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MSC3567: Allow requesting events from the start/end of the room history #3567

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MSC3567: Allow requesting events from the start/end of the room history #3567

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6125ffc
Initial cut of MSC to standardize from being optiona on /messages req…
clokep Dec 14, 2021
074773b
Fix typo.
clokep Dec 14, 2021
af730c0
Clarifications of start/end.
clokep Dec 14, 2021
c63cd31
Add information about back-paginating excluded events.
clokep Dec 17, 2021
ca5afae
Merge remote-tracking branch 'origin/main' into clokep/messages-from
clokep Dec 28, 2021
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# MSC3567: Allow requesting events from the start/end of the room history
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Is there a reason why synapse supports pagination from the start of the room history?


It can be useful to request the latest events in a room directly without calling
`/_matrix/client/v3/sync` first to fetch the room state. Some use-casse include:
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* Requesting events using a different filter after receiving a `/sync` response.
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* A client which does not need to fully sync an account, but wishes to inspect a
specific room's history (perhaps for exporting or auditing).
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## Proposal

The `from` field on the [`/_matrix/client/v3/rooms/{roomId}/messages`](https://spec.matrix.org/v1.1/client-server-api/#get_matrixclientv3roomsroomidmessages)
becomes optional. If it is not provided, the homeserver shall return a list of
messages from the start or end (per the value of the `dir` parameter) of the
room history.
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Note that Synapse already implements this, but it is not spec-compliant. It is
known to be used by Element Android [^1], and there are other cases involving
threads [^2], which shows real-world usage that this would be valuable.
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## Potential issues

None.


## Alternatives

The alternative is today's status quo: a client must first make a request to
`/_matrix/client/v3/sync` and then follow-up that request with queries to
`/_matrix/client/v3/rooms/{roomId}/messages`. This is clunky if the client is
going to throw away most of the information received from the `/sync` request.
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The behavior also seems undefined if a different `filter` parameter is provided
for the call to `/sync` compared to the one used for `/messages`.


## Security considerations

None.


## Unstable prefix

Since this is modifying the endpoint to support not including a field, no unstable
prefix is necessary.


## Dependencies

N/A

[^1]: https://github.com/matrix-org/synapse/issues/5538

[^2]: In order to list all threads in a room without pulling the history locally
it uses `/messages` to push the filtering onto the homeserver. See https://github.com/matrix-org/matrix-js-sdk/pull/2065