bpo-33725: multiprocessing uses spawn by default on macOS (GH-13603)

On macOS, the multiprocessing module now uses the "spawn" start
method by default.

Doc/library/multiprocessing.rst

@@ -102,7 +102,7 @@ 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*.
 
-    Available on Unix and Windows.  The default on Windows.
+    Available on Unix and Windows.  The default on Windows and macOS.
 
   *fork*
     The parent process uses :func:`os.fork` to fork the Python
@@ -124,6 +124,11 @@ to start a process.  These *start methods* are
     Available on Unix platforms which support passing file descriptors
     over Unix pipes.
 
+.. versionchanged:: 3.8
+
+   On macOS, *spawn* start method is now the default: *fork* start method is no
+   longer reliable on macOS, see :issue:`33725`.
+
 .. versionchanged:: 3.4
    *spawn* added on all unix platforms, and *forkserver* added for
    some unix platforms.

Doc/whatsnew/3.8.rst

@@ -469,6 +469,16 @@ access the ``madvise()`` system call.
 (Contributed by Zackery Spytz in :issue:`32941`.)
 
 
+multiprocessing
+---------------
+
+Added new :mod:`multiprocessing.shared_memory` module.
+(Contributed Davin Potts in :issue:`35813`.)
+
+On macOS, the *spawn* start method is now used by default.
+(Contributed by Victor Stinner in :issue:`33725`.)
+
+
 os
 --
 

Lib/multiprocessing/context.py

@@ -309,7 +309,12 @@ if sys.platform != 'win32':
         'spawn': SpawnContext(),
         'forkserver': ForkServerContext(),
     }
-    _default_context = DefaultContext(_concrete_contexts['fork'])
+    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['fork'])
 
 else:
 

Misc/NEWS.d/next/Library/2019-05-28-01-17-42.bpo-33725.fFZoDG.rst

@@ -0,0 +1,2 @@
+On macOS, the :mod:`multiprocessing` module now uses *spawn* start method by
+default.