test_asyncio.test_ssl: fix slow-socket transport race

Summary:
test_remote_shutdown_receives_trailing_data_on_slow_socket reads
`writer.transport._ssl_protocol._transport` and wraps its `write`. On a slow /
loaded socket the remote shutdown can complete first, and connection_lost
clears `_ssl_protocol._transport` to None, so the test raised
`AttributeError: 'NoneType' object has no attribute 'write'`. Bail out of the
delayed-write-path check when the underlying transport is already gone.

Reviewed By: Yhg1s

Differential Revision: D108567107

fbshipit-source-id: e06835bc46be1183c89aec21b04f693090d6aff4

Author: itamaro

Lib/test/test_asyncio/test_ssl.py

@@ -1544,6 +1544,13 @@ async def client(addr):
             # This triggers bug gh-115514, also tested using mocks in
             # test.test_asyncio.test_selector_events.SelectorSocketTransportTests.test_write_buffer_after_close
             socket_transport = writer.transport._ssl_protocol._transport
+            if socket_transport is None:
+                # On a slow socket the remote shutdown may have already torn
+                # down the underlying transport (connection_lost clears
+                # _ssl_protocol._transport) before we get here. The delayed
+                # write path exercised below is then moot, so skip it instead
+                # of dereferencing None.
+                return
 
             class SocketWrapper:
                 def __init__(self, sock) -> None: