"disconnecting properly" in tests still hangs on macOS

Right, so, I've been trying to get the technique in 
https://jml.io/pages/how-to-disconnect-in-twisted-really.html to work 
for me.

No hating please, most of my testing in the past has involved hitting a 
relational database, so there's already a TCP connection flying around, 
one more won't make any difference.

jml's example, exactly as-is on that page, hangs around 30-40% of the 
time when running on my macOS laptop. From changing the teardown to look 
like this:

     def tearDown(self):
         ds = defer.maybeDeferred(self.serverPort.stopListening)
         dc = defer.maybeDeferred(self.clientConnection.disconnect)
         print()

         ds.addCallback(lambda _: print('serverPort.stopListening'))
         dc.addCallback(lambda _: print('self.clientConnection.disconnect'))
         self.clientDisconnected.addCallback(lambda _: 
print('self.clientDisconnected'))
         self.serverDisconnected.addCallback(lambda _: 
print('self.serverDisconnected'))
         self.serverDisconnected.addErrback(lambda _: 
print('self.serverDisconnected:', _))
         return defer.gatherResults([ds, dc, self.clientDisconnected, 
self.serverDisconnected])

...it appears that it's the serverDisconnected deferred that's failing 
to fire. I can't reproduce this on Linux as of yet, so I'm guessing this 
is a difference between the SelectReactor used on macOS and the 
EPollReactor used on Linux.

What's the best way to go about debugging a non-firing deferred like this?

Anyone know what this might be?

cheers,

Chris

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