feat: Add first-instance-ack event to the app.requestSingleInstanceLock() flow (#31460)

* feat: Add onFirstInstanceAck event for requestSingleInstanceLock

* Add tests

* Apply patch fix

* Add back missing docs

* Rebase

* Listen for exit earlier in test

* Rebase

spec/fixtures/api/singleton-data/main.js

@@ -1,12 +1,18 @@
 const { app } = require('electron');
 
-// Send data from the second instance to the first instance.
-const sendAdditionalData = app.commandLine.hasSwitch('send-data');
-
 app.whenReady().then(() => {
   console.log('started'); // ping parent
 });
 
+// Send data from the second instance to the first instance.
+const sendAdditionalData = app.commandLine.hasSwitch('send-data');
+
+// Prevent the default behaviour of second-instance, which sends back an empty ack.
+const preventDefault = app.commandLine.hasSwitch('prevent-default');
+
+// Send an object back for the ack rather than undefined.
+const sendAck = app.commandLine.hasSwitch('send-ack');
+
 let obj = {
   level: 1,
   testkey: 'testvalue1',
@@ -15,20 +21,45 @@ let obj = {
     testkey: 'testvalue2'
   }
 };
+let ackObj = {
+  level: 1,
+  testkey: 'acktestvalue1',
+  inner: {
+    level: 2,
+    testkey: 'acktestvalue2'
+  }
+};
+
 if (app.commandLine.hasSwitch('data-content')) {
   obj = JSON.parse(app.commandLine.getSwitchValue('data-content'));
   if (obj === 'undefined') {
     obj = undefined;
   }
 }
+if (app.commandLine.hasSwitch('ack-content')) {
+  ackObj = JSON.parse(app.commandLine.getSwitchValue('ack-content'));
+  if (ackObj === 'undefined') {
+    ackObj = undefined;
+  }
+}
+
+app.on('first-instance-ack', (event, additionalData) => {
+  console.log(JSON.stringify(additionalData));
+});
 
 const gotTheLock = sendAdditionalData
   ? app.requestSingleInstanceLock(obj) : app.requestSingleInstanceLock();
 
-app.on('second-instance', (event, args, workingDirectory, data) => {
+app.on('second-instance', (event, args, workingDirectory, data, ackCallback) => {
+  if (preventDefault) {
+    event.preventDefault();
+  }
   setImmediate(() => {
     console.log([JSON.stringify(args), JSON.stringify(data)].join('||'));
-    app.exit(0);
+    sendAck ? ackCallback(ackObj) : ackCallback();
+    setImmediate(() => {
+      app.exit(0);
+    });
   });
 });