MSC 3840: Ignore invites

Signed-off: David Teller <davidt@element.io>

proposals/3840-ignore-invites.md

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+# MSC3840: Ignoring invites
+
+Matrix supports an event "m.ignored_user_list" to entirely ignore some users.
+
+In some cases, ignoring the user is the wrong granularity, e.g. we may wish to:
+
+- ignore invites to rooms entirely;
+- ignore invites from some servers entirely.
+
+While both features are pretty easy to implement client-side, standardizing how this preference
+is stored as part of the account is necessary to make sure that users can share this ignore list
+across devices and sessions.
+
+## Proposal
+
+
+Matrix currently supports a type `m.ignored_users_list` with content:
+
+| Content | Type | Description |
+|---------|------|-------------|
+| `ignored_users` | A maps of user ids to `{}` | A list of users to ignore |
+
+
+We adopt a similar `m.ignored_invites` with content:
+
+| Content | Type | Description |
+|---------|------|-------------|
+| `ignored_user_ids`   | optional `string[]` | Ignore invites from these users. |
+| `ignored_servers`    | optional `string[]` | Ignore invites from users in these homeservers. |
+| `ignored_room_ids`   | optional `string[]` | Ignore invites towards these rooms. |
+
+### Client behaviour
+
+If a user modifies `m.ignored_invites`, discard silently any matching pending invite
+currently displayed.
+
+### Server behaviour
+
+Wherever the server could filter out an event because of a `m.ignored_users_list`:
+- if the event is an invite; and
+- if `m.ignored_invites` is present in the recipient user's account; and
+- if the issuer is part of `ignored_user_ids` or the issuer is an account on `ignored_servers` or the invite room is part of `ignored_room_ids`, then filter out the event silently.
+
+
+The behavior of both client and server is expanded to also ignore any event issued from a server
+in `ignore_servers` wherever we would ignore an event from a user in `ignore_users`.
+
+## Potential issues
+
+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.
+
+## Alternatives
+
+Can't think of any right now.
+
+## Security considerations
+
+Can't think of any right now.
+
+## Unstable prefix
+
+During testing, `m.ignored_invites` should be prefixed `org.matrix.msc3840.ignored_invites`.
+
+Fields `ignored_user_ids`, `ignored_servers`, `ignored_room_ids` of this new event should remain unprefixed.
+