From 8d58051ac6ea3032cf673f3ba75d79ff8b8457a1 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Fri, 27 Mar 2020 17:50:42 +0100
Subject: [PATCH] bpo-40089: Fix threading._after_fork() (GH-19191)

If fork was not called by a thread spawned by threading.Thread,
threading._after_fork() now creates a _MainThread instance for
_main_thread, instead of a _DummyThread instance.
(cherry picked from commit d8ff44ce4cd6f3ec0fab5fccda6bf14afcb25c30)

Co-authored-by: Victor Stinner <vstinner@python.org>
---
 Lib/threading.py                                       | 10 +++++++++-
 .../Library/2020-03-27-17-22-34.bpo-40089.-lFsD0.rst   |  3 +++
 2 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-03-27-17-22-34.bpo-40089.-lFsD0.rst

diff --git a/Lib/threading.py b/Lib/threading.py
index b597336a150677..b961456fb792a2 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -1340,7 +1340,15 @@ def _after_fork():
 
     # fork() only copied the current thread; clear references to others.
     new_active = {}
-    current = current_thread()
+
+    try:
+        current = _active[get_ident()]
+    except KeyError:
+        # fork() was called in a thread which was not spawned
+        # by threading.Thread. For example, a thread spawned
+        # by thread.start_new_thread().
+        current = _MainThread()
+
     _main_thread = current
 
     # reset _shutdown() locks: threads re-register their _tstate_lock below
diff --git a/Misc/NEWS.d/next/Library/2020-03-27-17-22-34.bpo-40089.-lFsD0.rst b/Misc/NEWS.d/next/Library/2020-03-27-17-22-34.bpo-40089.-lFsD0.rst
new file mode 100644
index 00000000000000..f5335a33c066ca
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-03-27-17-22-34.bpo-40089.-lFsD0.rst
@@ -0,0 +1,3 @@
+Fix threading._after_fork(): if fork was not called by a thread spawned by
+threading.Thread, threading._after_fork() now creates a _MainThread instance
+for _main_thread, instead of a _DummyThread instance.