introduced mutex to avoid conflicts during tab closing

src/public/app/services/tab_manager.js

@@ -7,11 +7,14 @@ import treeService from "./tree.js";
 import utils from "./utils.js";
 import NoteContext from "./note_context.js";
 import appContext from "./app_context.js";
+import Mutex from "../utils/mutex.js";
 
 export default class TabManager extends Component {
     constructor() {
         super();
 
+        this.mutex = new Mutex();
+
         this.activeNtxId = null;
 
         // elements are arrays of note contexts for each tab (one main context + subcontexts [splits])
@@ -292,6 +295,9 @@ export default class TabManager extends Component {
     }
 
     async removeNoteContext(ntxId) {
+        // removing note context is async process which can take some time, if users presses CTRL-W quickly, two
+        // close events could interleave which would then lead to attempting to activate already removed context.
+        await this.mutex.runExclusively(async () => {
             const noteContextToRemove = this.getNoteContextById(ntxId);
 
             if (noteContextToRemove.isMainContext()) {
@@ -337,6 +343,7 @@ export default class TabManager extends Component {
             this.triggerEvent('noteContextRemoved', {ntxIds: ntxIdsToRemove});
 
             this.tabsUpdate.scheduleUpdate();
+        });
     }
 
     tabReorderEvent({ntxIdsInOrder}) {

src/public/app/utils/mutex.js

@@ -0,0 +1,28 @@
+export default class Mutex {
+    constructor() {
+        this.current = Promise.resolve();
+    }
+
+    /** @returns {Promise} */
+    lock() {
+        let resolveFun;
+        const subPromise = new Promise(resolve => resolveFun = () => resolve());
+        // Caller gets a promise that resolves when the current outstanding lock resolves
+        const newPromise = this.current.then(() => resolveFun);
+        // Don't allow the next request until the new promise is done
+        this.current = subPromise;
+        // Return the new promise
+        return newPromise;
+    };
+
+    async runExclusively(cb) {
+        const unlock = await this.lock();
+
+        try {
+            await cb();
+        }
+        finally {
+            unlock();
+        }
+    }
+}