gh-109263: Start process from spawn context in multiprocessing no longer have side effect (GH-135813)

(cherry picked from commit c2202a7)

Co-authored-by: AN Long <aisk@users.noreply.github.com>

Author: aisk

Lib/multiprocessing/spawn.py

@@ -184,7 +184,7 @@ def get_preparation_data(name):
         sys_argv=sys.argv,
         orig_dir=process.ORIGINAL_DIR,
         dir=os.getcwd(),
-        start_method=get_start_method(),
+        start_method=get_start_method(allow_none=True),
         )
 
     # Figure out whether to initialise main in the subprocess as a module

Lib/test/_test_multiprocessing.py

@@ -5689,6 +5689,26 @@ def test_context(self):
             self.assertRaises(ValueError, ctx.set_start_method, None)
             self.check_context(ctx)
 
+    @staticmethod
+    def _dummy_func():
+        pass
+
+    @warnings_helper.ignore_fork_in_thread_deprecation_warnings()
+    def test_spawn_dont_set_context(self):
+        # Run a process with spawn or forkserver context may change
+        # the global start method, see gh-109263.
+        for method in ('fork', 'spawn', 'forkserver'):
+            multiprocessing.set_start_method(None, force=True)
+
+            try:
+                ctx = multiprocessing.get_context(method)
+            except ValueError:
+                continue
+            process = ctx.Process(target=self._dummy_func)
+            process.start()
+            process.join()
+            self.assertIsNone(multiprocessing.get_start_method(allow_none=True))
+
     def test_context_check_module_types(self):
         try:
             ctx = multiprocessing.get_context('forkserver')

Misc/NEWS.d/next/Library/2025-06-22-18-57-19.gh-issue-109263.f92V95.rst

@@ -0,0 +1,2 @@
+Starting a process from spawn context in :mod:`multiprocessing` no longer
+sets the start method globally.