Prevent messages from being sent if other messages have failed to send

Fixes element-hq/element-web#5408

src/client.js

@@ -1186,6 +1186,13 @@ MatrixClient.prototype.sendEvent = function(roomId, eventType, content, txnId,
         room.addPendingEvent(localEvent, txnId);
     }
 
+    // addPendingEvent can change the state to NOT_SENT if it believes
+    // that there's other events that have failed. We won't bother to
+    // try sending the event if the state has changed as such.
+    if (localEvent.status === EventStatus.NOT_SENT) {
+        return Promise.reject(new Error("Event blocked by other events not yet sent"));
+    }
+
     return _sendEvent(this, room, localEvent, callback);
 };
 

src/models/room.js

@@ -999,6 +999,10 @@ Room.prototype.addPendingEvent = function(event, txnId) {
     this._txnToEvent[txnId] = event;
 
     if (this._opts.pendingEventOrdering == "detached") {
+        if (this._pendingEventList.some(e => e.status === EventStatus.NOT_SENT)) {
+            console.warn("Setting new event's status as " + EventStatus.NOT_SENT + " due to other similar messages");
+            event.status = EventStatus.NOT_SENT;
+        }
         this._pendingEventList.push(event);
     } else {
         for (let i = 0; i < this._timelineSets.length; i++) {