/messages can't paginate from the beginning of the room forwards - MSC3567

[MadLittleMods]

As discovered in matrix-org/matrix-spec#601 (comment)

Description

In MSC3567,

The from field on the /_matrix/client/v3/rooms/{roomId}/messages
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.

But Synapse doesn't seem to behave this way:

• ✅ GET /_matrix/client/v3/rooms/{roomId}/messages?dir=b: Gets the most recent events in the room
• ❌ GET /_matrix/client/v3/rooms/{roomId}/messages?dir=f: Gets no messages
  • Expected to get the first messages in the room (oldest)

Steps to reproduce

1. GET /_matrix/client/v3/rooms/{roomId}/messages?dir=f
2. Notice that the response doesn't include any messages and appears to just be the most recent position in the timeline looking forward

   {
       "chunk": [],
       "start": "s962_0_0_0_0_0_0_0_0",
       "end": "s962_0_0_0_0_0_0_0_0"
   }
   

Version information

• Homeserver: local dev homeserver

If not matrix.org:

• Version: Most recent develop (163fd68)

• Install method: Local development setup

• Platform: macOS 12.3