hydrogen-web/doc/impl-thoughts/PENDING_REPLIES.md

Replying to pending messages

The matrix spec requires clients capable of rich replies (that would be us once replies work) to include fallback (textual in body and structured in formatted_body) that can be rendered by clients that do not natively support rich replies (that would be us at the time of writing). The schema for the fallback is as follows:

<mx-reply>
  <blockquote>
    <a href="https://matrix.to/#/!somewhere:example.org/$event:example.org">In reply to</a>
    <a href="https://matrix.to/#/@alice:example.org">@alice:example.org</a>
    <br />
    <!-- This is where the related event's HTML would be. -->
  </blockquote>
</mx-reply>

There's a single complication here for pending events: we have $event:example.org in the schema (the In reply to link), and it must be present within the content, inside formatted_body. The issue is that, if we are queuing a reply to a pending event, we don't know its remote ID. All we know is its transaction ID on our end. If we were to use that while formatting the message, we'd be sending messages that contain our internal transaction IDs instead of proper matrix event identifiers.

To solve this, we'd need SendQueue, whenever it receives a remote echo, to update pending events that are replies with their relatedEventId. This already happens, and the event_id field in m.relates_to is updated. But we'd need to extend this to adjust the messages' formatted_body with the resolved remote ID, too.

How do we safely do this, without accidentally substituting event IDs into places in the body where they were not intended?