gh-137335: Fix unlikely nmae conflicts for named pipes in multiprocessing 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.

Author: serhiy-storchaka

Lib/asyncio/windows_utils.py

@@ -6,7 +6,6 @@
     raise ImportError('win32 only')
 
 import _winapi
-import itertools
 import msvcrt
 import os
 import subprocess
@@ -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(
-        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
@@ -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):
+                    raise
 
         h2 = _winapi.CreateFile(
             address, access, 0, _winapi.NULL, _winapi.OPEN_EXISTING,

Lib/multiprocessing/connection.py

@@ -11,7 +11,6 @@
 
 import errno
 import io
-import itertools
 import os
 import sys
 import socket
@@ -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']
 
@@ -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')
 
@@ -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):
@@ -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
@@ -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

Misc/NEWS.d/next/Library/2025-08-04-23-20-43.gh-issue-137335.IIjDJN.rst

@@ -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.