Add docs

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 accepts data to pass 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 +496,23 @@ 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:** In order to call the callback, first call `preventDefault` on the `event` object. Then, call the callback with the data to pass back to the second instance. Otherwise, null will be sent back.
+
 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:

shell/browser/api/electron_api_app.cc

@@ -1086,7 +1086,7 @@ void App::OnFirstInstanceAck(
   v8::HandleScope handle_scope(isolate);
   base::Value data_to_send;
   if (first_instance_data) {
-    // Don't send back the local directly, because it might be empty
+    // Don't send back the local directly, because it might be empty.
     v8::Local<v8::Value> data_local;
     data_local = DeserializeV8Value(isolate, *first_instance_data);
     if (!data_local.IsEmpty()) {
@@ -1124,7 +1124,7 @@ void App::OnSecondInstance(
   bool prevent_default =
       Emit("second-instance", cmd.argv(), cwd, data_value, cb);
   if (!prevent_default) {
-    // Call the callback ourselves and send back nothing.
+    // Call the callback ourselves, and send back nothing.
     ack_callback.Run(nullptr);
   }
 }

shell/browser/api/electron_api_app.h

@@ -196,7 +196,6 @@ class App : public ElectronBrowserClient::Delegate,
       const ProcessSingleton::NotificationAckCallback& ack_callback);
   bool HasSingleInstanceLock() const;
   bool RequestSingleInstanceLock(gin::Arguments* args);
-  v8::Local<v8::Promise> WaitSingleInstanceResponse(gin::Arguments* args);
   void ReleaseSingleInstanceLock();
   bool Relaunch(gin::Arguments* args);
   void DisableHardwareAcceleration(gin_helper::ErrorThrower thrower);