[3.9] bpo-39244: multiprocessing return default start method first on macOS (GH-18625)

Lib/multiprocessing/context.py

@@ -257,10 +257,11 @@ def get_all_start_methods(self):
         if sys.platform == 'win32':
             return ['spawn']
         else:
+            methods = ['spawn', 'fork'] if sys.platform == 'darwin' else ['fork', 'spawn']
             if reduction.HAVE_SEND_HANDLE:
-                return ['fork', 'spawn', 'forkserver']
-            else:
-                return ['fork', 'spawn']
+                methods.append('forkserver')
+            return methods
+
 
 #
 # Context types for fixed start method

Lib/test/_test_multiprocessing.py

@@ -5039,7 +5039,9 @@ def test_get_all(self):
             self.assertEqual(methods, ['spawn'])
         else:
             self.assertTrue(methods == ['fork', 'spawn'] or
-                            methods == ['fork', 'spawn', 'forkserver'])
+                            methods == ['spawn', 'fork'] or
+                            methods == ['fork', 'spawn', 'forkserver'] or
+                            methods == ['spawn', 'fork', 'forkserver'])
 
     def test_preload_resources(self):
         if multiprocessing.get_start_method() != 'forkserver':

Misc/NEWS.d/next/Library/2020-02-23-15-09-47.bpo-39244.aBK5IM.rst

@@ -0,0 +1,2 @@
+Fixed :class:`multiprocessing.context.get_all_start_methods`
+to properly return the default method first on macOS.