only set new live key when creating a fragment after the txn succeeds

when doing a limited sync, and a new fragment is created,
this._lastLiveKey is updated immediately. If the transaction
would then fail, the fragmentId in this._lastLiveKey was incremented
but the fragment wasn't written to the store, so if sync is resumed
and would subsequently succeed, fragmentIds would be assigned to events
that don't have a corresponding fragment in the timelineFragment store.

This would throw errors when trying to load the timeline,
breaking the whole app.

This changes SyncWriter to only update this._lastLiveKey in
the emit phase, when the transactions has been committed already.

src/matrix/room/room.js

@@ -22,19 +22,20 @@ export default class Room extends EventEmitter {
 
     async persistSync(roomResponse, membership, txn) {
 		const summaryChanged = this._summary.applySync(roomResponse, membership, txn);
-		const newTimelineEntries = await this._syncWriter.writeSync(roomResponse, txn);
+		const {entries, newLiveKey} = await this._syncWriter.writeSync(roomResponse, txn);
         let removedPendingEvents;
         if (roomResponse.timeline && roomResponse.timeline.events) {
             removedPendingEvents = this._sendQueue.removeRemoteEchos(roomResponse.timeline.events, txn);
         }
-        return {summaryChanged, newTimelineEntries, removedPendingEvents};
+        return {summaryChanged, newTimelineEntries: entries, newLiveKey, removedPendingEvents};
     }
 
-    emitSync({summaryChanged, newTimelineEntries, removedPendingEvents}) {
+    emitSync({summaryChanged, newTimelineEntries, newLiveKey, removedPendingEvents}) {
         if (summaryChanged) {
             this.emit("change");
             this._emitCollectionChange(this);
         }
+        this._syncWriter.setKeyOnCompleted(newLiveKey);
         if (this._timeline) {
             this._timeline.appendLiveEntries(newTimelineEntries);
         }

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

@@ -84,23 +84,23 @@ export default class SyncWriter {
     async writeSync(roomResponse, txn) {
         const entries = [];
         const timeline = roomResponse.timeline;
-        if (!this._lastLiveKey) {
+        let currentKey = this._lastLiveKey;
+        if (!currentKey) {
             // means we haven't synced this room yet (just joined or did initial sync)
             
             // as this is probably a limited sync, prev_batch should be there
             // (but don't fail if it isn't, we won't be able to back-paginate though)
             let liveFragment = await this._createLiveFragment(txn, timeline.prev_batch);
-            this._lastLiveKey = new EventKey(liveFragment.id, EventKey.defaultLiveKey.eventIndex);
+            currentKey = new EventKey(liveFragment.id, EventKey.defaultLiveKey.eventIndex);
             entries.push(FragmentBoundaryEntry.start(liveFragment, this._fragmentIdComparer));
         } else if (timeline.limited) {
             // replace live fragment for limited sync, *only* if we had a live fragment already
-            const oldFragmentId = this._lastLiveKey.fragmentId;
+            const oldFragmentId = currentKey.fragmentId;
-            this._lastLiveKey = this._lastLiveKey.nextFragmentKey();
+            currentKey = currentKey.nextFragmentKey();
-            const {oldFragment, newFragment} = await this._replaceLiveFragment(oldFragmentId, this._lastLiveKey.fragmentId, timeline.prev_batch, txn);
+            const {oldFragment, newFragment} = await this._replaceLiveFragment(oldFragmentId, currentKey.fragmentId, timeline.prev_batch, txn);
             entries.push(FragmentBoundaryEntry.end(oldFragment, this._fragmentIdComparer));
             entries.push(FragmentBoundaryEntry.start(newFragment, this._fragmentIdComparer));
         }
-        let currentKey = this._lastLiveKey;
         if (timeline.events) {
             const events = deduplicateEvents(timeline.events);
             for(const event of events) {
@@ -110,12 +110,6 @@ export default class SyncWriter {
                 entries.push(new EventEntry(entry, this._fragmentIdComparer));
             }
         }
-        // right thing to do? if the txn fails, not sure we'll continue anyways ...
-        // only advance the key once the transaction has succeeded 
-        txn.complete().then(() => {
-            this._lastLiveKey = currentKey;
-        })
-
         // persist state
         const state = roomResponse.state;
         if (state.events) {
@@ -132,7 +126,11 @@ export default class SyncWriter {
             }
         }
 
-        return entries;
+        return {entries, newLiveKey: currentKey};
+    }
+
+    setKeyOnCompleted(newLiveKey) {
+        this._lastLiveKey = newLiveKey;
     }
 }