Run CI against Twisted's "trunk" branch

[callahad]

We're a big / complex enough project to where testing against trunk would be genuinely valuable for the broader Twisted ecosystem.

This would likely take the form of a scheduled GitHub Action which runs our test suite once a day, using Twisted's trunk branch instead of whatever version is specified in our dependencies.

[ShadowJonathan]

How would the signalling of a failed job for this go? A matrix room?

[callahad]

I believe GitHub Actions sends repo owners a notification when a scheduled job fails, similar to how it notifies you when your own PR fails.

[richvdh]

We're a big / complex enough project to where testing against trunk would be genuinely valuable for the broader Twisted ecosystem.

This will also bring a more direct benefit to the Synapse project: it will give us early warning of any compatibility problems in the next version of Twisted.

[richvdh]

I've been wondering how we could best approach this. I'm assuming we want to run at least {mypy, trial, sytest, complement} on Synapse with pip install https://github.com/twisted/twisted/archive/trunk.zip.

Of those, I think the hardest is going to be sytest, because the incantations for setting up and running Synapse are all embedded within the sytest-synapse docker image.

Possibly we could do some sed on python-dependencies before we start the test runs, to replace the Twisted dependency with trunk instead of 18.whatever?