MSC3847: Ignoring invites with policy rooms

Author: Yoric

proposals/3840-ignore-invites.md

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+# MSC3847: Ignoring invites using individual policy rooms
+
+Receiving unwanted invites is something that users currently need to live
+with on the Matrix network. To alleviate this, Matrix needs a mechanism to
+let users specify that they are not interested in invites from specific
+users or towards specific rooms.
+
+In this proposal, we introduce the necessary extensions to let users do
+this and to let clients perform filtering of unwanted invites.
+
+We proceed as follows:
+
+1. We build upon the mechanism of policy rooms, defined in MSC2313, and define
+a user's individual policy room, which may be created on behalf of the user by
+a Matrix client, and which is shared across all devices and clients of the user.
+2. We build upon the mechanism of recommendations, also defined in MSC2313,
+and create a new recommendation for ignoring invites from a specific user, from
+a specific server or towards a specific room.
+
+
+## Proposal
+
+### Background
+
+MSC2313 defines policy rooms. A policy room is a room in which rules such
+as the following may be published:
+
+```jsonc
+{
+    "type": "m.policy.rule.user", // Or `m.policy.rule.server` or `m.policy.rule.room`.
+    "state_key": "rule_1",        // Arbitrary.
+    "content": {
+        "entity": "@alice:example.org",
+        "recommendation": "m.ban",
+        "reason": "undesirable behaviour"
+    }
+}
+```
+
+Policy rooms are designed to be associated with entire servers, communities,
+individual rooms or individual users, but there is no specification in MSC2313
+clarifying how to associate an issuer with a policy room.
+
+### Associating a policy room with a user
+
+For individual users, we introduce a new event `m.policies`, designed
+to be used as part of account data. This event has content:
+
+| Content | Type | Description |
+|---------|------|-------------|
+| `room`  | Room ID or Alias | The main room in which a user may publish policies to be applied on their behalf. |
+
+We expect that future MSCs will expand upon this event `m.policies` and add
+other rooms where policies are published by other users or communities but
+that the current user also wish to apply, e.g. for distributing trust. This
+is, however, beyond the scope of the current proposal.
+
+The expected behavior is that if a user Alice has a `m.policies` with `room` R,
+then:
+
+- whenever Alice issues a new policy from their client or another trusted agent,
+    this policy will be stored in room R;
+- any client or trusted agent acting on behalf of Alice will monitor room R for
+    new policies and apply the recommendations on behalf of Alice.
+
+### A recommendation for ignoring invites
+
+We expand the **`enum`** `recommendation` with the following value
+
+| Value | Description |
+|---------|-------------|
+| `m.invites.ignore`  | The user's client should not display any invite from/to the entity specified in `entity`. |
+
+In particular, if Alice has a policy with `recommendation` `m.invites.ignore`:
+
+- if `type` is `m.policy.rule.user` and `entity` is Bob's User ID, Alice's clients will not display any invite issued by Bob;
+- if `type` is `m.policy.rule.room` and `entity` is the Room ID or alias of room Bobroom, Alice's clients will not display any invite issued to Bobroom;
+- if `type` is `m.policy.rule.server` and `entity` is the server name of server Bobverse, Alice's clients will not display any invite issued from any account from Bobverse or towards a room alias on the Bobverse.
+
+### Client behaviour
+
+If a new policy `m.invites.ignore` appears in Alice's individual policy room:
+
+- any pending invite currently displayed that matches the `entity` is removed from display;
+- any incoming invite that matches the `entity` is not displayed among invites.
+
+However, clients are expected to offer the ability to look at any ignored invite,
+in a manner similar to a trashcan/recycle bin/undelete mechanism for desktop file
+systems.
+
+Similarly, if a policy `m.invites.ignore` is redacted/amended, clients should show any
+invite that doesn't match any `m.invites.ignore` entity anymore.
+
+### Server behavior
+
+As recommended in MSC2313, if a policy `m.ban` appears in Alice's individual policy room:
+
+- if `type` is `m.policy.rule.user`, ignore any message or invite from the user `entity`, as per `m.ignored_users`;
+- if `type` is `m.policy.rule.room`, ignore any message in the room or invite from the room `entity`;
+- if `type` is `m.policy.rule.server`, ignore any message in any room on server `entity`, any message from any user on server `entity`, any invite towards any room on server `entity`, any invite from any user on server `entity`.
+
+## Potential issues
+
+### Number of events
+
+There is a risk that the list of ignored invites of some users may grow a lot, which might have
+performance impact, both during initial sync and during filtering. We believe that this risk is
+limited. If necessary, clients may decide to cleanup ignored invites after some time.
+
+### Sync size
+
+With this proposal, any invite ignored with `m.invites.ignore` will still show up at each `/sync`.
+In time, this can grow expensive.
+
+If necessary, clients may decide to convert ignored invites into rejected invites or `m.ban`
+after a while.
+
+## Alternatives
+
+### Server-side filtering
+
+Just as `m.ignored_users_list` is handled mostly on the server, we could handle `m.invites.ignore`
+largely on the server. However, this would make it much harder to undo erroneous ignores (i.e.
+implementing some sort of recovery from trashcan) on the client.
+
+So we prefer handling things on the client, even if this may require more traffic.
+
+## Security considerations
+
+Can't think of any right now.
+
+## Unstable prefix
+
+During testing:
+
+- `m.invites.ignore` should be prefixed `org.matrix.msc3847.invites.ignore`;
+- `m.policies` should be prefixed `org.matrix.msc3847.policies`.