fix: Avoid CancelledError from being propagated to lifespan's receive()

Currently `LifeSpanOn.main()` is excepting `BaseException`, which
also catches `CancelledError`.

`CancelledError` is used by `asyncio` to signal that a task has been
cancelled and is not an error condition.

This commit changes the behavior to catch `CancelledError` separately
and log it as a "Lifespan  task cancelled" instead of letting the error
propagate.

Author: bellini666

tests/test_lifespan.py

@@ -1,4 +1,5 @@
 import asyncio
+from unittest import mock
 
 import pytest
 
@@ -126,6 +127,44 @@ async def test():
     loop.close()
 
 
+@pytest.mark.parametrize("mode", ("auto", "on"))
+def test_lifespan_on_when_app_is_cancelled(mode):
+    def get_app(started_event: asyncio.Event):
+        async def app(scope, receive, send):
+            started_event.set()
+            await receive()
+            pytest.fail("Should be cancelled before it gets here.")
+
+        return app
+
+    async def test():
+        # Python 3.8/3.9 doesn't allow us to instantiate an asyncio.Event object
+        # while outside of a running event loop context
+        started_event = asyncio.Event()
+
+        config = Config(app=get_app(started_event), lifespan=mode)
+        lifespan = LifespanOn(config)
+
+        main_lifespan_task = asyncio.create_task(lifespan.main())
+
+        started_response = await started_event.wait()
+        assert started_response
+        assert not lifespan.shutdown_event.is_set()
+
+        with mock.patch.object(lifespan, "logger") as logger:
+            main_lifespan_task.cancel()
+            await main_lifespan_task
+
+        assert not lifespan.error_occured
+        logger.info.assert_called_with("Lifespan task cancelled.")
+        assert lifespan.startup_event.is_set()
+        assert lifespan.shutdown_event.is_set()
+
+    loop = asyncio.new_event_loop()
+    loop.run_until_complete(test())
+    loop.close()
+
+
 @pytest.mark.parametrize("mode", ("auto", "on"))
 @pytest.mark.parametrize("raise_exception", (True, False))
 def test_lifespan_with_failed_startup(mode, raise_exception, caplog):

uvicorn/lifespan/on.py

@@ -84,6 +84,11 @@ async def main(self) -> None:
                 "state": self.state,
             }
             await app(scope, self.receive, self.send)
+        except asyncio.CancelledError:
+            # Lifespan task was cancelled, likely due to the server shutting down.
+            # This is not an error that should be handled by BaseException above,
+            # so we just log it and exit.
+            self.logger.info("Lifespan task cancelled.")
         except BaseException as exc:
             self.asgi = None
             self.error_occured = True