Unit tests for GapWriter, using a new timeline mock utility

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

@@ -47,28 +47,30 @@ export class GapWriter {
                 }
                 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, duplicateEventId);
-                    if (neighbourEvent.fragmentId === fragmentEntry.fragmentId) {
-                        log.log("hit #160, prevent fragment linking to itself", log.level.Warn);
-                    } else {
+                    // Only link fragment if this is the first overlapping fragment we discover.
+                    // TODO is this sufficient? Might we get "out of order" fragments from events?
+                    if (!neighbourFragmentEntry) {
+                        // 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, 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);
-                }
+                } 
+                // If more events remain, or if this wasn't the expected overlapping event,
+                // 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;
             }
         }
+        if (neighbourFragmentEntry?.fragmentId === fragmentEntry.fragmentId) {
+            log.log("hit #160, prevent fragment linking to itself", log.level.Warn);
+            neighbourFragmentEntry = null;
+        } 
         return {nonOverlappingEvents, neighbourFragmentEntry};
     }