bpo-39651: Fix asyncio proactor _write_to_self() (GH-22197)

Fix a race condition in the call_soon_threadsafe() method of
asyncio.ProactorEventLoop: do nothing if the self-pipe socket has
been closed.
(cherry picked from commit 1b0f0e3d7d03155da1cf9769a847874d559e57e3)

Co-authored-by: Victor Stinner <vstinner@python.org>

1 files changed, 10 insertions, 8 deletions

diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index a05cbb6..6ba3071 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -133,14 +133,16 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
         # a socket is closed, send() raises OSError (with errno set to
         # EBADF, but let's not rely on the exact error code).
         csock = self._csock
-        if csock is not None:
-            try:
-                csock.send(b'\0')
-            except OSError:
-                if self._debug:
-                    logger.debug("Fail to write a null byte into the "
-                                 "self-pipe socket",
-                                 exc_info=True)
+        if csock is None:
+            return
+
+        try:
+            csock.send(b'\0')
+        except OSError:
+            if self._debug:
+                logger.debug("Fail to write a null byte into the "
+                             "self-pipe socket",
+                             exc_info=True)
 
     def _start_serving(self, protocol_factory, sock,
                        sslcontext=None, server=None, backlog=100,