User directory definition of "public room"

[timokoesters]

The spec says

[...] the homeserver MUST at a minimum consider the users the requesting user shares a room with and those who reside in public rooms (known to the homeserver).

https://spec.matrix.org/v1.2/client-server-api/#post_matrixclientv3user_directorysearch

It is unclear what is meant by "public room". Does it have to be published to the room directory? Does it have to have join_rules=public? How about knock? And history_visibility?

If this is left as an implementation detail, then that should be explicit. Currently the spec uses the term "public room" without giving a definition.

[ShadowJonathan]

Considering that some more features are starting to rely on this (such as MSC2666), it'd be useful to have a concrete spec definition, so that clients know what to expect.

[richvdh]

This is really a duplicate of #633, but in this case I know the answer: it is specifically rooms with join_rules=public, which is to say that anyone could join the room if they wanted.

But frankly it's unclear why we have chosen that definition here, rather than (say):

• extending it to rooms with world_readable history_visibility, or
• extending the directory search to cover rooms that the calling user could join (either because they have an invite, or because it has restricted join rules where the calling user meets the restrictions, cf stripped state)

If this is left as an implementation detail, then that should be explicit.

Agreed.

[richvdh]

ok, turns out I don't know the answer in this case. Per https://github.com/matrix-org/synapse/blob/v1.74.0/synapse/storage/databases/main/user_directory.py#L281 and https://github.com/matrix-org/synapse/blob/774e20b57047b9f8700e62e7f4689717f4fa094c/synapse/storage/databases/main/user_directory.py#L457-L474, Synapse considers a room to be public if it has join_rules=public or it has history_visibility=world_readable.