MSC2676: Message editing

proposals/2676-message-editing.md

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+# MSC2676: Message editing
+
+Users may wish to edit previously sent messages, for example to correct typos.
+This can be done by sending a new message with an indication that it replaces
+the previously sent message.
+
+This proposal is one in a series of proposals that defines a mechanism for
+events to relate to each other.  Together, these proposals replace
+[MSC1849](https://github.com/matrix-org/matrix-doc/pull/1849).
+
+* [MSC2674](https://github.com/matrix-org/matrix-doc/pull/2674) defines a
+  standard shape for indicating events which relate to other events.
+* [MSC2675](https://github.com/matrix-org/matrix-doc/pull/2675) defines APIs to
+  let the server calculate the aggregations on behalf of the client, and so
+  bundle the related events with the original event where appropriate.
+* This proposal defines how users can edit messages using this mechanism.
+* [MSC2677](https://github.com/matrix-org/matrix-doc/pull/2677) defines how
+  users can annotate events, such as reacting to events with emoji, using this
+  mechanism.
+
+## Proposal
+
+A new `rel_type` of `m.replace` is defined for use with the `m.relates_to`
+field as defined in
+[MSC2674](https://github.com/matrix-org/matrix-doc/pull/2674).  This is
+intended primarily for handling edits, and lets you define an event which
+replaces an existing event.  When aggregated (as in
+[MSC2675](https://github.com/matrix-org/matrix-doc/pull/2675)), returns the
+most recent replacement event (as determined by `origin_server_ts`). The
+replacement event must contain an `m.new_content` which defines the replacement
+content (allowing the normal `body` fields to be used for a fallback for
+clients who do not understand replacement events).
+
+For instance, an `m.room.message` which replaces an existing event looks like:
+
+```json
+{
+    "type": "m.room.message",
+    "content": {
+        "body": "s/foo/bar/",
+        "msgtype": "m.text",
+        "m.new_content": {
+            "body": "Hello! My name is bar",
+            "msgtype": "m.text"
+        },
+        "m.relates_to": {
+            "rel_type": "m.replace",
+            "event_id": "$some_event_id"
+        }
+    }
+}
+```
+
+The `m.new_content` includes any fields that would normally be found in an
+event's `content` field, such as `formatted_body`.  In addition, the `msgtype`
+field need not be the same as in the original event.  For example, if a user
+intended to send a message beginning with "/me", but their client sends an
+`m.emote` event instead, they could edit the message to send be an `m.text`
+event as they had originally intended.
+
+Permalinks to edited events should capture the event ID that the sender is
+viewing at that point (which might be an edit ID).  The client viewing the
+permalink should resolve this ID to the source event ID, and then display the
+most recent version of that event.
+
+### Redactions
+
+When a message using a `rel_type` of `m.replace` is redacted, it removes that
+edit revision.
+
+In the UI, the act of redacting an edited message in the timeline should
+remove the message entirely from the timeline.  It can do this by redacting the
+original msg, while ensuring that clients locally discard any edits to a
+redacted message on receiving a redaction.
+
+When a specific revision of an event is redacted, the client should manually
+refresh the parent event via `/events` to grab whatever the replacement
+revision is.
+
+## Edge Cases
+
+How do you handle racing edits?
+ * The edits could form a DAG of relations for robustness.
+    * Tie-break between forward DAG extremities based on origin_ts
+    * this should be different from the target event_id in the relations, to
+      make it easier to know what is being replaced.
+    * hard to see who is responsible for linearising the DAG when receiving.
+      Nasty for the client to do it, but the server would have to buffer,
+      meaning relations could get stuck if an event in the DAG is unavailable.
+ * ...or do we just always order by on origin_ts, and rely on a social problem
+   for it not to be abused?
+    * problem is that other relation types might well need a more robust way of
+      ordering. XXX: can we think of any?
+    * could add the DAG in later if it's really needed?
+    * the abuse vector is for a malicious moderator to edit a message with origin_ts
+      of MAX_INT. the mitigation is to redact such malicious messages, although this
+      does mean the original message ends up being vandalised... :/
+ * Conclusion: let's do it for origin_ts as a first cut, but use event shapes which
+   could be switched to DAG in future is/as needed.  Good news is that it only
+   affects the server implementation; the clients can trust the server to linearise
+   correctly.
+
+What can we edit?
+ * Only non-state events for now.
+ * We can't change event types, or anything else which is in an E2E payload
+ * We can't change relation types either.
+
+How do diffs work on edits if you are missing intermediary edits?
+ * We just have to ensure that the UI for visualising diffs makes it clear
+   that diffs could span multiple edits rather than strictly be per-edit-event.
+
+What happens when we edit a reply?
+ * We just send an m.replace which refers to the m.reference target; nothing
+   special is needed.  i.e. you cannot change who the event is replying to.
+ * The edited reply should ditch the fallback representation of the reply itself
+   however from `m.new_content` (specifically the `<mx-reply>` tag in the
+   HTML, and the chevron prefixed text in the plaintext which we don't know
+   whether to parse as we don't know whether this is a reply or not), as we
+   can assume that any client which can handle edits can also display replies
+   natively.
+
+What power level do you need to be able to edit other people's messages, and how
+does it fit in with federation event auth rules?
+ * 50, by default?
+
+    XXX: Synapse doesn't impose this currently - it lets anyone send an edit,
+    but then filters them out of bundled data.
+
+"Editing other people's messages is evil; we shouldn't allow it"
+ * Sorry, we have to bridge with systems which support cross-user edits.
+ * When it happens, we should make it super clear in the timeline that a message
+   was edited by a specific user.
+ * We do not recommend that native Matrix clients expose this as a feature.
+
+## Future considerations
+
+In future we may wish to consider ordering replacements (or relations in
+general) via a DAG rather than using `origin_server_ts` to determine ordering -
+particularly to mitigate potential abuse of edits applied by moderators.
+Whatever, care must be taken by the server to ensure that if there are multiple
+replacement events, the server must consistently choose the same one as all
+other servers.