPipe(
address, openmode, _winapi.PIPE_WAIT,
1, obsize, ibsize, _winapi.NMPWAIT_WAIT_FOREVER, _winapi.NULL)
while True:
address = prefix + next(names)
try:
h1 = _winapi.CreateNamedPipe(
address, openmode, _winapi.PIPE_WAIT,
1, obsize, ibsize, _winapi.NMPWAIT_WAIT_FOREVER, _winapi.NULL)
break
except OSError as e:
if e.winerror not in (_winapi.ERROR_PIPE_BUSY,
_winapi.ERROR_ACCESS_DENIED):
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@gpshead gpshead Feb 10, 2026

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an infinite loop retrying on this error could potentially hide an error if the system simply doesn't allow this process to create a pipe with the given address? (here and below)

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@serhiy-storchaka serhiy-storchaka Feb 10, 2026

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I do not know if this is even possible (\\.\pipe\ is not a real directory which you can forbid access to), but it will not harm if limit the number of attempts.

raise

h2 = _winapi.CreateFile(
address, access, 0, _winapi.NULL, _winapi.OPEN_EXISTING,
Expand Down
55 changes: 34 additions & 21 deletions Lib/multiprocessing/connection.py
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,6 @@

import errno
import io
import itertools
import os
import sys
import socket
Expand Down Expand Up @@ -45,8 +44,6 @@
# A very generous timeout when it comes to local connections...
CONNECTION_TIMEOUT = 20.

_mmap_counter = itertools.count()

default_family = 'AF_INET'
families = ['AF_INET']

Expand Down Expand Up @@ -78,8 +75,7 @@ def arbitrary_address(family):
elif family == 'AF_UNIX':
return tempfile.mktemp(prefix='sock-', dir=util.get_temp_dir())
elif family == 'AF_PIPE':
return tempfile.mktemp(prefix=r'\\.\pipe\pyc-%d-%d-' %
(os.getpid(), next(_mmap_counter)), dir="")
return fr'\\.\pipe\pyc-{os.getpid()}-{next(tempfile._get_candidate_names())}'
else:
raise ValueError('unrecognized family')

Expand Down Expand Up @@ -472,17 +468,27 @@ class Listener(object):
def __init__(self, address=None, family=None, backlog=1, authkey=None):
family = family or (address and address_type(address)) \
or default_family
address = address or arbitrary_address(family)

_validate_family(family)
if authkey is not None and not isinstance(authkey, bytes):
raise TypeError('authkey should be a byte string')

if family == 'AF_PIPE':
self._listener = PipeListener(address, backlog)
if address:
self._listener = PipeListener(address, backlog)
else:
while True:
address = arbitrary_address(family)
try:
self._listener = PipeListener(address, backlog)
break
except OSError as e:
if e.winerror not in (_winapi.ERROR_PIPE_BUSY,
_winapi.ERROR_ACCESS_DENIED):
raise
else:
address = address or arbitrary_address(family)
self._listener = SocketListener(address, family, backlog)

if authkey is not None and not isinstance(authkey, bytes):
raise TypeError('authkey should be a byte string')

self._authkey = authkey

def accept(self):
Expand Down Expand Up @@ -570,7 +576,6 @@ def Pipe(duplex=True):
'''
Returns pair of connection objects at either end of a pipe
'''
address = arbitrary_address('AF_PIPE')
if duplex:
openmode = _winapi.PIPE_ACCESS_DUPLEX
access = _winapi.GENERIC_READ | _winapi.GENERIC_WRITE
Expand All @@ -580,15 +585,23 @@ def Pipe(duplex=True):
access = _winapi.GENERIC_WRITE
obsize, ibsize = 0, BUFSIZE

h1 = _winapi.CreateNamedPipe(
address, openmode | _winapi.FILE_FLAG_OVERLAPPED |
_winapi.FILE_FLAG_FIRST_PIPE_INSTANCE,
_winapi.PIPE_TYPE_MESSAGE | _winapi.PIPE_READMODE_MESSAGE |
_winapi.PIPE_WAIT,
1, obsize, ibsize, _winapi.NMPWAIT_WAIT_FOREVER,
# default security descriptor: the handle cannot be inherited
_winapi.NULL
)
while True:
address = arbitrary_address('AF_PIPE')
try:
h1 = _winapi.CreateNamedPipe(
address, openmode | _winapi.FILE_FLAG_OVERLAPPED |
_winapi.FILE_FLAG_FIRST_PIPE_INSTANCE,
_winapi.PIPE_TYPE_MESSAGE | _winapi.PIPE_READMODE_MESSAGE |
_winapi.PIPE_WAIT,
1, obsize, ibsize, _winapi.NMPWAIT_WAIT_FOREVER,
# default security descriptor: the handle cannot be inherited
_winapi.NULL
)
break
except OSError as e:
if e.winerror not in (_winapi.ERROR_PIPE_BUSY,
_winapi.ERROR_ACCESS_DENIED):
raise
h2 = _winapi.CreateFile(
address, access, 0, _winapi.NULL, _winapi.OPEN_EXISTING,
_winapi.FILE_FLAG_OVERLAPPED, _winapi.NULL
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2025-08-04-23-20-43.gh-issue-137335.IIjDJN.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Get rid of any possibility of a name conflict for named pipes in
:mod:`miltiprocessing` and :mod:`asyncio` on Windows, no matter how small.
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