fix: callback inappropriately suppresses asyncio logs (#725)

Author: BobTheBuidler

async_lru/__init__.py

@@ -178,7 +178,10 @@ def _cache_miss(self, key: Hashable) -> None:
         self.__misses += 1
 
     def _task_done_callback(self, key: Hashable, task: "asyncio.Task[_R]") -> None:
-        if task.cancelled() or task.exception() is not None:
+        # We must use the private attribute instead of `exception()`
+        # so asyncio does not set `task.__log_traceback = False` on
+        # the false assumption that the caller read the task Exception
+        if task.cancelled() or task._exception is not None:
             self.__cache.pop(key, None)
             return
 

tests/test_internals.py

@@ -1,10 +1,12 @@
 import asyncio
+import gc
+import logging
 from functools import partial
 from unittest import mock
 
 import pytest
 
-from async_lru import _LRUCacheWrapper
+from async_lru import _CacheItem, _LRUCacheWrapper
 
 
 async def test_done_callback_cancelled() -> None:
@@ -41,6 +43,42 @@ async def test_done_callback_exception() -> None:
     assert task not in wrapped._LRUCacheWrapper__tasks  # type: ignore[attr-defined]
 
 
+async def test_done_callback_exception_logs(caplog: pytest.LogCaptureFixture) -> None:
+    caplog.set_level(logging.ERROR, logger="asyncio")
+
+    wrapped = _LRUCacheWrapper(mock.ANY, None, False, None)
+    loop = asyncio.get_running_loop()
+
+    async def boom() -> None:
+        await asyncio.sleep(0)
+        raise RuntimeError("boom")
+
+    key = object()
+    task = loop.create_task(boom())
+    wrapped._LRUCacheWrapper__cache[key] = _CacheItem(task, None, 1)  # type: ignore[attr-defined]
+    task.add_done_callback(partial(wrapped._task_done_callback, key))
+
+    while not task.done():
+        await asyncio.sleep(0)
+    await asyncio.sleep(0)
+
+    assert key not in wrapped._LRUCacheWrapper__cache  # type: ignore[attr-defined]
+    # asyncio disables logging when exception() is called; keep logging enabled.
+    assert task._log_traceback
+
+    caplog.clear()
+
+    del task  # Remove reference so task get garbage collected.
+    for _ in range(5):  # pragma: no branch
+        gc.collect()
+        await asyncio.sleep(0)
+        if "Task exception was never retrieved" in caplog.text:  # pragma: no branch
+            break
+
+    assert "Task exception was never retrieved" in caplog.text
+    assert "RuntimeError: boom" in caplog.text
+
+
 async def test_cache_invalidate_typed() -> None:
     wrapped = _LRUCacheWrapper(mock.AsyncMock(return_value=1), None, True, None)