pythongh-84559: Remove the new multiprocessing warning, too disruptiv…

…e. (python#101551)

This reverts the core of python#100618 while leaving relevant documentation
improvements and minor refactorings in place.

Doc/library/concurrent.futures.rst

@@ -281,18 +281,18 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
 
       Added the *initializer* and *initargs* arguments.
 
+      .. note::
+         The default :mod:`multiprocessing` start method
+         (see :ref:`multiprocessing-start-methods`) will change away from
+         *fork* in Python 3.14.  Code that requires *fork* be used for their
+         :class:`ProcessPoolExecutor` should explicitly specify that by
+         passing a ``mp_context=multiprocessing.get_context("fork")``
+         parameter.
+
    .. versionchanged:: 3.11
       The *max_tasks_per_child* argument was added to allow users to
       control the lifetime of workers in the pool.
 
-   .. versionchanged:: 3.12
-      The implicit use of the :mod:`multiprocessing` *fork* start method as a
-      platform default (see :ref:`multiprocessing-start-methods`) now raises a
-      :exc:`DeprecationWarning`. The default will change in Python 3.14.
-      Code that requires *fork* should explicitly specify that when creating
-      their :class:`ProcessPoolExecutor` by passing a
-      ``mp_context=multiprocessing.get_context('fork')`` parameter.
-
 .. _processpoolexecutor-example:
 
 ProcessPoolExecutor Example

Doc/library/multiprocessing.rst

@@ -126,6 +126,11 @@ to start a process.  These *start methods* are
 
     Available on POSIX systems.  Currently the default on POSIX except macOS.
 
+    .. note::
+       The default start method will change away from *fork* in Python 3.14.
+       Code that requires *fork* should explicitly specify that via
+       :func:`get_context` or :func:`set_start_method`.
+
   *forkserver*
     When the program starts and selects the *forkserver* start method,
     a server process is spawned.  From then on, whenever a new process
@@ -138,11 +143,6 @@ to start a process.  These *start methods* are
     Available on POSIX platforms which support passing file descriptors
     over Unix pipes such as Linux.
 
-.. versionchanged:: 3.12
-   Implicit use of the *fork* start method as the default now raises a
-   :exc:`DeprecationWarning`. Code that requires it should explicitly
-   specify *fork* via :func:`get_context` or :func:`set_start_method`.
-   The default will change away from *fork* in 3.14.
 
 .. versionchanged:: 3.8
 
@@ -1107,6 +1107,7 @@ Miscellaneous
    launched (before creating a :class:`Pool` or starting a :class:`Process`).
 
    Only meaningful when using the ``'forkserver'`` start method.
+   See :ref:`multiprocessing-start-methods`.
 
    .. versionadded:: 3.4
 

Doc/whatsnew/3.12.rst

@@ -440,12 +440,6 @@ Deprecated
   warning at compile time. This field will be removed in Python 3.14.
   (Contributed by Ramvikrams and Kumar Aditya in :gh:`101193`. PEP by Ken Jin.)
 
-* Use of the implicit default ``'fork'`` start method for
-  :mod:`multiprocessing` and :class:`concurrent.futures.ProcessPoolExecutor`
-  now emits a :exc:`DeprecationWarning` on Linux and other non-macOS POSIX
-  systems. Avoid this by explicitly specifying a start method.
-  See :ref:`multiprocessing-start-methods`.
-
 Pending Removal in Python 3.13
 ------------------------------
 
@@ -510,9 +504,13 @@ Pending Removal in Python 3.14
 * Testing the truth value of an :class:`xml.etree.ElementTree.Element`
   is deprecated and will raise an exception in Python 3.14.
 
-* The default :mod:`multiprocessing` start method will change to one of either
-  ``'forkserver'`` or ``'spawn'`` on all platforms for which ``'fork'`` remains
-  the default per :gh:`84559`.
+* The default :mod:`multiprocessing` start method will change to a safer one on
+  Linux, BSDs, and other non-macOS POSIX platforms where ``'fork'`` is currently
+  the default (:gh:`84559`). Adding a runtime warning about this was deemed too
+  disruptive as the majority of code is not expected to care. Use the
+  :func:`~multiprocessing.get_context` or
+  :func:`~multiprocessing.set_start_method` APIs to explicitly specify when
+  your code *requires* ``'fork'``.  See :ref:`multiprocessing-start-methods`.
 
 Pending Removal in Future Versions
 ----------------------------------

Lib/concurrent/futures/process.py

@@ -57,7 +57,6 @@
 import itertools
 import sys
 from traceback import format_exception
-import warnings
 
 
 _threads_wakeups = weakref.WeakKeyDictionary()
@@ -651,22 +650,6 @@ def __init__(self, max_workers=None, mp_context=None,
                 mp_context = mp.get_context("spawn")
             else:
                 mp_context = mp.get_context()
-        if (mp_context.get_start_method() == "fork" and
-            mp_context == mp.context._default_context._default_context):
-            warnings.warn(
-                "The default multiprocessing start method will change "
-                "away from 'fork' in Python >= 3.14, per GH-84559. "
-                "ProcessPoolExecutor uses multiprocessing. "
-                "If your application requires the 'fork' multiprocessing "
-                "start method, explicitly specify that by passing a "
-                "mp_context= parameter. "
-                "The safest start method is 'spawn'.",
-                category=mp.context.DefaultForkDeprecationWarning,
-                stacklevel=2,
-            )
-            # Avoid the equivalent warning from multiprocessing itself via
-            # a non-default fork context.
-            mp_context = mp.get_context("fork")
         self._mp_context = mp_context
 
         # https://github.com/python/cpython/issues/90622

Lib/multiprocessing/context.py

@@ -23,9 +23,6 @@ class TimeoutError(ProcessError):
 class AuthenticationError(ProcessError):
     pass
 
-class DefaultForkDeprecationWarning(DeprecationWarning):
-    pass
-
 #
 # Base type for contexts. Bound methods of an instance of this type are included in __all__ of __init__.py
 #
@@ -284,23 +281,6 @@ def _Popen(process_obj):
             from .popen_fork import Popen
             return Popen(process_obj)
 
-    _warn_package_prefixes = (os.path.dirname(__file__),)
-
-    class _DeprecatedForkProcess(ForkProcess):
-        @classmethod
-        def _Popen(cls, process_obj):
-            import warnings
-            warnings.warn(
-                "The default multiprocessing start method will change "
-                "away from 'fork' in Python >= 3.14, per GH-84559. "
-                "Use multiprocessing.get_context(X) or .set_start_method(X) to "
-                "explicitly specify it when your application requires 'fork'. "
-                "The safest start method is 'spawn'.",
-                category=DefaultForkDeprecationWarning,
-                skip_file_prefixes=_warn_package_prefixes,
-            )
-            return super()._Popen(process_obj)
-
     class SpawnProcess(process.BaseProcess):
         _start_method = 'spawn'
         @staticmethod
@@ -324,9 +304,6 @@ class ForkContext(BaseContext):
         _name = 'fork'
         Process = ForkProcess
 
-    class _DefaultForkContext(ForkContext):
-        Process = _DeprecatedForkProcess
-
     class SpawnContext(BaseContext):
         _name = 'spawn'
         Process = SpawnProcess
@@ -342,16 +319,13 @@ def _check_available(self):
         'fork': ForkContext(),
         'spawn': SpawnContext(),
         'forkserver': ForkServerContext(),
-        # Remove None and _DefaultForkContext() when changing the default
-        # in 3.14 for https://github.com/python/cpython/issues/84559.
-        None: _DefaultForkContext(),
     }
     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'])
     else:
-        _default_context = DefaultContext(_concrete_contexts[None])
+        _default_context = DefaultContext(_concrete_contexts['fork'])
 
 else:
 

Lib/test/_test_multiprocessing.py

@@ -4098,10 +4098,9 @@ def test_shared_memory_SharedMemoryServer_ignores_sigint(self):
     def test_shared_memory_SharedMemoryManager_reuses_resource_tracker(self):
         # bpo-36867: test that a SharedMemoryManager uses the
         # same resource_tracker process as its parent.
-        cmd = f'''if 1:
+        cmd = '''if 1:
             from multiprocessing.managers import SharedMemoryManager
-            from multiprocessing import set_start_method
-            set_start_method({multiprocessing.get_start_method()!r})
+
 
             smm = SharedMemoryManager()
             smm.start()

Lib/test/test_concurrent_futures.py

@@ -18,7 +18,6 @@
 import threading
 import time
 import unittest
-import warnings
 import weakref
 from pickle import PicklingError
 
@@ -572,24 +571,6 @@ def test_shutdown_no_wait(self):
         assert all([r == abs(v) for r, v in zip(res, range(-5, 5))])
 
 
-@unittest.skipIf(mp.get_all_start_methods()[0] != "fork", "non-fork default.")
-class ProcessPoolExecutorDefaultForkWarning(unittest.TestCase):
-    def test_fork_default_warns(self):
-        with self.assertWarns(mp.context.DefaultForkDeprecationWarning):
-            with futures.ProcessPoolExecutor(2):
-                pass
-
-    def test_explicit_fork_does_not_warn(self):
-        with warnings.catch_warnings(record=True) as ws:
-            warnings.simplefilter("ignore")
-            warnings.filterwarnings(
-                'always', category=mp.context.DefaultForkDeprecationWarning)
-            ctx = mp.get_context("fork")  # Non-default fork context.
-            with futures.ProcessPoolExecutor(2, mp_context=ctx):
-                pass
-        self.assertEqual(len(ws), 0, msg=[str(x) for x in ws])
-
-
 create_executor_tests(ProcessPoolShutdownTest,
                       executor_mixins=(ProcessPoolForkMixin,
                                        ProcessPoolForkserverMixin,

Lib/test/test_logging.py

@@ -4759,9 +4759,8 @@ def test_multiprocessing(self):
             # In other processes, processName is correct when multiprocessing in imported,
             # but it is (incorrectly) defaulted to 'MainProcess' otherwise (bpo-38762).
             import multiprocessing
-            mp = multiprocessing.get_context('spawn')
-            parent_conn, child_conn = mp.Pipe()
-            p = mp.Process(
+            parent_conn, child_conn = multiprocessing.Pipe()
+            p = multiprocessing.Process(
                 target=self._extract_logrecord_process_name,
                 args=(2, LOG_MULTI_PROCESSING, child_conn,)
             )

Lib/test/test_re.py

@@ -2431,8 +2431,7 @@ def test_regression_gh94675(self):
         input_js = '''a(function() {
             ///////////////////////////////////////////////////////////////////
         });'''
-        mp = multiprocessing.get_context('spawn')
-        p = mp.Process(target=pattern.sub, args=('', input_js))
+        p = multiprocessing.Process(target=pattern.sub, args=('', input_js))
         p.start()
         p.join(SHORT_TIMEOUT)
         try: