From 59ace509f3a67a11ab5482b0edd57ef760d9bfb9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?=
 <marmarek@invisiblethingslab.com>
Date: Tue, 7 Mar 2023 23:53:40 +0100
Subject: [PATCH 1/3] Add regression test for gh-102512

---
 Lib/test/test_threading.py | 49 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index a39a267b403d83..a40de2f9daeefa 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -683,6 +683,55 @@ def func():
         self.assertEqual(err.decode('utf-8'), "")
         self.assertEqual(data, "Thread-1 (func)\nTrue\nTrue\n")
 
+    @unittest.skipIf(sys.platform in platforms_to_skip, "due to known OS bug")
+    @support.requires_fork()
+    @unittest.skipUnless(hasattr(os, 'waitpid'), "test needs os.waitpid()")
+    def test_main_thread_after_fork_from_foreign_thread(self):
+        code = """if 1:
+            import os, threading, sys, traceback, _thread
+            from test import support
+
+            def func(lock):
+                # call current_thread() before fork to allocate DummyThread
+                current = threading.current_thread()
+                print(current.name)
+                # flush before fork, so child won't flush it again
+                sys.stdout.flush()
+                pid = os.fork()
+                if pid == 0:
+                    main = threading.main_thread()
+                    print(main.ident == threading.current_thread().ident)
+                    print(main.ident == threading.get_ident())
+                    # stdout is fully buffered because not a tty,
+                    # we have to flush before exit.
+                    sys.stdout.flush()
+                    try:
+                        threading._shutdown()
+                        os._exit(0)
+                    except:
+                        os._exit(1)
+                else:
+                    try:
+                        support.wait_process(pid, exitcode=0)
+                    except Exception as e:
+                        # avoid 'could not acquire lock for
+                        # <_io.BufferedWriter name='<stderr>'> at interpreter shutdown,'
+                        traceback.print_exc()
+                        sys.stderr.flush()
+                    finally:
+                        lock.release()
+
+            join_lock = _thread.allocate_lock()
+            join_lock.acquire()
+            th = _thread.start_new_thread(func, (join_lock,))
+            join_lock.acquire()
+        """
+        # "DeprecationWarning: This process is multi-threaded, use of fork() may lead to deadlocks in the child"
+        _, out, err = assert_python_ok("-W", "ignore::DeprecationWarning", "-c", code)
+        data = out.decode().replace('\r', '')
+        self.assertEqual(err, b"")
+        self.assertEqual(data, "Dummy-1\nTrue\nTrue\n")
+
     def test_main_thread_during_shutdown(self):
         # bpo-31516: current_thread() should still point to the main thread
         # at shutdown

From 43c9f3b2c85fadd038e4ceaef8859154512fdfcd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?=
 <marmarek@invisiblethingslab.com>
Date: Tue, 7 Mar 2023 23:56:18 +0100
Subject: [PATCH 2/3] Initialize tstate_lock in DummyThread too

If os.fork() is called from a DummyThread (a thread started not by the
threading module, in which somebody called threading.current_thread()),
it becomes main thread. Threading shutdown logic relies on the main
thread having tstate_lock, so initialize it for the DummyThread too.

Fixes #102512
---
 Lib/threading.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Lib/threading.py b/Lib/threading.py
index df273870fa4273..99f87c70353bd5 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -1439,7 +1439,7 @@ class _DummyThread(Thread):
     def __init__(self):
         Thread.__init__(self, name=_newname("Dummy-%d"),
                         daemon=_daemon_threads_allowed())
-
+        self._set_tstate_lock()
         self._started.set()
         self._set_ident()
         if _HAVE_THREAD_NATIVE_ID:

From 9d186b3ed544e0f43f424e8bf511994b2256ce9c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?=
 <marmarek@invisiblethingslab.com>
Date: Wed, 8 Mar 2023 00:02:34 +0100
Subject: [PATCH 3/3] Add NEWS entry

---
 .../next/Library/2023-03-08-00-02-30.gh-issue-102512.LiugDr.rst | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2023-03-08-00-02-30.gh-issue-102512.LiugDr.rst

diff --git a/Misc/NEWS.d/next/Library/2023-03-08-00-02-30.gh-issue-102512.LiugDr.rst b/Misc/NEWS.d/next/Library/2023-03-08-00-02-30.gh-issue-102512.LiugDr.rst
new file mode 100644
index 00000000000000..81f95a06ffb941
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-03-08-00-02-30.gh-issue-102512.LiugDr.rst
@@ -0,0 +1,2 @@
+Fix handling threading after os.fork() called from a foreign thread (aka
+DummyThread).