GH-84559: Deprecate fork being the multiprocessing default. (#100618)

This starts the process. Users who don't specify their own start method
and use the default on platforms where it is 'fork' will see a
DeprecationWarning upon multiprocessing.Pool() construction or upon
multiprocessing.Process.start() or concurrent.futures.ProcessPool use.

See the related issue and documentation within this change for details.

1 files changed, 19 insertions, 0 deletions

diff --git a/Lib/test/test_concurrent_futures.py b/Lib/test/test_concurrent_futures.py
index b3520ae3994..4493cd31252 100644
--- a/Lib/test/test_concurrent_futures.py
+++ b/Lib/test/test_concurrent_futures.py
@@ -18,6 +18,7 @@ import sys
 import threading
 import time
 import unittest
+import warnings
 import weakref
 from pickle import PicklingError
 
@@ -571,6 +572,24 @@ class ProcessPoolShutdownTest(ExecutorShutdownTest):
         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,