MSC: Inhibit profile propagation

proposals/0000-inhibit-profile-propagation.md

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+# MSC0000: Inhibit profile propagation
+
+The current [`PUT /avatar_url`](https://spec.matrix.org/v1.8/client-server-api/#put_matrixclientv3profileuseridavatar_url)
+and [`PUT /displayname`](https://spec.matrix.org/v1.8/client-server-api/#put_matrixclientv3profileuseriddisplayname)
+endpoints modify the "global" profile for the user. This profile is typically exposed through the
+user directory and other similar room-less places. Servers are additionally
+[required to](https://spec.matrix.org/v1.8/client-server-api/#events-on-change-of-profile-information)
+update the `m.room.member` (and similar) events for the user with the new profile information.
+
+Some applications prefer to manually manage the profile updates in-room and over presence, as they
+may need to enable per-space, per-room, or other sufficiently limited profile updates. Instead of
+waiting for the server to update the user's profile everywhere before "fixing" the events, it could
+ask the server to not update the events in the first place. This proposal introduces an optional
+flag to enable that exact functionality.
+
+A somewhat more niche usage of this MSC might be a project like [matrix-media-repo](https://github.com/turt2live/matrix-media-repo)
+aiming to implement [MSC3911](https://github.com/matrix-org/matrix-spec-proposals/pull/3911): when
+the user's avatar URL is changed, the media repo can intercept the request, proxy it forward to the
+homeserver as inhibited, then manually copy the media enough times to update all the user's joined
+rooms. This approach would reduce the number of custom endpoints required by a homeserver for MMR
+integration.
+
+## Proposal
+
+The following two endpoints gain an *optional* new query parameter: `propagate`. The value MUST be a
+boolean, and defaults to `true` if not provided. If the value is *not* a boolean, a normal 400 Bad
+Request error code is returned.
+
+* [`PUT /avatar_url`](https://spec.matrix.org/v1.8/client-server-api/#put_matrixclientv3profileuseridavatar_url)
+* [`PUT /displayname`](https://spec.matrix.org/v1.8/client-server-api/#put_matrixclientv3profileuseriddisplayname)
+
+With respect to [MSC3911](https://github.com/matrix-org/matrix-spec-proposals/pull/3911), when the
+user updates their profile's avatar with `?propagate=false`, the media they use is *not* cloned nor
+used in event updates. The client can manually clone the media if they prefer using MSC3911's copy
+API.
+
+Some servers use different rate limits depending on whether the client is using the profile APIs or
+state APIs. This proposal suggests no changes to the rate limits, though servers *may* wish to apply
+intelligent rate limits to state event changes. For example, detecting that the user is updating their
+profile information in a subset of rooms shortly after updating their global profile.
+
+## Potential issues
+
+This adds a bit of API bloat for a relatively uncommon feature, though may enable other features to
+be more readily possible. For example, a client *introducing* per-space memberships might use this
+new query parameter to make it easier to manually apply `m.room.member` event changes.
+
+## Alternatives
+
+Currently Discord bridges will update the global user profile, wait for that to complete (which can
+take minutes), then manually fix all of the `m.room.member` events for the user to accomplish proper
+platform bridging. While this does work, it is often slow and unreliable when the server takes an
+eternity to complete the profile change request due to a large number of rooms needing updates. Some
+servers, like t2bot.io, instead decide to [patch](https://github.com/t2bot/synapse/commit/049cacfc2d9e98fc602b85978c70363ce3c4f52f)
+their server software to enforce this MSC's behaviour with a more blunt instrument.
+
+## Security considerations
+
+No major security considerations required. This MSC effectively reduces spam, though may affect a
+user's perceivable privacy. Clients should be careful to explain *where* a user's profile updates
+will be visible, regardless of this MSC.
+
+## Unstable prefix
+
+While this MSC is not considered stable, clients can use `?org.matrix.msc0000.propagate` instead of
+`?propagate`. To ensure the server supports the functionality before a spec release, clients should
+look for the unstable feature `org.matrix.msc0000`.
+
+Once this MSC has successfully passed a merge FCP, clients can (and should) send *both* `?propagate`
+and `?org.matrix.msc0000.propagate` to ensure backwards compatibility with slightly old servers.
+
+Once released in the specification, clients should be checking for server support through advertised
+spec versions instead.
+
+## Dependencies
+
+This MSC has no dependencies.