Use a cross-process lock to signal port destruction. (#34)

We used an `unload` listener for this before, but this event is both
deprecated and unreliable.  Instead, switch to using an exclusive lock
that the parent process acquires first and releases only when the tab
is destroyed.  The worker tries to acquire the same lock on port init
and assumes the process on the other side of the port has been
destroyed once the lock is successfully acquired.

Author: pkaminski

dist/firetruss.es2015.js

@@ -1,6 +1,6 @@
 {
   "name": "firetruss",
-  "version": "7.6.5",
+  "version": "7.7.0",
   "description": "Advanced data sync layer for Firebase and Vue.js",
   "scripts": {
     "update": "yarn upgrade-interactive --latest",

dist/firetruss.es2015.js.map

@@ -1,7 +1,7 @@
 import {unescapeKey} from './utils/paths.js';
 import _ from 'lodash';
 
-const MIN_WORKER_VERSION = '3.0.0';
+const MIN_WORKER_VERSION = '4.0.0';
 
 
 class Snapshot {
@@ -51,10 +51,9 @@ export default class Bridge {
     this._shared = !!webWorker.port;
     Object.seal(this);
     this._port.onmessage = this._receive.bind(this);
-    window.addEventListener('unload', () => {this._send({msg: 'destroy'});});
   }
 
-  init(webWorker, config) {
+  init(lockName, config) {
     const items = [];
     try {
       const storage = window.localStorage || window.sessionStorage;
@@ -66,7 +65,7 @@ export default class Bridge {
     } catch {
       // Some browsers don't like us accessing local storage -- nothing we can do.
     }
-    return this._send({msg: 'init', storage: items, config}).then(response => {
+    return this._send({msg: 'init', storage: items, config, lockName}).then(response => {
       const workerVersion = response.version.match(/^(\d+)\.(\d+)\.(\d+)(-.*)?$/);
       if (workerVersion) {
         const minVersion = MIN_WORKER_VERSION.match(/^(\d+)\.(\d+)\.(\d+)(-.*)?$/);

dist/firetruss.umd.js

@@ -241,11 +241,15 @@ export default class Truss {
     if (_.isString(webWorker)) {
       const Worker = window.SharedWorker || window.Worker;
       if (!Worker) throw new Error('Browser does not implement Web Workers');
+      if (!navigator.locks) throw new Error('Browser does not implement locks');
       webWorker = new Worker(webWorker);
+      const random = window.crypto.getRandomValues(new Uint32Array(1))[0];
+      webWorker.lockName = `truss_worker_lock_${Date.now()}.${random}`;
+      navigator.locks.request(webWorker.lockName, () => new Promise(_.noop));
     }
     bridge = new Bridge(webWorker);
     if (logging) bridge.enableLogging(logging);
-    return bridge.init(webWorker, config).then(
+    return bridge.init(webWorker.lockName, config).then(
       ({exposedFunctionNames, firebaseSdkVersion}) => {
         Object.defineProperty(Truss, 'FIREBASE_SDK_VERSION', {value: firebaseSdkVersion});
         for (const name of exposedFunctionNames) Truss.preExpose(name);