From aebbbaffe8c5e65acb937612d369c44a0e15fc1c Mon Sep 17 00:00:00 2001
From: Noam Cohen <noam@noam.me>
Date: Tue, 20 May 2025 20:03:19 +0300
Subject: [PATCH] [3.13] gh-132542: Set native thread ID after fork (GH-132701)
 (GH-134361)

(cherry picked from commit 6b735023132a4ac9dc5b849d982104eeb1e8bdad)
---
 Lib/test/test_threading.py                    | 28 +++++++++++++++++++
 Lib/threading.py                              |  2 ++
 ...-04-19-17-16-46.gh-issue-132542.7T_TY_.rst |  2 ++
 3 files changed, 32 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-04-19-17-16-46.gh-issue-132542.7T_TY_.rst

diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 51e3f4a66510..31b52e14e506 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -1208,6 +1208,34 @@ def f():
         self.assertEqual(err, b'')
 
 
+    @skip_unless_reliable_fork
+    def test_native_id_after_fork(self):
+        script = """if True:
+            import threading
+            import os
+            from test import support
+
+            parent_thread_native_id = threading.current_thread().native_id
+            print(parent_thread_native_id, flush=True)
+            assert parent_thread_native_id == threading.get_native_id()
+            childpid = os.fork()
+            if childpid == 0:
+                print(threading.current_thread().native_id, flush=True)
+                assert threading.current_thread().native_id == threading.get_native_id()
+            else:
+                try:
+                    assert parent_thread_native_id == threading.current_thread().native_id
+                    assert parent_thread_native_id == threading.get_native_id()
+                finally:
+                    support.wait_process(childpid, exitcode=0)
+            """
+        rc, out, err = assert_python_ok('-c', script)
+        self.assertEqual(rc, 0)
+        self.assertEqual(err, b"")
+        native_ids = out.strip().splitlines()
+        self.assertEqual(len(native_ids), 2)
+        self.assertNotEqual(native_ids[0], native_ids[1])
+
 class ThreadJoinOnShutdown(BaseTestCase):
 
     def _run_and_join(self, script):
diff --git a/Lib/threading.py b/Lib/threading.py
index d6b0e2b4fbf1..11f1b5af6ac1 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -932,6 +932,8 @@ def _after_fork(self, new_ident=None):
             # This thread is alive.
             self._ident = new_ident
             assert self._handle.ident == new_ident
+            if _HAVE_THREAD_NATIVE_ID:
+                self._set_native_id()
         else:
             # Otherwise, the thread is dead, Jim.  _PyThread_AfterFork()
             # already marked our handle done.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-04-19-17-16-46.gh-issue-132542.7T_TY_.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-04-19-17-16-46.gh-issue-132542.7T_TY_.rst
new file mode 100644
index 000000000000..c69ce5efdedc
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-04-19-17-16-46.gh-issue-132542.7T_TY_.rst
@@ -0,0 +1,2 @@
+Update :attr:`Thread.native_id <threading.Thread.native_id>` after
+:manpage:`fork(2)` to ensure accuracy. Patch by Noam Cohen.