Handle sync ordering vs back-fill ordering (see synapse #7164) resulting in dupe events

Also extra robustness while filling a gap

ignore duplicate events from synapse instead considering them an
overlapping event with the adjacent fragment

src/matrix/room/timeline/Direction.js

@@ -17,6 +17,10 @@ export default class Direction {
         return this.isForward ? "f" : "b";
     }
 
+    reverse() {
+        return this.isForward ? Direction.Backward : Direction.Forward
+    }
+
     static get Forward() {
         return _forward;
     }

src/matrix/room/timeline/EventKey.js

@@ -37,7 +37,11 @@ export default class EventKey {
     }
 
     static get defaultLiveKey() {
-        return new EventKey(Platform.minStorageKey, Platform.middleStorageKey);
+        return EventKey.defaultFragmentKey(Platform.minStorageKey);
+    }
+
+    static defaultFragmentKey(fragmentId) {
+        return new EventKey(fragmentId, Platform.middleStorageKey);
     }
 
     toString() {

src/matrix/room/timeline/persistence/GapWriter.js

@@ -10,30 +10,75 @@ export default class GapWriter {
     }
     // events is in reverse-chronological order (last event comes at index 0) if backwards
     async _findOverlappingEvents(fragmentEntry, events, txn) {
-        const eventIds = events.map(e => e.event_id);
-        let nonOverlappingEvents = events;
+        let expectedOverlappingEventId;
+        if (fragmentEntry.hasLinkedFragment) {
+            expectedOverlappingEventId = await this._findExpectedOverlappingEventId(fragmentEntry, txn);
+        }
+        let remainingEvents = events;
+        let nonOverlappingEvents = [];
         let neighbourFragmentEntry;
-        const neighbourEventId = await txn.timelineEvents.findFirstOccurringEventId(this._roomId, eventIds);
-        if (neighbourEventId) {
-            // trim overlapping events
-            const neighbourEventIndex = events.findIndex(e => e.event_id === neighbourEventId);
-            nonOverlappingEvents = events.slice(0, neighbourEventIndex);
+        while (remainingEvents && remainingEvents.length) {
+            const eventIds = remainingEvents.map(e => e.event_id);
+            const duplicateEventId = await txn.timelineEvents.findFirstOccurringEventId(this._roomId, eventIds);
+            if (duplicateEventId) {
+                const duplicateEventIndex = remainingEvents.findIndex(e => e.event_id === duplicateEventId);
+                // should never happen, just being defensive as this *can't* go wrong
+                if (duplicateEventIndex === -1) {
+                    throw new Error(`findFirstOccurringEventId returned ${duplicateEventIndex} which wasn't ` +
+                        `in [${eventIds.join(",")}] in ${this._roomId}`);
+                }
+                nonOverlappingEvents.push(...remainingEvents.slice(0, duplicateEventIndex));
+                if (!expectedOverlappingEventId || duplicateEventId === expectedOverlappingEventId) {
+                    // TODO: check here that the neighbourEvent is at the correct edge of it's fragment
                     // get neighbour fragment to link it up later on
-            const neighbourEvent = await txn.timelineEvents.getByEventId(this._roomId, neighbourEventId);
+                    const neighbourEvent = await txn.timelineEvents.getByEventId(this._roomId, duplicateEventId);
                     const neighbourFragment = await txn.timelineFragments.get(this._roomId, neighbourEvent.fragmentId);
                     neighbourFragmentEntry = fragmentEntry.createNeighbourEntry(neighbourFragment);
+                    // trim overlapping events
+                    remainingEvents = null;
+                } else {
+                    // we've hit https://github.com/matrix-org/synapse/issues/7164, 
+                    // e.g. the event id we found is already in our store but it is not
+                    // the adjacent fragment id. Ignore the event, but keep processing the ones after.
+                    remainingEvents = remainingEvents.slice(duplicateEventIndex + 1);
+                }
+            } else {
+                nonOverlappingEvents.push(...remainingEvents);
+                remainingEvents = null;
+            }
         }
         return {nonOverlappingEvents, neighbourFragmentEntry};
     }
 
-    async _findLastFragmentEventKey(fragmentEntry, txn) {
+    async _findExpectedOverlappingEventId(fragmentEntry, txn) {
+        const eventEntry = await this._findFragmentEdgeEvent(
+            fragmentEntry.linkedFragmentId,
+            // reverse because it's the oppose edge of the linked fragment
+            fragmentEntry.direction.reverse(),
+            txn);
+        if (eventEntry) {
+            return eventEntry.event.event_id;
+        }
+    }
+
+    async _findFragmentEdgeEventKey(fragmentEntry, txn) {
         const {fragmentId, direction} = fragmentEntry;
+        const event = await this._findFragmentEdgeEvent(fragmentId, direction, txn);
+        if (event) {
+            return new EventKey(event.fragmentId, event.eventIndex);
+        } else {
+            // no events yet in the fragment ... odd, but let's not fail and take the default key
+            return EventKey.defaultFragmentKey(fragmentEntry.fragmentId);
+        }
+    }
+
+    async _findFragmentEdgeEvent(fragmentId, direction, txn) {
         if (direction.isBackward) {
             const [firstEvent] = await txn.timelineEvents.firstEvents(this._roomId, fragmentId, 1);
-            return new EventKey(firstEvent.fragmentId, firstEvent.eventIndex);
+            return firstEvent;
         } else {
             const [lastEvent] = await txn.timelineEvents.lastEvents(this._roomId, fragmentId, 1);
-            return new EventKey(lastEvent.fragmentId, lastEvent.eventIndex);
+            return lastEvent;
         }
     }
 
@@ -57,8 +102,22 @@ export default class GapWriter {
         directionalAppend(entries, fragmentEntry, direction);
         // set `end` as token, and if we found an event in the step before, link up the fragments in the fragment entry
         if (neighbourFragmentEntry) {
+            // the throws here should never happen and are only here to detect client or unhandled server bugs
+            // and a last measure to prevent corrupting fragment links
+            if (!fragmentEntry.hasLinkedFragment) {
                 fragmentEntry.linkedFragmentId = neighbourFragmentEntry.fragmentId;
+            } else if (fragmentEntry.linkedFragmentId !== neighbourFragmentEntry.fragmentId) {
+                throw new Error(`Prevented changing fragment ${fragmentEntry.fragmentId} ` +
+                    `${fragmentEntry.direction.asApiString()} link from ${fragmentEntry.linkedFragmentId} ` +
+                    `to ${neighbourFragmentEntry.fragmentId} in ${this._roomId}`);
+            }
+            if (!neighbourFragmentEntry.hasLinkedFragment) {
                 neighbourFragmentEntry.linkedFragmentId = fragmentEntry.fragmentId;
+            } else if (neighbourFragmentEntry.linkedFragmentId !== fragmentEntry.fragmentId) {
+                throw new Error(`Prevented changing fragment ${neighbourFragmentEntry.fragmentId} ` +
+                    `${neighbourFragmentEntry.direction.asApiString()} link from ${neighbourFragmentEntry.linkedFragmentId} ` +
+                    `to ${fragmentEntry.fragmentId} in ${this._roomId}`);
+            }
             // if neighbourFragmentEntry was found, it means the events were overlapping,
             // so no pagination should happen anymore.
             neighbourFragmentEntry.token = null;
@@ -100,7 +159,7 @@ export default class GapWriter {
             throw new Error("start is not equal to prev_batch or next_batch");
         }
         // find last event in fragment so we get the eventIndex to begin creating keys at
-        let lastKey = await this._findLastFragmentEventKey(fragmentEntry, txn);
+        let lastKey = await this._findFragmentEdgeEventKey(fragmentEntry, txn);
         // find out if any event in chunk is already present using findFirstOrLastOccurringEventId
         const {
             nonOverlappingEvents,