gh-84559: Change the default multiprocessing start method.

We drop 'fork' in favor of 'forkserver' or 'spawn'. See the issue for details.
python · gpshead · Sep 26, 2024 · Feb 4, 2023 · Sep 19, 2024 · Sep 19, 2024
commit c5277b7549f6451cb0de79ac7d60770ad3deb67c
@@ -259,13 +259,12 @@ 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 += (
+                name for name in _concrete_contexts if name != default
+        )
+        return start_method_names
 
 
 #
@@ -325,7 +324,11 @@ def _check_available(self):
         # 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'])
+        # gh-84559: We changed the default to a thread safe one in 3.14.
+        if reduction.HAVE_SEND_HANDLE:
+            _default_context = DefaultContext(_concrete_contexts['forkserver'])
+        else:
+            _default_context = DefaultContext(_concrete_contexts['spawn'])
 
 else:
 

@@ -5261,15 +5261,26 @@ def test_set_get(self):
             multiprocessing.set_start_method(old_method, force=True)
         self.assertGreaterEqual(count, 1)
 
-    def test_get_all(self):
+    def test_get_all_start_methods(self):
         methods = multiprocessing.get_all_start_methods()
+        self.assertIn('spawn', methods)
         if sys.platform == 'win32':
             self.assertEqual(methods, ['spawn'])
+        if sys.platform == 'darwin':
+            self.assertEqual(methods[0], 'spawn')  # The default is first.
+            # Whether these work or not, they remain available on macOS.
+            self.assertIn('fork', methods)
+            self.assertIn('forkserver', methods)
         else:
-            self.assertTrue(methods == ['fork', 'spawn'] or
-                            methods == ['spawn', 'fork'] or
-                            methods == ['fork', 'spawn', 'forkserver'] or
-                            methods == ['spawn', 'fork', 'forkserver'])
+            # POSIX
+            self.assertIn('fork', methods)
+            if other_methods := set(methods) - {'fork', 'spawn'}:
+                self.assertEqual({'forkserver'}, other_methods)
+            # >=3.14 Defaults to forkserver if the platform supports it.
+            self.assertIn(methods[0], {'forkserver', 'spawn'},
+                          msg='3.14+ default must not be fork')
+            if methods[0] == 'spawn':
+                self.assertNotIn('forkserver', methods)
 
     def test_preload_resources(self):
         if multiprocessing.get_start_method() != 'forkserver':

diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
@@ -2161,7 +2161,13 @@ def skip_if_broken_multiprocessing_synchronize():
             # bpo-38377: On Linux, creating a semaphore fails with OSError
             # if the current user does not have the permission to create
             # a file in /dev/shm/ directory.
-            synchronize.Lock(ctx=None)
+            import multiprocessing
+            synchronize.Lock(ctx=multiprocessing.get_context('fork'))
+            # The explicit fork mp context is required as relying on the
+            # default breaks TestResourceTracker.test_resource_tracker_reused
+            # when the default start method is not fork as synchronize creates
+            # a new multiprocessing.resource_tracker process at module import
+            # time via the aboe call in that scenario. This enables gh-84559.
         except OSError as exc:
             raise unittest.SkipTest(f"broken multiprocessing SemLock: {exc!r}")