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

* Initial cut of MSC to standardize from being optiona on /messages requests.

* Fix typo.

Co-authored-by: Travis Ralston <travisr@matrix.org>

* Clarifications of start/end.

* Add information about back-paginating excluded events.

Co-authored-by: Travis Ralston <travisr@matrix.org>

proposals/3567-optional-from-on-messages.md

@@ -0,0 +1,67 @@
+# MSC3567: Allow requesting events from the start/end 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-cases include:
+
+* Requesting events using a different filter after receiving a `/sync` response.
+* 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).
+
+
+## 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 first or last (per the value of the `dir` parameter) visible
+event in the room history for the requesting user.
+
+Note that Synapse already implements this, but it is not spec-compliant. It is
+known to be used by Element Android [^1] and Element Web, and there are other
+use-cases involving threads [^2], which shows real-world usage that this would
+be valuable.
+
+Ideally this would not be necessary and the `prev_batch` token received from
+calling `/sync` could be provided as the pagination token to `/messages`, but this
+will not work if you `/sync` with a filter that excludes a given class of event
+(such as threaded replies), and all the events taking place in a room are of that
+class. This will result in your `/sync` not returning an update for that room,
+which means that your most recent `prev_batch` token precedes all the excluded
+events. Trying to back-paginate from `prev_batch` using `/messages` will not
+result in seeing the excluded events.
+
+
+## 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.
+
+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