feat: Add first-instance-ack event to the `app.requestSingleInstanc…

…eLock()` flow (electron#31460)

* feat: Add onFirstInstanceAck event for requestSingleInstanceLock

* Add tests

* Apply patch fix

* Add back missing docs

* Rebase

* Listen for exit earlier in test

* Rebase

docs/api/app.md

@@ -484,6 +484,7 @@ Returns:
 * `argv` string[] - An array of the second instance's command line arguments
 * `workingDirectory` string - The second instance's working directory
 * `additionalData` unknown - A JSON object of additional data passed from the second instance
+* `ackCallback` unknown - A function that can be used to send data back to the second instance
 
 This event will be emitted inside the primary instance of your application
 when a second instance has been executed and calls `app.requestSingleInstanceLock()`.
@@ -495,12 +496,35 @@ non-minimized.
 
 **Note:** If the second instance is started by a different user than the first, the `argv` array will not include the arguments.
 
+**Note:** `ackCallback` allows the user to send data back to the
+second instance during the `app.requestSingleInstanceLock()` flow.
+This callback can be used for cases where the second instance
+needs to obtain additional information from the first instance
+before quitting.
+
+Currently, the limit on the message size is kMaxMessageLength,
+or around 32kB. To be safe, keep the amount of data passed to 31kB at most.
+
+In order to call the callback, `event.preventDefault()` must be called, first.
+If the callback is not called in either case, `null` will be sent back.
+If `event.preventDefault()` is not called, but `ackCallback` is called
+by the user in the event, then the behaviour is undefined.
+
 This event is guaranteed to be emitted after the `ready` event of `app`
 gets emitted.
 
 **Note:** Extra command line arguments might be added by Chromium,
 such as `--original-process-start-time`.
 
+### Event: 'first-instance-ack'
+
+Returns:
+
+* `event` Event
+* `additionalData` unknown - A JSON object of additional data passed from the first instance, in response to the first instance's `second-instance` event.
+
+This event will be emitted within the second instance during the call to `app.requestSingleInstanceLock()`, when the first instance calls the `ackCallback` provided by the `second-instance` event handler.
+
 ## Methods
 
 The `app` object has the following methods:
@@ -959,21 +983,33 @@ starts:
 const { app } = require('electron')
 let myWindow = null
 
+app.on('first-instance-ack', (event, additionalData) => {
+  // Print out the ack received from the first instance.
+  // Note this event handler must come before the requestSingleInstanceLock call.
+  // Expected output: '{"myAckKey":"myAckValue"}'
+  console.log(JSON.stringify(additionalData))
+})
+
 const additionalData = { myKey: 'myValue' }
 const gotTheLock = app.requestSingleInstanceLock(additionalData)
 
 if (!gotTheLock) {
   app.quit()
 } else {
   app.on('second-instance', (event, commandLine, workingDirectory, additionalData) => {
+    // We must call preventDefault if we're sending back data.
+    event.preventDefault()
     // Print out data received from the second instance.
-    console.log(additionalData)
+    // Expected output: '{"myKey":"myValue"}'
+    console.log(JSON.stringify(additionalData))
 
     // Someone tried to run a second instance, we should focus our window.
     if (myWindow) {
       if (myWindow.isMinimized()) myWindow.restore()
       myWindow.focus()
     }
+    const ackData = { myAckKey: 'myAckValue' }
+    ackCallback(ackData)
   })
 
   // Create myWindow, load the rest of the app, etc...

patches/chromium/.patches

@@ -106,8 +106,8 @@ chore_do_not_use_chrome_windows_in_cryptotoken_webrequestsender.patch
 process_singleton.patch
 fix_expose_decrementcapturercount_in_web_contents_impl.patch
 add_ui_scopedcliboardwriter_writeunsaferawdata.patch
-feat_add_data_parameter_to_processsingleton.patch
 mas_gate_private_enterprise_APIs.patch
 load_v8_snapshot_in_browser_process.patch
 fix_patch_out_permissions_checks_in_exclusive_access.patch
 fix_aspect_ratio_with_max_size.patch
+feat_add_data_transfer_to_requestsingleinstancelock.patch