id summary reporter owner description type status component version severity resolution keywords cc stage has_patch needs_docs needs_tests needs_better_patch easy ui_ux 36939 Connecting the same signal multiple times with weak=True allocates new memory that is never deallocated phantom-jacob Juho Hautala "We recently observed steadily increasing memory usage in our Django application running under uwsgi with celery workers, which we tracked down to multiple calls to connect the same signal handlers. We eventually figured out that when `weak=True` on signal connection (the default behavior), each call to `connect()` allocates new memory (which `tracemalloc` indicates is largely from `weakref`) on every call. This memory is not cleared by disconnecting the signal, and in practice we never saw the memory free before the system eventually OOMed. Anyway, I've developed a standalone reproduction script, which can be run with `uv run django_mem_leak.py`. If the `--weak` argument is provided, you can see memory grow drastically: {{{ uv run django_mem_leak.py Installed 3 packages in 92ms Running memory audit with 100000 iterations, weak=False Starting memory usage: 28.88 MiB [100000/100000] Memory usage: 30.48 MiB Final memory usage: 30.52 MiB ---------------------------------------------------------------------------------------------------- [ Top 10 memory allocations ] :784: size=178 KiB (+178 KiB), count=1678 (+1678), average=109 B :488: size=119 KiB (+119 KiB), count=1379 (+1379), average=89 B :801: size=8156 B (+8156 B), count=68 (+68), average=120 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/pathlib/_local.py:59: size=4133 B (+4133 B), count=18 (+18), average=230 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/pathlib/_abc.py:403: size=3595 B (+3595 B), count=10 (+10), average=360 B :133: size=3479 B (+3479 B), count=21 (+21), average=166 B ~/.cache/uv/environments-v2/django-mem-leak-5f36d9b360a80206/lib/python3.13/site-packages/django/conf/global_settings.py:433: size=3264 B (+3264 B), count=1 (+1), average=3264 B :1704: size=3086 B (+3086 B), count=33 (+33), average=94 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/pathlib/_abc.py:55: size=2915 B (+2915 B), count=11 (+11), average=265 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/pathlib/_local.py:482: size=2802 B (+2802 B), count=13 (+13), average=216 B uv run django_mem_leak.py --weak Running memory audit with 100000 iterations, weak=True Starting memory usage: 27.36 MiB [100000/100000] Memory usage: 102.73 MiB Final memory usage: 102.73 MiB ---------------------------------------------------------------------------------------------------- [ Top 10 memory allocations ] ~/.cache/uv/environments-v2/django-mem-leak-5f36d9b360a80206/lib/python3.13/site-packages/django/dispatch/dispatcher.py:120: size=9375 KiB (+9375 KiB), count=200001 (+200001), average=48 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/weakref.py:576: size=7813 KiB (+7813 KiB), count=100001 (+100001), average=80 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/weakref.py:575: size=7812 KiB (+7812 KiB), count=100000 (+100000), average=80 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/weakref.py:582: size=5120 KiB (+5120 KiB), count=1 (+1), average=5120 KiB ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/weakref.py:581: size=2727 KiB (+2727 KiB), count=99743 (+99743), average=28 B :784: size=233 KiB (+233 KiB), count=2363 (+2363), average=101 B :488: size=40.9 KiB (+40.9 KiB), count=328 (+328), average=128 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/pathlib/_local.py:59: size=4133 B (+4133 B), count=18 (+18), average=230 B ~/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/lib/python3.13/pathlib/_abc.py:403: size=3715 B (+3715 B), count=11 (+11), average=338 B :133: size=3479 B (+3479 B), count=21 (+21), average=166 B }}} Use this script to reproduce: {{{ # /// script # requires-python = "">=3.13"" # dependencies = [ # ""django"", # ] # /// import argparse import resource import sys import tracemalloc from django.dispatch import Signal def signal_handler(sender, **kwargs): pass def parse_args() -> argparse.Namespace: parser = argparse.ArgumentParser( description=""Run a memory audit to demonstrate signal handler leaks"" ) parser.add_argument( ""num"", type=int, default=100000, nargs=""?"", help=""Number of iterations to run the memory audit for"", ) parser.add_argument( ""--weak"", action=""store_true"", help=""Whether to use weak references for audit signals"", ) return parser.parse_args() def _get_memory_usage() -> float: maxrss = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss # Mac reports in bytes, Linux reports in KiB if sys.platform == ""darwin"": return maxrss / 1024 / 1024 return maxrss / 1024 def main( *, num: int, weak: bool, ) -> None: signal = Signal() print(f""Running memory audit with {num} iterations, {weak=}"") print(f""Starting memory usage: {_get_memory_usage():.2f} MiB"") tracemalloc.start() start_snapshot = tracemalloc.take_snapshot() for i in range(1, num + 1): print( f""[{i}/{num}] Memory usage: {_get_memory_usage():.2f} MiB"", end=""\r"", flush=True, ) signal.connect(signal_handler, weak=weak, dispatch_uid=""test_signal"") signal.disconnect(signal_handler, dispatch_uid=""test_signal"") print("""") print(f""Final memory usage: {_get_memory_usage():.2f} MiB"") end_snapshot = tracemalloc.take_snapshot() top_stats = end_snapshot.compare_to(start_snapshot, ""lineno"") print(""\n\n"") print(""-"" * 100) print(""\n\n[ Top 10 memory allocations ]"") for stat in top_stats[:10]: print(stat) tracemalloc.stop() if __name__ == ""__main__"": args = parse_args() main(**vars(args)) }}} " Bug closed Core (Other) 6.0 Normal fixed signal connect weak weakref phantom-jacob Ready for checkin 1 0 0 0 0 0