Stabilize support for MSC2675: /relations endpoint

[clokep]

MSC2675 is currently in FCP (to complete tomorrow, barring any concerns). We have a few bits of work to do to stabilize support in Synapse for it:

• Fix-up redaction support:
  • Ensure that redacted relations are not bundled nor returned from /relations.
  • Support fetching relations of a redacted event. (Currently this returns nothing.)
  • Bundle aggregations for a redacted event.
• Properly handle relations for ignored users in:
  • /relations
  • Bundled aggregations.
• Support accepting pagination tokens from /sync and /messages.
  • Clean-up backwards compatibility with the old pagination token format.
• Add the v1 prefix for /relations.
• Remove the unstable prefix for /relations.

Those were all the differences that I found by reading through it, but there could be more.

[clokep]

It is not explicit in the MSC, but it seems we also don't return any bundled aggregations for a redacted event. We likely should since that would match the behavior of being able to call /relations on it.

[clokep]

Another piece of this that seems broken is that currently you cannot paginate this API using the results of a /sync or /messages API.

[clokep]

Another piece of this that seems broken is that currently you cannot paginate this API using the results of a /sync or /messages API.

This is due to the usage of RelationPaginationToken instead of StreamToken by the returned APIs. I think a way forward would be to:

• Switch the APIs to accept a serialized StreamToken or RelationPaginationToken (in which case we generate a StreamToken with only room_key filled in from the parsed RelationPaginationToken).
• Update the generated tokens to be serialied StreamTokens.

This all seems fairly straightforward (although finicky), but the similar code in the PaginationHandler does some rather complicated bits with topological ordering.

[clokep]

This is done, minus removing support for the unstable /relations endpoint, which I think needs to wait for the next spec release.

[clokep]

This is done, minus removing support for the unstable /relations endpoint, which I think needs to wait for the next spec release.

I think Matrix v1.3 included this endpoint, so we can probably finish up this work now. Also see #14104.