feat: MessagePorts in the main process (#22404)

docs/api/ipc-renderer.md

@@ -57,7 +57,7 @@ Removes all listeners, or those of the specified `channel`.
 
 Send an asynchronous message to the main process via `channel`, along with
 arguments. Arguments will be serialized with the [Structured Clone
-Algorithm][SCA], just like [`postMessage`][], so prototype chains will not be
+Algorithm][SCA], just like [`window.postMessage`][], so prototype chains will not be
 included. Sending Functions, Promises, Symbols, WeakMaps, or WeakSets will
 throw an exception.
 
@@ -68,6 +68,10 @@ throw an exception.
 The main process handles it by listening for `channel` with the
 [`ipcMain`](ipc-main.md) module.
 
+If you need to transfer a [`MessagePort`][] to the main process, use [`ipcRenderer.postMessage`](#ipcrendererpostmessagechannel-message-transfer).
+
+If you want to receive a single response from the main process, like the result of a method call, consider using [`ipcRenderer.invoke`](#ipcrendererinvokechannel-args).
+
 ### `ipcRenderer.invoke(channel, ...args)`
 
 * `channel` String
@@ -77,7 +81,7 @@ Returns `Promise<any>` - Resolves with the response from the main process.
 
 Send a message to the main process via `channel` and expect a result
 asynchronously. Arguments will be serialized with the [Structured Clone
-Algorithm][SCA], just like [`postMessage`][], so prototype chains will not be
+Algorithm][SCA], just like [`window.postMessage`][], so prototype chains will not be
 included. Sending Functions, Promises, Symbols, WeakMaps, or WeakSets will
 throw an exception.
 
@@ -102,6 +106,10 @@ ipcMain.handle('some-name', async (event, someArgument) => {
 })
 ```
 
+If you need to transfer a [`MessagePort`][] to the main process, use [`ipcRenderer.postMessage`](#ipcrendererpostmessagechannel-message-transfer).
+
+If you do not need a respons to the message, consider using [`ipcRenderer.send`](#ipcrenderersendchannel-args).
+
 ### `ipcRenderer.sendSync(channel, ...args)`
 
 * `channel` String
@@ -111,7 +119,7 @@ Returns `any` - The value sent back by the [`ipcMain`](ipc-main.md) handler.
 
 Send a message to the main process via `channel` and expect a result
 synchronously. Arguments will be serialized with the [Structured Clone
-Algorithm][SCA], just like [`postMessage`][], so prototype chains will not be
+Algorithm][SCA], just like [`window.postMessage`][], so prototype chains will not be
 included. Sending Functions, Promises, Symbols, WeakMaps, or WeakSets will
 throw an exception.
 
@@ -127,6 +135,35 @@ and replies by setting `event.returnValue`.
 > last resort. It's much better to use the asynchronous version,
 > [`invoke()`](ipc-renderer.md#ipcrendererinvokechannel-args).
 
+### `ipcRenderer.postMessage(channel, message, [transfer])`
+
+* `channel` String
+* `message` any
+* `transfer` MessagePort[] (optional)
+
+Send a message to the main process, optionally transferring ownership of zero
+or more [`MessagePort`][] objects.
+
+The transferred `MessagePort` objects will be available in the main process as
+[`MessagePortMain`](message-port-main.md) objects by accessing the `ports`
+property of the emitted event.
+
+For example:
+```js
+// Renderer process
+const { port1, port2 } = new MessageChannel()
+ipcRenderer.postMessage('port', { message: 'hello' }, [port1])
+
+// Main process
+ipcMain.on('port', (e, msg) => {
+  const [port] = e.ports
+  // ...
+})
+```
+
+For more information on using `MessagePort` and `MessageChannel`, see the [MDN
+documentation](https://developer.mozilla.org/en-US/docs/Web/API/MessageChannel).
+
 ### `ipcRenderer.sendTo(webContentsId, channel, ...args)`
 
 * `webContentsId` Number
@@ -150,4 +187,5 @@ in the [`ipc-renderer-event`](structures/ipc-renderer-event.md) structure docs.
 
 [event-emitter]: https://nodejs.org/api/events.html#events_class_eventemitter
 [SCA]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm
-[`postMessage`]: https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage
+[`window.postMessage`]: https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage
+[`MessagePort`]: https://developer.mozilla.org/en-US/docs/Web/API/MessagePort

docs/api/message-channel-main.md

@@ -0,0 +1,30 @@
+# MessageChannelMain
+
+`MessageChannelMain` is the main-process-side equivalent of the DOM
+[`MessageChannel`][] object. Its singular function is to create a pair of
+connected [`MessagePortMain`](message-port-main.md) objects.
+
+See the [Channel Messaging API][] documentation for more information on using
+channel messaging.
+
+## Class: MessageChannelMain
+
+Example:
+```js
+const { port1, port2 } = new MessageChannelMain()
+w.webContents.postMessage('port', null, [port2])
+port1.postMessage({ some: 'message' })
+```
+
+### Instance Properties
+
+#### `channel.port1`
+
+A [`MessagePortMain`](message-port-main.md) property.
+
+#### `channel.port2`
+
+A [`MessagePortMain`](message-port-main.md) property.
+
+[`MessageChannel`]: https://developer.mozilla.org/en-US/docs/Web/API/MessageChannel
+[Channel Messaging API]: https://developer.mozilla.org/en-US/docs/Web/API/Channel_Messaging_API

docs/api/message-port-main.md

@@ -0,0 +1,49 @@
+# MessagePortMain
+
+`MessagePortMain` is the main-process-side equivalent of the DOM
+[`MessagePort`][] object. It behaves similarly to the DOM version, with the
+exception that it uses the Node.js `EventEmitter` event system, instead of the
+DOM `EventTarget` system. This means you should use `port.on('message', ...)`
+to listen for events, instead of `port.onmessage = ...` or
+`port.addEventListener('message', ...)`
+
+See the [Channel Messaging API][] documentation for more information on using
+channel messaging.
+
+`MessagePortMain` is an [EventEmitter][event-emitter].
+
+## Class: MessagePortMain
+
+### Instance Methods
+
+#### `port.postMessage(message, [transfer])`
+
+* `message` any
+* `transfer` MessagePortMain[] (optional)
+
+Sends a message from the port, and optionally, transfers ownership of objects
+to other browsing contexts.
+
+#### `port.start()`
+
+Starts the sending of messages queued on the port. Messages will be queued
+until this method is called.
+
+#### `port.close()`
+
+Disconnects the port, so it is no longer active.
+
+### Instance Events
+
+#### Event: 'message'
+
+Returns:
+
+* `messageEvent` Object
+  * `data` any
+  * `ports` MessagePortMain[]
+
+Emitted when a MessagePortMain object receives a message.
+
+[`MessagePort`]: https://developer.mozilla.org/en-US/docs/Web/API/MessagePort
+[Channel Messaging API]: https://developer.mozilla.org/en-US/docs/Web/API/Channel_Messaging_API

docs/api/structures/ipc-main-event.md

@@ -3,6 +3,7 @@
 * `frameId` Integer - The ID of the renderer frame that sent this message
 * `returnValue` any - Set this to the value to be returned in a synchronous message
 * `sender` WebContents - Returns the `webContents` that sent the message
+* `ports` MessagePortMain[] - A list of MessagePorts that were transferred with this message
 * `reply` Function - A function that will send an IPC message to the renderer frame that sent the original message that you are currently handling.  You should use this method to "reply" to the sent message in order to guarantee the reply will go to the correct process and frame.
   * `channel` String
   * `...args` any[]

docs/api/structures/ipc-renderer-event.md

@@ -2,5 +2,6 @@
 
 * `sender` IpcRenderer - The `IpcRenderer` instance that emitted the event originally
 * `senderId` Integer - The `webContents.id` that sent the message, you can call `event.sender.sendTo(event.senderId, ...)` to reply to the message, see [ipcRenderer.sendTo][ipc-renderer-sendto] for more information. This only applies to messages sent from a different renderer. Messages sent directly from the main process set `event.senderId` to `0`.
+* `ports` MessagePort[] - A list of MessagePorts that were transferred with this message
 
 [ipc-renderer-sendto]: #ipcrenderersendtowindowid-channel--arg1-arg2-

docs/api/web-contents.md

@@ -1593,6 +1593,32 @@ ipcMain.on('ping', (event) => {
 })
 ```
 
+#### `contents.postMessage(channel, message, [transfer])`
+
+* `channel` String
+* `message` any
+* `transfer` MessagePortMain[] (optional)
+
+Send a message to the renderer process, optionally transferring ownership of
+zero or more [`MessagePortMain`][] objects.
+
+The transferred `MessagePortMain` objects will be available in the renderer
+process by accessing the `ports` property of the emitted event. When they
+arrive in the renderer, they will be native DOM `MessagePort` objects.
+
+For example:
+```js
+// Main process
+const { port1, port2 } = new MessageChannelMain()
+webContents.postMessage('port', { message: 'hello' }, [port1])
+
+// Renderer process
+ipcRenderer.on('port', (e, msg) => {
+  const [port] = e.ports
+  // ...
+})
+```
+
 #### `contents.enableDeviceEmulation(parameters)`
 
 * `parameters` Object