[Twisted-Python] TB in tls.py

[Scott, Barry]

I'm see this TB often and I'm wondering how to fix it.

  File "/usr/local/lib/python2.7/site-packages/twisted/internet/base.py", line 
902, in runUntilCurrent
    call.func(*call.args, **call.kw)
  File "/usr/local/lib/python2.7/site-packages/twisted/web/http.py", line 
2333, in forceAbortClient
    self.transport.abortConnection()
  File "/usr/local/lib/python2.7/site-packages/twisted/protocols/tls.py", line 
435, in abortConnection
    self._shutdownTLS()
  File "/usr/local/lib/python2.7/site-packages/twisted/protocols/tls.py", line 
338, in _shutdownTLS
    shutdownSuccess = self._tlsConnection.shutdown()
exceptions.AttributeError: 'NoneType' object has no attribute 'shutdown'

I can see in TLSMemoryBIOProtocol.connectionLost that _tlsConnection
is set to None. Which I think is racing with the call later that calls 
forceAbortClient().

If that is the case then I think this is a possible fix?
Thoughts?

--- twisted-upstream/src/twisted/protocols/tls.py	2019-06-26 
16:10:54.329822916 +0100
+++ _diff_.tmp	2019-06-26 16:12:17.992418082 +0100
@@ -334,7 +334,7 @@
         Initiate, or reply to, the shutdown handshake of the TLS layer.
         """
         try:
-            shutdownSuccess = self._tlsConnection.shutdown()
+            shutdownSuccess = self._tlsConnection is not None and 
self._tlsConnection.shutdown()
         except Error:
             # Mid-handshake, a call to shutdown() can result in a
             # WantWantReadError, or rather an SSL_ERR_WANT_READ; but 
pyOpenSSL

Barry