MSC2716: Incrementally importing history into existing rooms

A proposal for letting ASes specify event parents and timestamps when
submitting events, letting them much more effectively insert past
conversation history.

cc @tulir for feedback, as the main consumer of the ?ts= API today...

Author: ara4n

proposals/2716-importing-history-into-existing-rooms.md

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+# MSC2716: Incrementally importing history into existing rooms
+
+## Problem
+
+Matrix has historically been unable to easily import existing history into a
+room that already exists. This is a major problem when bridging existing
+conversations into Matrix, particularly if the scrollback is being
+incrementally or lazily imported.
+
+For instance, an NNTP bridge might work by letting a user join a room that
+maps to a given newsgroup, first showing an empty room, and then importing the
+most recent 1000 newsgroup posts for that room to flesh out some history.  The
+bridge might then choose to slowly import additional posts for that newsgroup
+in the background, until however many decades of backfill were complete.
+Finally, as more archives surface, they might also need to be manually
+gradually added into the history of the room - slowly building up the complete
+history of the conversations over time.
+
+This is currently not supported because:
+ * There is no way to set historical room state in a room via the CS or AS API -
+   you can only edit current room state.
+ * There is no way to create messages in the context of historical room state in
+   a room via CS or AS API - you can only create events relative to current room
+   state.
+ * There is currently no way to override the timestamp on an event via the AS API.
+   (We used to have the concept of [timestamp
+   massaging](https://matrix.org/docs/spec/application_service/r0.1.2#timestamp-massaging),
+   but it never got properly specified)
+
+## Proposal
+
+ 1. We let the AS API override the parent(s) of an event when injecting it into
+    the room, thus letting bridges consciously specify the topological ordering of
+    the room DAG.  We do this by adding a `parent` querystring parameter on the
+    `PUT /_matrix/client/r0/rooms/{roomId}/send/{eventType}/{txnId}` and
+    `PUT /_matrix/client/r0/rooms/{roomId}/state/{eventType}/{stateKey}` endpoints.
+    The `parent` parameter can be repeated multiple times to specify multiple parent
+    event IDs of the event being submitted.  An event must not have more than 20 parents.
+    If a `parent` parameter is not presented, the server assumes the event is being
+    appended to the current timeline and calculates the parents as normal.  If an
+    unrecognised event ID is specified as a `parent`, the request fails with a 404.
+
+ 2. We also let the AS API override ('massage') the `origin_server_ts` timestamp applied
+    to sent events.  We do this by adding a `ts` querystring parameter on the
+    `PUT /_matrix/client/r0/rooms/{roomId}/send/{eventType}/{txnId}` and
+    `PUT /_matrix/client/r0/rooms/{roomId}/state/{eventType}/{stateKey}`endpoints, specifying
+    the value to apply to `origin_server_ts` on the event (UNIX epoch milliseconds).
+
+ 3. Finally, we can add a optional `"m.historical": true` field to events to
+    indicate that they are historical at the point of being added to a room, and
+    as such servers should not serve them to clients via the CS `/sync` API -
+    instead preferring clients to discover them by paginating scrollback via
+    `/messages`.
+
+This lets history be injected at the right place topologically in the room.  For instance, different eras of the room could
+end up as branches off the original `m.room.create` event, each first setting up the contextual room state for that era before
+the block of imported history.  So, you could end up with something like this:
+
+```
+m.room.create
+     |\
+     | \___________________________________
+     |            \                        \
+     |             \                        \
+live timeline    previous 1000 messages   another block of ancient history
+```
+
+We consciously don't support the new `parent` and `ts` parameters on the
+various helper syntactic-sugar APIs like `/kick` and `/ban`.  If a bridge/bot is
+smart enough to be faking history, it is already in the business of dealing
+with raw events, and should not be using the syntactic sugar APIs.
+
+## Potential issues
+
+There are a bunch of security considerations here - see below.
+
+## Alternatives
+
+We could insist that we use the SS API to import history history in this manner rather than
+extending the AS API.  However, it seems unnecessarily burdensome to make bridge authors
+understand the SS API, especially when we already have so many AS API bridges.  Hence these
+minor extensions to the existing AS API.
+
+Another way of doing this might be to store the different eras of the room as
+different versions of the room, using `m.room.tombstone` events to form a
+linked list of the eras. This has the advantage of isolating room state
+between different eras of the room, simplifying state resolution calculations
+and avoiding risk of any cross-talk.  It's also easier to reason about, and
+avoids exposing the DAG to bridge developers.  However, it would require
+better presentation of room versions in clients, and it would require support
+for retrospectively specifying the `predecessor` of the current room when you
+retrospectively import history.  Currently `predecessor` is in the immutable
+`m.room.create` event of a room, so cannot be changed retrospectively - and
+doing so in a safe and race-free manner sounds Hard.
+
+## Security considerations
+
+This allows an AS to tie the room DAG in knots by specifying inappropriate
+event IDs as parents, potentially DoSing the state resolution algorithm, or
+triggering undesired state resolution results. This is already possible by the
+SS API today however, and given AS API requires the homeserver admin to
+explicitly authorise the AS in question, this doesn't feel too bad.
+
+This also makes it much easier for an AS to maliciously spoof history.  This
+is a bit unavoidable given the nature of the feature, and is also possible
+today via SS API.
+
+If the state changes from under us due to importing history, we have no way to
+tell the client about it.  This is an [existing
+bug](https://github.com/matrix-org/synapse/issues/4508) that can be triggered
+today by SS API traffic, so is orthogonal to this proposal.
+
+## Unstable prefix
+
+Feels unnecessary.