Implement MSC3051 to support multiple relations per event

[chayleaf]

It seems there's considerable interest in MSC3051 - scalable relations (multiple relations per event in an array) as the limits of the current model become more and more apparent. This is a WIP attempt to implement it (there's no tests yet).

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[clokep]

Don't forget to sign-off on your changes. Shout if you want us to run CI, but seems like this is very much WIP so far.

[chayleaf]

Yes, I'll rebase it later anyway. Is there a simple way to debug internal server errors in tests?

[clokep]

Yes, I'll rebase it later anyway. Is there a simple way to debug internal server errors in tests?

Look in the _trial_temp/test.log file. Feel free to ask questions in #synapse-dev:matrix.org.

[chayleaf]

This passes tests now, but I'm not sure which new tests I should add. More importantly, there are a bunch of "XXX" comments where I'm not confident whether the existing checks should be relaxed with the introduction of multiple relations per an event. Overall I suppose this isn't "too big" of a change in terms of backwards compatibility.

[clokep]

@chayleaf Are you looking for a review on this PR? (I'm guessing no since it was marked as a draft.)

[chayleaf]

yes, sorry, I thought it's fine but it still has some bugs which I found by adding new tests