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, 1 deletions

diff --git a/Lib/asyncio/proactor_events.py b/Lib/asyncio/proactor_events.py
index 54e4deb..3e0a14f 100644
--- a/Lib/asyncio/proactor_events.py
+++ b/Lib/asyncio/proactor_events.py
@@ -791,8 +791,17 @@ class BaseProactorEventLoop(base_events.BaseEventLoop):
             f.add_done_callback(self._loop_self_reading)
 
     def _write_to_self(self):
+        # This may be called from a different thread, possibly after
+        # _close_self_pipe() has been called or even while it is
+        # running.  Guard for self._csock being None or closed.  When
+        # 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 None:
+            return
+
         try:
-            self._csock.send(b'\0')
+            csock.send(b'\0')
         except OSError:
             if self._debug:
                 logger.debug("Fail to write a null byte into the "