gh-140814: Fix freeze_support() setting start method as side effect

freeze_support() called get_start_method() without allow_none=True,
which locked in the default start method context. This caused a
subsequent set_start_method() call to raise "context has already been
set". Use allow_none=True and accept None as a matching value, since
spawn.freeze_support() independently detects spawned child processes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gpshead

Lib/multiprocessing/context.py

@@ -145,7 +145,13 @@ def freeze_support(self):
         '''Check whether this is a fake forked process in a frozen executable.
         If so then run code specified by commandline and exit.
         '''
-        if self.get_start_method() == 'spawn' and getattr(sys, 'frozen', False):
+        # gh-140814: allow_none=True avoids locking in the default start
+        # method, which would cause a later set_start_method() to fail.
+        # None is safe to pass through: spawn.freeze_support()
+        # independently detects whether this process is a spawned
+        # child, so the start method check here is only an optimization.
+        if (getattr(sys, 'frozen', False)
+                and self.get_start_method(allow_none=True) in ('spawn', None)):
             from .spawn import freeze_support
             freeze_support()
 

Misc/NEWS.d/next/Library/2026-02-08-22-04-06.gh-issue-140814.frzSpn.rst

@@ -0,0 +1,3 @@
+:func:`multiprocessing.freeze_support` no longer sets the default start method
+as a side effect, which previously caused a subsequent
+:func:`multiprocessing.set_start_method` call to raise ``RuntimeError``.