Clarify that /invite will respond with 200 if the user is already invited to the room

[Half-Shot]

Potentially fixes #1083

I believe this might fall within the scope of minor text clarification rather than requiring a full MSC, as it's a behavior that has been in action for a long time (for Synapse at least, and possibly other implementations?). This doesn't seek to correct the response.

Preview: https://pr1084--matrix-spec-previews.netlify.app

[turt2live]

This needs a changelog entry, and evidence that this is in fact the case please :)

[turt2live]

@Half-Shot are you able to take a look at this?

[Half-Shot]

Yep, let me sort that out.

In terms of proof, Synapse would be my evidence. Synapse has a very complex set of rules around how it manages membership but if we assume that it allows a transition of invite -> invite then the handler code will return a 200 response.

In practice you can test this yourself by doing:

# Create a room, out of band
curl -H "Authorization: Bearer $AT" https://chaotic.half-shot.uk/_matrix/client/v3/rooms/!your:room/invite --data '{"user_id": "@another:user"}'
# Note the membership of the user is now "invite"
curl -H "Authorization: Bearer $AT" https://chaotic.half-shot.uk/_matrix/client/v3/rooms/!your:room/members
# The second request returns a 200 response.
curl -H "Authorization: Bearer $AT" https://chaotic.half-shot.uk/_matrix/client/v3/rooms/!your:room/invite --data '{"user_id": "@another:user"}'

This was true in my testing:

➜  ~ curl -H "Authorization: Bearer $AT" https://chaotic.half-shot.uk/_matrix/client/v3/rooms/\!HNuxrSCzjoKTvWBIuS:half-shot.uk/invite --data '{"user_id": "@h:half-shot.uk"}'
{

}

➜  ~ curl -H "Authorization: Bearer $AT" https://chaotic.half-shot.uk/_matrix/client/v3/rooms/\!HNuxrSCzjoKTvWBIuS:half-shot.uk/members
{
    "chunk": [
...
        {
            "content": {
                "membership": "invite"
            },
            "room_id": "!HNuxrSCzjoKTvWBIuS:half-shot.uk",
            "sender": "@Half-Shot:half-shot.uk",
            "state_key": "@h:half-shot.uk",
            "type": "m.room.member",
            "age": 807733
        }
    ]
}

➜  ~ curl -H "Authorization: Bearer $AT" https://chaotic.half-shot.uk/_matrix/client/v3/rooms/\!HNuxrSCzjoKTvWBIuS:half-shot.uk/invite --data '{"user_id": "@h:half-shot.uk"}'
{

}

[turt2live]

otherwise lgtm