Use WeakKeyDictionary for WRITE_LOCKS to prevent file object leaks

CHANGELOG.md

@@ -27,6 +27,13 @@ You can find our backwards-compatibility policy [here](https://github.com/hynek/
   [#802](https://github.com/hynek/structlog/pull/802)
 
 
+### Fixed
+
+- `structlog.BytesLogger`, `structlog.PrintLogger`, and `structlog.WriteLogger` now hold *weak* references to the files they use for output.
+  This prevents their leakage in long-running processes that open many logfiles, such as task executors that create a per-task `BytesLogger` or `WriteLogger`.
+  [#807](https://github.com/hynek/structlog/pull/807)
+
+
 ### Changed
 
 - `structlog.dev.ConsoleRenderer` does not warn anymore when the `exception` key has a rendered value despite having a fancy formatter configured.

src/structlog/_output.py

@@ -12,13 +12,16 @@
 import copy
 import sys
 import threading
+import weakref
 
 from pickle import PicklingError
 from sys import stderr, stdout
 from typing import IO, Any, BinaryIO, TextIO
 
 
-WRITE_LOCKS: dict[IO[Any], threading.Lock] = {}
+WRITE_LOCKS: weakref.WeakKeyDictionary[IO[Any], threading.Lock] = (
+    weakref.WeakKeyDictionary()
+)
 
 
 def _get_lock_for_file(file: IO[Any]) -> threading.Lock:

tests/test_output.py

@@ -4,6 +4,7 @@
 # repository for complete details.
 
 import copy
+import gc
 import pickle
 
 from io import BytesIO, StringIO
@@ -23,6 +24,35 @@
 from .helpers import stdlib_log_methods
 
 
+@pytest.mark.parametrize(
+    ("logger_cls", "mode"),
+    [(PrintLogger, "w"), (WriteLogger, "w"), (BytesLogger, "wb")],
+)
+def test_write_locks_released_on_gc(logger_cls, mode, tmp_path):
+    """
+    WRITE_LOCKS entry is removed automatically when the file object is GC'd.
+
+    Closing the file is not enough -- the entry persists until the file object
+    itself is collected.
+    """
+    gc.collect()
+    size_before = len(WRITE_LOCKS)
+    f = (tmp_path / "test.log").open(mode)
+    logger = logger_cls(f)
+
+    assert len(WRITE_LOCKS) == size_before + 1
+
+    # close() alone does not remove the entry
+    f.close()
+
+    assert len(WRITE_LOCKS) == size_before + 1
+
+    del logger, f
+    gc.collect()
+
+    assert len(WRITE_LOCKS) == size_before
+
+
 class TestLoggers:
     """
     Tests common to the Print and WriteLoggers.