8000 [3.9] bpo-37788: Fix reference leak when Thread is never joined (GH-26103) by pitrou · Pull Request #26142 · python/cpython · GitHub
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[3.9] bpo-37788: Fix reference leak when Thread is never joined (GH-26103) #26142

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Merged
merged 2 commits into from
May 15, 2021
Merged

[3.9] bpo-37788: Fix reference leak when Thread is never joined (GH-26103) #26142

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[3.9] bpo-37788: Fix reference leak when Thread is never joined (GH-2…
pitrou May 14, 2021
51ac31f
Fix test
pitrou May 15, 2021
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8 changes: 8 additions & 0 deletions Lib/test/test_threading.py
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Original file line number Diff line number Diff line change
Expand Up @@ -805,6 +805,14 @@ def __del__(self):
""")
self.assertEqual(out.rstrip(), b"thread_dict.atexit = 'value'")

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):

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19 changes: 18 additions & 1 deletion Lib/threading.py
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Expand Up @@ -755,12 +755,27 @@ def _newname(template="Thread-%d"):
_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:
Expand Down Expand Up @@ -932,6 +947,7 @@ 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):
Expand Down Expand Up @@ -987,7 +1003,8 @@ 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."
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2021-05-13-19-07-28.bpo-37788.adeFcf.rst
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@@ -0,0 +1 @@
Fix a reference leak when a Thread object is never joined.
0