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.

Lib/asyncio/proactor_events.py

@@ -793,8 +793,17 @@ 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 "

Lib/asyncio/selector_events.py

@@ -133,7 +133,9 @@ 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:
+        if csock is None:
+            return
+
         try:
             csock.send(b'\0')
         except OSError:

Misc/NEWS.d/next/Library/2020-09-11-12-38-55.bpo-39651.JMp9l2.rst

@@ -0,0 +1,3 @@
+Fix a race condition in the ``call_soon_threadsafe()`` method of
+``asyncio.ProactorEventLoop``: do nothing if the self-pipe socket has been
+closed.