From 4b1e41f715521bc5f7e8583d2dc9d1d14b5e63bf Mon Sep 17 00:00:00 2001
From: Viktor Vorobev <viktor.vorobev@emlid.com>
Date: Wed, 8 Sep 2021 14:10:46 +0300
Subject: [PATCH] bpo-37788: Fix reference leak when Thread is never joined
 (GH-26103)

When a Thread is not joined after it has stopped, its lock may remain in the _shutdown_locks set until interpreter shutdown.  If many threads are created this way, the _shutdown_locks set could therefore grow endlessly.  To avoid such a situation, purge expired locks each time a new one is added or removed.
(cherry picked from commit c10c2ec)

Co-authored-by: @pitrou
---
 Lib/test/test_threading.py                       |  7 +++++++
 Lib/threading.py                                 | 16 +++++++++++++++-
 .../2021-05-13-19-07-28.bpo-37788.adeFcf.rst     |  1 +
 3 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2021-05-13-19-07-28.bpo-37788.adeFcf.rst

diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index ac4e7a7e0f53b3..9ca273a5707092 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -762,6 +762,13 @@ def test_shutdown_locks(self):
                 # Daemon threads must never add it to _shutdown_locks.
                 self.assertNotIn(tstate_lock, threading._shutdown_locks)
 
+    def test_leak_without_join(self):
+        # bpo-37788: Test that a thread which is not joined explicitly
+        # does not leak. Test written for reference leak checks.
+        def noop(): pass
+        with support.wait_threads_exit():
+            threading.Thread(target=noop).start()
+            # Thread.join() is not called
 
 class ThreadJoinOnShutdown(BaseTestCase):
 
diff --git a/Lib/threading.py b/Lib/threading.py
index 813dae2aa9f8e5..84cda94024a569 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -745,6 +745,18 @@ def _newname(template="Thread-%d"):
 _shutdown_locks_lock = _allocate_lock()
 _shutdown_locks = set()
 
+
+def _maintain_shutdown_locks():
+    """
+    Drop any shutdown locks that don't correspond to running threads anymore.
+    Calling this from time to time avoids an ever-growing _shutdown_locks
+    set when Thread objects are not joined explicitly. See bpo-37788.
+    This must be called with _shutdown_locks_lock acquired.
+    """
+    # If a lock was released, the corresponding thread has exited
+    to_remove = [lock for lock in _shutdown_locks if not lock.locked()]
+    _shutdown_locks.difference_update(to_remove)
+
 # Main class for threads
 
 class Thread:
@@ -910,6 +922,7 @@ def _set_tstate_lock(self):
 
         if not self.daemon:
             with _shutdown_locks_lock:
+                _maintain_shutdown_locks()
                 _shutdown_locks.add(self._tstate_lock)
 
     def _bootstrap_inner(self):
@@ -965,7 +978,8 @@ def _stop(self):
         self._tstate_lock = None
         if not self.daemon:
             with _shutdown_locks_lock:
-                _shutdown_locks.discard(lock)
+                # Remove our lock and other released locks from _shutdown_locks
+                _maintain_shutdown_locks()
 
     def _delete(self):
         "Remove current thread from the dict of currently running threads."
diff --git a/Misc/NEWS.d/next/Library/2021-05-13-19-07-28.bpo-37788.adeFcf.rst b/Misc/NEWS.d/next/Library/2021-05-13-19-07-28.bpo-37788.adeFcf.rst
new file mode 100644
index 00000000000000..0c33923e992452
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-05-13-19-07-28.bpo-37788.adeFcf.rst
@@ -0,0 +1 @@
+Fix a reference leak when a Thread object is never joined.