26 lines
1.6 KiB
Markdown
26 lines
1.6 KiB
Markdown
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# Replying to pending messages
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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
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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:
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```
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<mx-reply>
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<blockquote>
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<a href="https://matrix.to/#/!somewhere:example.org/$event:example.org">In reply to</a>
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<a href="https://matrix.to/#/@alice:example.org">@alice:example.org</a>
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<br />
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<!-- This is where the related event's HTML would be. -->
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</blockquote>
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</mx-reply>
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```
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There's a single complication here for pending events: we have `$event:example.org` in the schema (the `In reply to` link), and it must
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be present _within the content_, inside `formatted_body`. The issue is that, if we are queuing a reply to a pending event,
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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,
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we'd be sending messages that contain our internal transaction IDs instead of proper matrix event identifiers.
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To solve this, we'd need `SendQueue`, whenever it receives a remote echo, to update pending events that are replies with their
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`relatedEventId`. This already happens, and the `event_id` field in `m.relates_to` is updated. But we'd need to extend this
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to adjust the messages' `formatted_body` with the resolved remote ID, too.
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How do we safely do this, without accidentally substituting event IDs into places in the body where they were not intended?
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