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

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.

Lib/asyncio/windows_utils.py

@@ -10,7 +10,6 @@ import itertools
 import msvcrt
 import os
 import subprocess
-import tempfile
 import warnings
 
 
@@ -24,6 +23,7 @@ BUFSIZE = 8192
 PIPE = subprocess.PIPE
 STDOUT = subprocess.STDOUT
 _mmap_counter = itertools.count()
+_MAX_PIPE_ATTEMPTS = 20
 
 
 # Replacement for os.pipe() using handles instead of fds
@@ -31,10 +31,6 @@ _mmap_counter = itertools.count()
 
 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
@@ -56,9 +52,20 @@ def pipe(*, duplex=False, overlapped=(True, True), bufsize=BUFSIZE):
 
     h1 = h2 = None
     try:
-        h1 = _winapi.CreateNamedPipe(
-            address, openmode, _winapi.PIPE_WAIT,
-            1, obsize, ibsize, _winapi.NMPWAIT_WAIT_FOREVER, _winapi.NULL)
+        for attempts in itertools.count():
+            address = r'\\.\pipe\python-pipe-{:d}-{:d}-{}'.format(
+                os.getpid(), next(_mmap_counter), os.urandom(8).hex())
+            try:
+                h1 = _winapi.CreateNamedPipe(
+                    address, openmode, _winapi.PIPE_WAIT,
+                    1, obsize, ibsize, _winapi.NMPWAIT_WAIT_FOREVER, _winapi.NULL)
+                break
+            except OSError as e:
+                if attempts >= _MAX_PIPE_ATTEMPTS:
+                    raise
+                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

@@ -46,6 +46,7 @@ BUFSIZE = 64 * 1024
 CONNECTION_TIMEOUT = 20.
 
 _mmap_counter = itertools.count()
+_MAX_PIPE_ATTEMPTS = 100
 
 default_family = 'AF_INET'
 families = ['AF_INET']
@@ -78,8 +79,8 @@ 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 (r'\\.\pipe\pyc-%d-%d-%s' %
+                (os.getpid(), next(_mmap_counter), os.urandom(8).hex()))
     else:
         raise ValueError('unrecognized family')
 
@@ -472,17 +473,29 @@ 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 family == 'AF_PIPE':
-            self._listener = PipeListener(address, backlog)
-        else:
-            self._listener = SocketListener(address, family, backlog)
-
         if authkey is not None and not isinstance(authkey, bytes):
             raise TypeError('authkey should be a byte string')
 
+        if family == 'AF_PIPE':
+            if address:
+                self._listener = PipeListener(address, backlog)
+            else:
+                for attempts in itertools.count():
+                    address = arbitrary_address(family)
+                    try:
+                        self._listener = PipeListener(address, backlog)
+                        break
+                    except OSError as e:
+                        if attempts >= _MAX_PIPE_ATTEMPTS:
+                            raise
+                        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)
+
         self._authkey = authkey
 
     def accept(self):
@@ -570,7 +583,6 @@ else:
         '''
         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 +592,25 @@ else:
             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
-            )
+        for attempts in itertools.count():
+            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 attempts >= _MAX_PIPE_ATTEMPTS:
+                    raise
+                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:`multiprocessing` and :mod:`asyncio` on Windows, no matter how small.