Fix macOS fork-safety crash: default to spawn on darwin

billiard/context.py

@@ -368,7 +368,14 @@ def _check_available(self):
         'spawn': SpawnContext(),
         'forkserver': ForkServerContext(),
     }
-    _default_context = DefaultContext(_concrete_contexts['fork'])
+    if sys.platform == 'darwin':
+        # bpo-33725: running arbitrary code after fork() is no longer
+        # reliable on macOS since macOS 10.14 (Mojave). Use spawn by
+        # default instead.
+        # See https://github.com/celery/celery/issues/9894
+        _default_context = DefaultContext(_concrete_contexts['spawn'])
+    else:
+        _default_context = DefaultContext(_concrete_contexts['fork'])
 
 else:
 

t/unit/test_context.py

@@ -0,0 +1,55 @@
+import subprocess
+import sys
+
+
+def _get_default_context_attr(platform, attr):
+    """Run a subprocess that patches sys.platform before importing billiard.
+
+    This exercises the actual import-time conditional without corrupting
+    the module state of the current process.
+    """
+    result = subprocess.run(
+        [sys.executable, '-c',
+         'import unittest.mock; '
+         f'unittest.mock.patch("sys.platform", {platform!r}).start(); '
+         'import billiard.context; '
+         f'print(getattr(billiard.context._default_context, {attr!r})())'],
+        capture_output=True, text=True,
+    )
+    assert result.returncode == 0, result.stderr
+    return result.stdout.strip()
+
+
+class test_default_context_darwin:
+    """Tests that macOS defaults to spawn, matching CPython (bpo-33725)."""
+
+    def test_default_start_method_is_spawn_on_darwin(self):
+        method = _get_default_context_attr('darwin', 'get_start_method')
+        assert method == 'spawn'
+
+    def test_default_start_method_is_fork_on_linux(self):
+        method = _get_default_context_attr('linux', 'get_start_method')
+        assert method == 'fork'
+
+    def test_set_start_method_override_on_darwin(self):
+        result = subprocess.run(
+            [sys.executable, '-c',
+             'import unittest.mock; '
+             'unittest.mock.patch("sys.platform", "darwin").start(); '
+             'import billiard.context; '
+             'ctx = billiard.context._default_context; '
+             'assert ctx.get_start_method() == "spawn"; '
+             'ctx.set_start_method("fork", force=True); '
+             'print(ctx.get_start_method())'],
+            capture_output=True, text=True,
+        )
+        assert result.returncode == 0, result.stderr
+        assert result.stdout.strip() == 'fork'
+
+    def test_forking_is_enabled_false_on_darwin(self):
+        enabled = _get_default_context_attr('darwin', 'forking_is_enabled')
+        assert enabled == 'False'
+
+    def test_forking_is_enabled_true_on_linux(self):
+        enabled = _get_default_context_attr('linux', 'forking_is_enabled')
+        assert enabled == 'True'