gh-125714: Doc: Explain that multiprocessing only supports unpicklable functions through the fork method

Doc/library/concurrent.futures.rst

@@ -412,7 +412,8 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
    .. versionchanged:: 3.14
       The default process start method (see
       :ref:`multiprocessing-start-methods`) changed away from *fork*. If you
-      require the *fork* start method for :class:`ProcessPoolExecutor` you must
+      require the *fork* start method for :class:`ProcessPoolExecutor` or need to run
+      :ref:`unpicklable functions <pickle-picklable>`, you must
       explicitly pass ``mp_context=multiprocessing.get_context("fork")``.
 
 .. _processpoolexecutor-example:

Doc/library/multiprocessing.rst

@@ -115,6 +115,8 @@ to start a process.  These *start methods* are
     will not be inherited.  Starting a process using this method is
     rather slow compared to using *fork* or *forkserver*.
 
+    Functions run with this method must be :ref:`picklable <pickle-picklable>`.
+
     Available on POSIX and Windows platforms.  The default on Windows and macOS.
 
   *fork*
@@ -124,6 +126,9 @@ to start a process.  These *start methods* are
     inherited by the child process.  Note that safely forking a
     multithreaded process is problematic.
 
+    Functions run with this method do *not* need to be
+    :ref:`picklable <pickle-picklable>`.
+
     Available on POSIX systems.
 
     .. versionchanged:: 3.14
@@ -146,6 +151,8 @@ to start a process.  These *start methods* are
     side-effect so it is generally safe for it to use :func:`os.fork`.
     No unnecessary resources are inherited.
 
+    Functions run with this method must be :ref:`picklable <pickle-picklable>`.
+
     Available on POSIX platforms which support passing file descriptors over
     Unix pipes such as Linux.  The default on those.
 

Doc/whatsnew/3.14.rst

@@ -487,14 +487,6 @@ Deprecated
   arguments are parsed.
   (Contributed by Serhiy Storchaka in :gh:`58032`.)
 
-* :mod:`multiprocessing` and :mod:`concurrent.futures`:
-  The default start method (see :ref:`multiprocessing-start-methods`) changed
-  away from *fork* to *forkserver* on platforms where it was not already
-  *spawn* (Windows & macOS). If you require the threading incompatible *fork*
-  start method you must explicitly specify it when using :mod:`multiprocessing`
-  or :mod:`concurrent.futures` APIs.
-  (Contributed by Gregory P. Smith in :gh:`84559`.)
-
 * :mod:`os`:
   :term:`Soft deprecate <soft deprecated>` :func:`os.popen` and
   :func:`os.spawn* <os.spawnl>` functions. They should no longer be used to
@@ -661,6 +653,8 @@ Others
   :meth:`~object.__index__`. (Contributed by Mark Dickinson in :gh:`119743`.)
 
 
+.. _whatsnew314-porting-to-python314:
+
 Porting to Python 3.14
 ======================
 
@@ -679,6 +673,19 @@ Changes in the Python API
   This temporary change affects other threads.
   (Contributed by Serhiy Storchaka in :gh:`69998`.)
 
+* :mod:`multiprocessing` and :mod:`concurrent.futures`:
+  The default start method (see :ref:`multiprocessing-start-methods`) changed
+  away from *fork* to *forkserver* on POSIX platforms which support passing file
+  descriptors over Unix pipes, such as Linux. Windows and macOS keep *spawn*
+  as their default method.
+  If you require the threading-incompatible *fork* start method,
+  which can run :ref:`unpicklable functions <pickle-picklable>`,
+  you must explicitly specify it when using :mod:`multiprocessing`
+  or :mod:`concurrent.futures` APIs.
+  Code running picklable functions on these platforms without explicitly opting
+  into the *fork* method will now raise :exc:`pickle.PicklingError`.
+  (Contributed by Gregory P. Smith in :gh:`84559`.)
+
 
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