Stabilize the threads API (MSC3856) by supporting (only) the v1
path for the endpoint.
This also marks the API as safe for workers since it is a read-only
API.
Implement the /threads endpoint from MSC3856.
This is currently unstable and behind an experimental configuration
flag.
It includes a background update to backfill data, results from
the /threads endpoint will be partial until that finishes.
Keep the old behavior (of including the original_event field) for any
requests to the /unstable version of the endpoint, but do not include
the field when the /v1 version is used.
This should avoid new clients from depending on this field, but will
not help with current dependencies.
Since MSC3715 has passed FCP, the stable parameter can be used.
This currently falls back to the unstable parameter if the stable
parameter is not provided (and MSC3715 support is enabled in
the configuration).
This endpoint was removed from MSC2675 before it was approved.
It is currently unspecified (even in any MSCs) and therefore subject to
removal. It is not implemented by any known clients.
This also changes the bundled aggregation format for `m.annotation`,
which previously included pagination tokens for the `/aggregations`
endpoint, which are no longer useful.
This is allowed per MSC2675, although the original implementation did
not allow for it and would return an empty chunk / not bundle aggregations.
The main thing to improve is that the various caches get cleared properly
when an event is redacted, and that edits must not leak if the original
event is redacted (as that would presumably leak something similar to
the original event content).
This makes the serialization of events synchronous (and it no
longer access the database), but we must manually calculate and
provide the bundled aggregations.
Overall this should cause no change in behavior, but is prep work
for other improvements.
Due to updates to MSC2675 this includes a few fixes:
* Include bundled aggregations for /sync.
* Do not include bundled aggregations for /initialSync and /events.
* Do not bundle aggregations for state events.
* Clarifies comments and variable names.
Patrick Cloke