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Go into more depth about membership changes surrounding knocking

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Andrew Morgan 2020-10-01 16:19:33 +01:00
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proposals/2403-knock.md
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@ -50,6 +50,62 @@ that user can be transitioned to the following possible states:
- `ban`: In this case, the knock was rejected and the user has been prevented
from sending further knocks.
Let's talk about each one of these in more detail.
### Membership change to `invite`
The knock has been accepted by someone.
This user must have the power level to perform invites. The user's homeserver
will then send an invite - over federation if necessary - to the knocking
user. The knocking user may then join the room as if they had been invited
normally.
### Membership change to `leave`
The knock has been rejected by someone in the room.
This is made a bit tricky in that it is very difficult to have knock
rejections, aka leave events from the room directed towards you, propagate
over federation if you're not in the room at the same time. This is a problem
that currently affects other similar operations, such as disinviting or
unbanning a federated user. In both cases, they won't be notified as their homeserver is not in the room.
While we could send easily send the leave event as part of a generic
transaction to the remote homeserver, that homeserver would have no way to
validate the `prev_events` and `auth_events` that the event references. We
could send those events over as well, but those will also reference other
events that require validation and so on.
A dumb thing we could easily do here is to trust the leave event implicitly
if it is sent by the homeserver we originally knocked through. We know this
homeserver is (or at least was) in the room, so they're probably telling the
truth. This is almost an edge case though, as while you'll knock through one
homeserver in the room, there's no guarantee that the admin that denies your
knock will be on the same homeserver you knocked through. Perhaps the homeserver you knocked through could listen for this and then send the event back to you - but what if it goes offline in the meantime?
As such, this feature working over federation should be de-scoped for now,
and left to a future MSC which can solve this problem across the board for
all affected features in a proper way. Rejections should still work for the
homeservers that are in the room however.
XXX: There is also an open question here about who should be able to reject a
knock. To disinvite a user, perhaps counter-intuitively, you need to have a
high enough power level to kick users, rather than invite them. You also need
to have a higher power level than them. Should the same be done for knocking,
assuming the knocking user has the default power level?
### Membership change to `ban`
The knock has been rejected by someone.
This one is fairly straightforward. Someone with the appropriate power levels
in the room bans the user. This will have the same effect as rejecting the
knock, and in addition prevent any further knocks by this user from being
allowed into the room.
If the user is unbanned, then knocks will be accepted again.
## Client-Server API
Two new endpoints are introduced in the Client-Server API (similarly to
join): `POST /_matrix/client/r0/rooms/{roomId}/knock` and `POST