youknowone · Jun 1, 2026 · May 29, 2026 · May 29, 2026 · May 29, 2026 · May 29, 2026
diff --git a/Lib/multiprocessing/connection.py b/Lib/multiprocessing/connection.py
@@ -11,13 +11,12 @@
 
 import errno
 import io
+import itertools
 import os
 import sys
 import socket
 import struct
 import time
-import tempfile
-import itertools
 
 
 from . import util
@@ -39,11 +38,14 @@
 #
 #
 
-BUFSIZE = 8192
+# 64 KiB is the default PIPE buffer size of most POSIX platforms.
+BUFSIZE = 64 * 1024
+
 # A very generous timeout when it comes to local connections...
 CONNECTION_TIMEOUT = 20.
 
 _mmap_counter = itertools.count()
+_MAX_PIPE_ATTEMPTS = 100
 
 default_family = 'AF_INET'
 families = ['AF_INET']
@@ -74,10 +76,14 @@ def arbitrary_address(family):
     if family == 'AF_INET':
         return ('localhost', 0)
     elif family == 'AF_UNIX':
-        return tempfile.mktemp(prefix='sock-', dir=util.get_temp_dir())
+        # NOTE: util.get_temp_dir() is a 0o700 per-process directory. A
+        # mktemp-style ToC vs ToU concern is not important; bind() surfaces
+        # the extremely unlikely collision as EADDRINUSE.
+        return os.path.join(util.get_temp_dir(),
+                            f'sock-{os.urandom(6).hex()}')
     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')
 
@@ -179,6 +185,10 @@ def close(self):
             finally:
                 self._handle = None
 
+    def _detach(self):
+        """Stop managing the underlying file descriptor or handle."""
+        self._handle = None
+
     def send_bytes(self, buf, offset=0, size=None):
         """Send the bytes data from a bytes-like object"""
         self._check_closed()
@@ -316,22 +326,32 @@ def _recv_bytes(self, maxsize=None):
                 try:
                     ov, err = _winapi.ReadFile(self._handle, bsize,
                                                 overlapped=True)
+
+                    sentinel = object()
+                    return_value = sentinel
                     try:
-                        if err == _winapi.ERROR_IO_PENDING:
-                            waitres = _winapi.WaitForMultipleObjects(
-                                [ov.event], False, INFINITE)
-                            assert waitres == WAIT_OBJECT_0
+                        try:
+                            if err == _winapi.ERROR_IO_PENDING:
+                                waitres = _winapi.WaitForMultipleObjects(
+                                    [ov.event], False, INFINITE)
+                                assert waitres == WAIT_OBJECT_0
+                        except:
+                            ov.cancel()
+                            raise
+                        finally:
+                            nread, err = ov.GetOverlappedResult(True)
+                            if err == 0:
+                                f = io.BytesIO()
+                                f.write(ov.getbuffer())
+                                return_value = f
+                            elif err == _winapi.ERROR_MORE_DATA:
+                                return_value = self._get_more_data(ov, maxsize)
                     except:
-                        ov.cancel()
-                        raise
-                    finally:
-                        nread, err = ov.GetOverlappedResult(True)
-                        if err == 0:
-                            f = io.BytesIO()
-                            f.write(ov.getbuffer())
-                            return f
-                        elif err == _winapi.ERROR_MORE_DATA:
-                            return self._get_more_data(ov, maxsize)
+                        if return_value is sentinel:
+                            raise
+
+                    if return_value is not sentinel:
+                        return return_value
                 except OSError as e:
                     if e.winerror == _winapi.ERROR_BROKEN_PIPE:
                         raise EOFError
@@ -392,7 +412,8 @@ def _recv(self, size, read=_read):
         handle = self._handle
         remaining = size
         while remaining > 0:
-            chunk = read(handle, remaining)
+            to_read = min(BUFSIZE, remaining)
+            chunk = read(handle, to_read)
             n = len(chunk)
             if n == 0:
                 if remaining == size:
@@ -455,17 +476,29 @@ 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 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:
+                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)
 
-        if authkey is not None and not isinstance(authkey, bytes):
-            raise TypeError('authkey should be a byte string')
-
         self._authkey = authkey
 
     def accept(self):
@@ -553,7 +586,6 @@ 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
@@ -563,15 +595,25 @@ 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
-            )
+        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

diff --git a/Lib/multiprocessing/context.py b/Lib/multiprocessing/context.py
@@ -145,7 +145,13 @@ def freeze_support(self):
         '''Check whether this is a fake forked process in a frozen executable.
         If so then run code specified by commandline and exit.
         '''
-        if self.get_start_method() == 'spawn' and getattr(sys, 'frozen', False):
+        # gh-140814: allow_none=True avoids locking in the default start
+        # method, which would cause a later set_start_method() to fail.
+        # None is safe to pass through: spawn.freeze_support()
+        # independently detects whether this process is a spawned
+        # child, so the start method check here is only an optimization.
+        if (getattr(sys, 'frozen', False)
+                and self.get_start_method(allow_none=True) in ('spawn', None)):
             from .spawn import freeze_support
             freeze_support()
 
@@ -167,7 +173,7 @@ def allow_connection_pickling(self):
         '''
         # This is undocumented.  In previous versions of multiprocessing
         # its only effect was to make socket objects inheritable on Windows.
-        from . import connection
+        from . import connection  # noqa: F401
 
     def set_executable(self, executable):
         '''Sets the path to a python.exe or pythonw.exe binary used to run
@@ -259,13 +265,12 @@ def get_start_method(self, allow_none=False):
 
     def get_all_start_methods(self):
         """Returns a list of the supported start methods, default first."""
-        if sys.platform == 'win32':
-            return ['spawn']
-        else:
-            methods = ['spawn', 'fork'] if sys.platform == 'darwin' else ['fork', 'spawn']
-            if reduction.HAVE_SEND_HANDLE:
-                methods.append('forkserver')
-            return methods
+        default = self._default_context.get_start_method()
+        start_method_names = [default]
+        start_method_names.extend(
+            name for name in _concrete_contexts if name != default
+        )
+        return start_method_names
 
 
 #
@@ -320,14 +325,15 @@ def _check_available(self):
         'spawn': SpawnContext(),
         'forkserver': ForkServerContext(),
     }
-    if sys.platform == 'darwin':
-        # bpo-33725: running arbitrary code after fork() is no longer reliable
-        # on macOS since macOS 10.14 (Mojave). Use spawn by default instead.
-        _default_context = DefaultContext(_concrete_contexts['spawn'])
+    # bpo-33725: running arbitrary code after fork() is no longer reliable
+    # on macOS since macOS 10.14 (Mojave). Use spawn by default instead.
+    # gh-84559: We changed everyones default to a thread safeish one in 3.14.
+    if reduction.HAVE_SEND_HANDLE and sys.platform != 'darwin':
+        _default_context = DefaultContext(_concrete_contexts['forkserver'])
     else:
-        _default_context = DefaultContext(_concrete_contexts['fork'])
+        _default_context = DefaultContext(_concrete_contexts['spawn'])
 
-else:
+else:  # Windows
 
     class SpawnProcess(process.BaseProcess):
         _start_method = 'spawn'

diff --git a/Lib/multiprocessing/dummy/__init__.py b/Lib/multiprocessing/dummy/__init__.py
@@ -33,7 +33,7 @@
 
 class DummyProcess(threading.Thread):
 
-    def __init__(self, group=None, target=None, name=None, args=(), kwargs={}):
+    def __init__(self, group=None, target=None, name=None, args=(), kwargs=None):
         threading.Thread.__init__(self, group, target, name, args, kwargs)
         self._pid = None
         self._children = weakref.WeakKeyDictionary()