MSC2666: Get rooms in common with another user

proposals/2666-get-rooms-in-common.md

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+# MSC 2666: Get rooms in common with another user
+
+It is useful to be able to fetch rooms you have in common with another user. Popular messaging
+services such as Telegram offer users the ability to show "groups in common", which allows users to
+determine what they have in common before participating in conversation.
+
+There are a variety of applications for this information. Some users may want to block invites from
+users they do not share a room with at the client level, and need a way to poll the homeserver for
+this information. Another use case would be trying to determine how a user came across your MXID, as
+invites on their own do not present much context. With this endpoint, a client could tell you what
+rooms you have in common before you accept an invite.
+
+While this information can be determined if the user has full access to member state for all rooms,
+modern clients often implement
+[lazy-loading of room members](https://spec.matrix.org/v1.3/client-server-api/#lazy-loading-room-members),
+so they often only have a subset of state for the rooms the user is in. Therefore, the homeserver
+should have a means to provide this information.
+
+This proposal aims to implement a simple mechanism to fetch rooms you have in common with another
+user.
+
+## Proposal
+
+Homeservers will implement a new endpoint `/_matrix/client/v1/mutual_rooms`.
+
+This endpoint will take a query parameter of `user_id` which will contain the MXID of the user
+matched against.
+
+This endpoint can be rate limited and requires authentication.
+Guests are not allowed to use the endpoint, as guests will usually not join multiple rooms.
+
+The response format will be an array containing all rooms where both the authenticated user and
+`user_id` have a membership of type `join`.
+
+If the `user_id` does not exist, or does not share any rooms with the authenticated user,
+an empty array should be returned.
+
+Handling invalid user IDs may result in an error, is likely implementation-specific,
+and is beyond the scope of this proposal. Implementations are also free to simply
+return a success response with an empty array for invalid user IDs.
+
+```http
+GET /_matrix/client/v1/user/mutual_rooms?user_id=%40bob%3Aexample.com
+```
+
+```json
+{
+  "joined": [
+    "!OGEhHVWSdvArJzumhm:matrix.org",
+    "!HYlSnuBHTxUPgyZPKC:half-shot.uk",
+    "!DueayyFpVTeVOQiYjR:example.com"
+  ]
+}
+```
+
+The server may decide that the response to this endpoint is too large, and thus an optional key
+`"next_batch"` can be inserted, which the client has to pass to `from` in the query
+parameters together with the original `user_id` to fetch the next batch of responses. This will
+continue until the server does no longer insert `"next_batch"`. The `next_batch` field follows
+the [standard for opaque identifiers](https://spec.matrix.org/v1.16/appendices/#opaque-identifiers).
+
+```json5
+{
+  "joined": [
+    // ...
+  ],
+  "next_batch": "<an opaque identifier, containing only the characters [0-9a-zA-Z._~-], non-empty if not omitted, and at most 255 characters>"
+}
+```
+
+The batch tokens this endpoint generates are only valid for this endpoint.
+
+The response error for when the given `from` batch token is invalid will be a response with HTTP code 400,
+with `M_INVALID_PARAM` as `errcode`.
+
+The response error for trying to get shared rooms with yourself will be an HTTP code 400, with
+`M_UNKNOWN` as the `errcode`. And the error description may be "you cannot request rooms in common with yourself".
+
+Tokens generated by this endpoint must be valid for at least 10 minutes, after which, tokens can expire.
+Expired tokens must be handled similar to invalid tokens, as described above.
+
+## Potential issues
+
+Homeserver performance and storage may be impacted by this endpoint. While a homeserver already
+stores membership information for each of its users, the information may not be stored in a way that
+is readily accessible. Homeservers that have implemented
+[POST /user_directory/search](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3user_directorysearch)
+may have started some of this work, if they are limiting users to searching for users for which they
+share rooms. While this is not a given by any means, it may mean that implementations of this API
+and /search may be complimentary.
+
+## Alternatives
+
+A client which holds full and current state can already see all membership for all rooms, and thus
+determine which of those rooms contains a "join" membership for the given user_id. Clients which "lazy-load"
+however do not have this information, as they will have only synced a subset of the full membership for
+all rooms. While a client *could* pull all membership for all rooms at the point of needing this information,
+it's computationally expensive for both the homeserver and the client, as well as a bandwidth waste for constrained
+clients.
+
+## Forward-compatibility considerations
+
+There possibly comes a time where it's desirable to query mutual rooms for more-than-one other user,
+where multiple people (including the self-user) are matched against for which rooms all of them
+share.
+
+Because of that, the endpoint accepts a query parameter, however, it will only accept *one* query
+parameter for the time being. In the future this restriction can be lifted to accept multiple query
+parameters under `user_id`
+
+## Security considerations
+
+The information provided in this endpoint is already accessible to the client if it has a copy of all
+state that the user can see. This endpoint only makes it possible to get this information without having
+to request all state ahead of time.
+
+As mentioned earlier, servers should return an empty array for non-existent users to avoid leaking
+existence of other users. If necessary, servers MAY also add random delays to responses to combat
+timing attacks.
+
+## Unstable prefix
+
+The implementation MUST use `/_matrix/client/unstable/uk.half-shot.msc2666/user/mutual_rooms`.
+
+The /versions endpoint MUST include a new key in `unstable_features` with the name
+`uk.half-shot.msc2666.query_mutual_rooms`.
+
+Previous iterations of this MSC has used the following `unstable_features` key(s):
+- `uk.half-shot.msc2666.mutual_rooms`
+- `uk.half-shot.msc2666`
+
+If the value is false or the key is not present, clients MUST assume the feature is not available.
+
+Once the MSC has been merged, and the homeserver has advertised support for the Matrix version that
+this endpoint is included in, clients should use `/_matrix/client/v1/mutual_rooms` and will no
+longer need to check for the `unstable_features` flag.