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.

1 files changed, 8 insertions, 0 deletions

diff --git a/Lib/asyncio/proactor_events.py b/Lib/asyncio/proactor_events.py
index d0b7100..4670bd6 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