[3.14] gh-109263: Start process from spawn context in multiprocessing no longer have side effect (GH-135813) (#143115)

miss-islington · aisk · web-flow · commit 64461f1ca563 · 2025-12-24T03:04:46.000Z
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>
diff --git a/Lib/multiprocessing/spawn.py b/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
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
@@ -5845,6 +5845,25 @@ def test_context(self):
             self.assertRaises(ValueError, ctx.set_start_method, None)
             self.check_context(ctx)
 
+    @staticmethod
+    def _dummy_func():
+        pass
+
+    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')
diff --git a/Misc/NEWS.d/next/Library/2025-06-22-18-57-19.gh-issue-109263.f92V95.rst b/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.

Original file line number	Diff line number	Diff line change
`@@ -184,7 +184,7 @@ def get_preparation_data(name):`
`184`	`184`	`sys_argv=sys.argv,`
`185`	`185`	`orig_dir=process.ORIGINAL_DIR,`
`186`	`186`	`dir=os.getcwd(),`
`187`		`- start_method=get_start_method(),`
	`187`	`+ start_method=get_start_method(allow_none=True),`
`188`	`188`	`)`
`189`	`189`
`190`	`190`	`# Figure out whether to initialise main in the subprocess as a module`
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Starting a process from spawn context in :mod:`multiprocessing` no longer
	`2`	`+sets the start method globally.`