Issue #18418: After fork(), reinit all threads states, not only active ones.

Patch by A. Jesse Jiryu Davis.

2 files changed, 22 insertions, 1 deletions

diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 3629749..0a78f8c 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -445,6 +445,27 @@ class ThreadTests(BaseTestCase):
         self.assertEqual(out, b'')
         self.assertEqual(err, b'')
 
+    @unittest.skipUnless(hasattr(os, 'fork'), "needs os.fork()")
+    def test_is_alive_after_fork(self):
+        # Try hard to trigger #18418: is_alive() could sometimes be True on
+        # threads that vanished after a fork.
+        old_interval = sys.getswitchinterval()
+        self.addCleanup(sys.setswitchinterval, old_interval)
+
+        # Make the bug more likely to manifest.
+        sys.setswitchinterval(1e-6)
+
+        for i in range(20):
+            t = threading.Thread(target=lambda: None)
+            t.start()
+            self.addCleanup(t.join)
+            pid = os.fork()
+            if pid == 0:
+                os._exit(1 if t.is_alive() else 0)
+            else:
+                pid, status = os.waitpid(pid, 0)
+                self.assertEqual(0, status)
+
 
 class ThreadJoinOnShutdown(BaseTestCase):
 
diff --git a/Lib/threading.py b/Lib/threading.py
index c7f919c..21bc781 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -940,7 +940,7 @@ def _after_fork():
     new_active = {}
     current = current_thread()
     with _active_limbo_lock:
-        for thread in _active.values():
+        for thread in _enumerate():
             # Any lock/condition variable may be currently locked or in an
             # invalid state, so we reinitialize them.
             thread._reset_internal_locks()