From b30b25b26663fb6070b8ed86fe3a20dcb557d05d Mon Sep 17 00:00:00 2001 From: Antoine Pitrou Date: Sat, 15 May 2021 11:51:20 +0200 Subject: [PATCH] [3.9] bpo-37788: Fix reference leak when Thread is never joined (GH-26103) (GH-26142) Reference:https://github.com/python/cpython/commit/b30b25b26663fb6070b8ed86fe3a20dcb557d05d 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 c10c2ec7a0e06975e8010c56c9c3270f8ea322ec) Co-authored-by: Antoine Pitrou Automerge-Triggered-By: GH:pitrou --- Lib/test/test_threading.py | 8 ++++++++ Lib/threading.py | 19 ++++++++++++++++++- .../2021-05-13-19-07-28.bpo-37788.adeFcf.rst | 1 + 3 files changed, 27 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 ac4e7a7..f2e427c 100644 --- a/Lib/test/test_threading.py +++ b/Lib/test/test_threading.py @@ -762,6 +762,14 @@ class ThreadTests(BaseTestCase): # 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 813dae2..f8f5954 100644 --- a/Lib/threading.py +++ b/Lib/threading.py @@ -739,12 +739,27 @@ _active_limbo_lock = _allocate_lock() _active = {} # maps thread id to Thread object _limbo = {} _dangling = WeakSet() + # Set of Thread._tstate_lock locks of non-daemon threads used by _shutdown() # to wait until all Python thread states get deleted: # see Thread._set_tstate_lock(). _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 +925,7 @@ class Thread: if not self.daemon: with _shutdown_locks_lock: + _maintain_shutdown_locks() _shutdown_locks.add(self._tstate_lock) def _bootstrap_inner(self): @@ -965,7 +981,8 @@ class Thread: 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 0000000..0c33923 --- /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. -- 2.25.1