CI run against Twisted trunk is failing

[matrixbot]

See https://github.com/element-hq/synapse/actions/runs/8643480271

[spaetz]

For ease of access:
The failure in the linked CI run is in task mypy:

synapse/http/server.py:157: error: "ITransport" has no attribute "abortConnection"; maybe "loseConnection"?  [attr-defined]
synapse/http/proxy.py:264: error: Item "ITransport" of "Optional[ITransport]" has no attribute "abortConnection"  [union-attr]
synapse/http/proxy.py:264: error: Item "None" of "Optional[ITransport]" has no attribute "abortConnection"  [union-attr]
synapse/http/site.py:152: error: Item "ITransport" of "Optional[ITransport]" has no attribute "abortConnection"  [union-attr]
synapse/http/site.py:152: error: Item "None" of "Optional[ITransport]" has no attribute "abortConnection"  [union-attr]
Found 5 errors in 3 files (checked 870 source files)

Some more clueless rooting in source (https://github.com/twisted/twisted/blob/d827ff43cc97495f54cf14d0b65a7af42f52c161/src/twisted/internet/interfaces.py#L2129)

ITransport is an abstract interface which indeed does not have a method "abortConnection" defined. It is implemented by 2 other classes ITCPTransport which does define abortConnection and by IUNIXTransport (a Transport for stream-oriented unix domain connections.) which does not.

So indeed, we can not be sure that abortConnection exists until we are sure that request.transport is of the ITCPTransport kind (which it will always be in synapse, I guess???)

To make mypy happy, one would either have to

1. silence the warnings to the calls to abortConnection(), or
2. test explicitely that we have a request.transport is a ITCPTransport subclass to make sure it is of the right kind, or simply
3. check for the existance of the abortConnection method.