bpo-39010: Fix errors logged on proactor loop restart (#22017)

Stopping and restarting a proactor event loop on windows can lead to
spurious errors logged (ConnectionResetError while reading from the
self pipe). This fixes the issue by ensuring that we don't attempt
to start multiple copies of the self-pipe reading loop.

2 files changed, 14 insertions, 2 deletions

diff --git a/Lib/asyncio/proactor_events.py b/Lib/asyncio/proactor_events.py
index d0b7100f5e0..4670bd683ab 100644
--- a/Lib/asyncio/proactor_events.py
+++ b/Lib/asyncio/proactor_events.py
@@ -768,6 +768,14 @@ class BaseProactorEventLoop(base_events.BaseEventLoop):
         try:
             if f is not None:
                 f.result()  # may raise
+            if self._self_reading_future is not f:
+                # When we scheduled this Future, we assigned it to
+                # _self_reading_future. If it's not there now, something has
+                # tried to cancel the loop while this callback was still in the
+                # queue (see windows_events.ProactorEventLoop.run_forever). In
+                # that case stop here instead of continuing to schedule a new
+                # iteration.
+                return
             f = self._proactor.recv(self._ssock, 4096)
         except exceptions.CancelledError:
             # _close_self_pipe() has been called, stop waiting for data
diff --git a/Lib/asyncio/windows_events.py b/Lib/asyncio/windows_events.py
index a6759b78bd8..5e7cd795895 100644
--- a/Lib/asyncio/windows_events.py
+++ b/Lib/asyncio/windows_events.py
@@ -318,8 +318,12 @@ class ProactorEventLoop(proactor_events.BaseProactorEventLoop):
             if self._self_reading_future is not None:
                 ov = self._self_reading_future._ov
                 self._self_reading_future.cancel()
-                # self_reading_future was just cancelled so it will never be signalled
-                # Unregister it otherwise IocpProactor.close will wait for it forever
+                # self_reading_future was just cancelled so if it hasn't been
+                # finished yet, it never will be (it's possible that it has
+                # already finished and its callback is waiting in the queue,
+                # where it could still happen if the event loop is restarted).
+                # Unregister it otherwise IocpProactor.close will wait for it
+                # forever
                 if ov is not None:
                     self._proactor._unregister(ov)
                 self._self_reading_future = None