rotate outbound megolm session when somebody leaves the room

src/matrix/e2ee/RoomEncryption.js

@@ -45,6 +45,9 @@ export class RoomEncryption {
     }
 
     async writeMemberChanges(memberChanges, txn) {
+        if (memberChanges.some(m => m.hasLeft)) {
+            this._megolmEncryption.discardOutboundSession(this._room.id, txn);
+        }
         return await this._deviceTracker.writeMemberChanges(this._room, memberChanges, txn);
     }
 

src/matrix/e2ee/megolm/Encryption.js

@@ -26,6 +26,10 @@ export class Encryption {
         this._ownDeviceId = ownDeviceId;
     }
 
+    discardOutboundSession(roomId, txn) {
+        txn.outboundGroupSessions.remove(roomId);
+    }
+
     /**
      * Encrypts a message with megolm
      * @param  {string} roomId           

src/matrix/room/members/RoomMember.js

@@ -134,4 +134,8 @@ export class MemberChange {
     get membership() {
         return this._memberEvent.content?.membership;
     }
+
+    get hasLeft() {
+        return this.previousMembership === "join" && this.membership !== "join";
+    }
 }

src/matrix/storage/idb/stores/OutboundGroupSessionStore.js

@@ -19,6 +19,10 @@ export class OutboundGroupSessionStore {
         this._store = store;
     }
 
+    remove(roomId) {
+        this._store.delete(roomId);
+    }
+
     get(roomId) {
         return this._store.get(roomId);
     }