MSC3925: m.replace aggregation with full event (#3925)
* init m.replace aggregation with full event Signed-off-by: benkuly <kontakt@folivo.net> * Rename tmp.md to 3925-replace-aggregation-with-full-event.md Signed-off-by: benkuly <kontakt@folivo.net> * Update 3925-replace-aggregation-with-full-event.md typos Signed-off-by: benkuly <kontakt@folivo.net> * Update 3925-replace-aggregation-with-full-event.md added MSC number Signed-off-by: benkuly <kontakt@folivo.net> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Jonas Platte <jplatte+git@posteo.de> Signed-off-by: benkuly <kontakt@folivo.net> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Jonas Platte <jplatte+git@posteo.de> Signed-off-by: benkuly <kontakt@folivo.net> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * redact instead of delete * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * word wrap 80 * remove immutable argument * add alternative from https://github.com/matrix-org/matrix-spec/issues/1299#issuecomment-1290332433 * add json example * shorter version of the actual proposal Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * describe the actual reason, why encrypted events cannot be replaced Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * mention discussion * typo * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * clarify inconsistent behavior with replaced contents * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * Update proposals/3925-replace-aggregation-with-full-event.md Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com> * clarify inconsistent behavior and put it into introduction * remove superfluous event content field * add some details to potential issues suggested by @richvdh --------- Signed-off-by: benkuly <kontakt@folivo.net> Co-authored-by: Jonas Platte <jplatte+git@posteo.de> Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>pull/3972/head
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# MSC3925: m.replace aggregation with full event
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As [currently specified](https://spec.matrix.org/v1.4/client-server-api/#server-side-replacement-of-content),
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servers should replace the content of events that have been replaced via an `m.replace`
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relation.
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There are some issues with this requirement:
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* Changing the fundamental concept of mostly immutable events is confusing. The server
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can respond with different event contents for the same `event_id`.
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* If an event with `m.replace` relation is redacted, clients need to
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detect if the original content was replaced, and possibly need to fetch the
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original content.
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* Servers cannot replace the original event content for encrypted events (because the
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replacement content is inside the encrypted body).
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See [matrix-spec#1299](https://github.com/matrix-org/matrix-spec/issues/1299).
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* Replacing the content can lead to inconsistent behavior on clients which don't
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support replacing events.
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Assume that we have this timeline on the server:
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- `{"event_id": "E1", "content": {"body": "1"}}`
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- `{"event_id": "E2", "content": {"body": "* 2", "m.new_content": {"body": "2"}}` // replaces `E1`
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A `/sync` is done after `E2`. We will have this timeline on a client:
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- `{"event_id": "E1", "content": {"body": "2"}}`
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- `{"event_id": "E2", "content": {"body": "* 2"}`
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Although `2` is a replaced body it does not have a `*`. This looks to the
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user as if it is the original content of the event.
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## Proposal
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The following two changes are proposed:
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1. The [server-side aggregation of `m.replace` relationships](https://spec.matrix.org/v1.4/client-server-api/#server-side-aggregation-of-mreplace-relationships)
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is extended to be the entire content of the most recent replacement event, formatted
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as described in [Room Event Format](https://spec.matrix.org/v1.4/client-server-api/#room-event-format).
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This ensures that the client will always have the most recent edit without having to
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fetch it from the server.
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2. Servers should no longer replace the original content of an event as described
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at https://spec.matrix.org/v1.4/client-server-api/#server-side-replacement-of-content.
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For an original event:
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```json5
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{
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"event_id": "$original_event",
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"type": "m.room.message",
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"content": {
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"body": "I really like cake",
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"msgtype": "m.text",
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"formatted_body": "I really like cake"
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}
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// irrelevant fields not shown
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}
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```
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With a replacing event:
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```json5
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{
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"event_id": "$edit_event",
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"type": "m.room.message",
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"content": {
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"body": "* I really like *chocolate* cake",
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"msgtype": "m.text",
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"m.new_content": {
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"body": "I really like *chocolate* cake",
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"msgtype": "m.text"
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},
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"m.relates_to": {
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"rel_type": "m.replace",
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"event_id": "$original_event_id"
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}
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}
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// irrelevant fields not shown
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}
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```
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This is how the original event would look like after the replacement:
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```json5
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{
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"event_id": "$original_event",
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"type": "m.room.message",
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"content": {
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"body": "I really like cake",
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"msgtype": "m.text",
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"formatted_body": "I really like cake"
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},
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"unsigned": {
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"m.relations": {
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"m.replace": {
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"event_id": "$edit_event",
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"type": "m.room.message",
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"content": {
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"body": "* I really like *chocolate* cake",
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"msgtype": "m.text",
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"m.new_content": {
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"body": "I really like *chocolate* cake",
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"msgtype": "m.text"
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},
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"m.relates_to": {
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"rel_type": "m.replace",
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"event_id": "$original_event_id"
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}
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}
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// irrelevant fields not shown
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}
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}
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}
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// irrelevant fields not shown
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}
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```
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## Potential issues
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* There could be clients which rely on the current behavior:
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* element-web relied on it until [matrix-org/matrix-js-sdk#3045](https://github.com/matrix-org/matrix-js-sdk/pull/3045)
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* The failure mode is: suppose we have an event E, which was subsequently replaced by an event E'. Now, if jumped back to a bit of timeline that contains E but not E', then clients unaware of this change will show the original event E rather than the edited content E'. But: That is an edge-case and it already happens for events other than `m.room.message` (including encrypted events) due to [matrix-org/synapse#12503](https://github.com/matrix-org/synapse/issues/12503).
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* It will be harder for clients which do not support replacing events to get
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the current content of an event: currently they can just look at `content.body`.
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However, there is an inconsistent behavior for clients that do not support
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replacing events (see above).
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## Alternatives
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One [suggestion](https://github.com/matrix-org/matrix-spec/issues/1299#issuecomment-1290332433) is
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to have the server return the complete replacement event instead of the original event. However, that would
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break compatibility with existing clients and is a more invasive change.
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## Security considerations
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None currently foreseen.
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## Unstable prefix
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No unstable prefix is currently proposed.
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