docs: add advanced specification for Squirrel updater (#43234)

* docs: add advanced specification for Squirrel updater

* address feedback

docs/api/auto-updater.md

@@ -32,22 +32,28 @@ This is a requirement of `Squirrel.Mac`.
 ### Windows
 
 On Windows, you have to install your app into a user's machine before you can
-use the `autoUpdater`, so it is recommended that you use the
-[electron-winstaller][installer-lib], [Electron Forge][electron-forge-lib] or the [grunt-electron-installer][installer] package to generate a Windows installer.
-
-When using [electron-winstaller][installer-lib] or [Electron Forge][electron-forge-lib] make sure you do not try to update your app [the first time it runs](https://github.com/electron/windows-installer#handling-squirrel-events) (Also see [this issue for more info](https://github.com/electron/electron/issues/7155)). It's also recommended to use [electron-squirrel-startup](https://github.com/mongodb-js/electron-squirrel-startup) to get desktop shortcuts for your app.
-
-The installer generated with Squirrel will create a shortcut icon with an
+use the `autoUpdater`, so it is recommended that you use
+[electron-winstaller][installer-lib] or [Electron Forge's Squirrel.Windows maker][electron-forge-lib] to generate a Windows installer.
+
+Apps built with Squirrel.Windows will trigger [custom launch events](https://github.com/Squirrel/Squirrel.Windows/blob/51f5e2cb01add79280a53d51e8d0cfa20f8c9f9f/docs/using/custom-squirrel-events-non-cs.md#application-startup-commands)
+that must be handled by your Electron application to ensure proper setup and teardown.
+
+Squirrel.Windows apps will launch with the `--squirrel-firstrun` argument immediately
+after installation. During this time, Squirrel.Windows will obtain a file lock on
+your app, and `autoUpdater` requests will fail until the lock is released. In practice,
+this means that you won't be able to check for updates on first launch for the first
+few seconds. You can work around this by not checking for updates when `process.argv`
+contains the `--squirrel-firstrun` flag or by setting a 10-second timeout on your
+update checks (see [electron/electron#7155](https://github.com/electron/electron/issues/7155)
+for more information).
+
+The installer generated with Squirrel.Windows will create a shortcut icon with an
 [Application User Model ID][app-user-model-id] in the format of
 `com.squirrel.PACKAGE_ID.YOUR_EXE_WITHOUT_DOT_EXE`, examples are
 `com.squirrel.slack.Slack` and `com.squirrel.code.Code`. You have to use the
 same ID for your app with `app.setAppUserModelId` API, otherwise Windows will
 not be able to pin your app properly in task bar.
 
-Like Squirrel.Mac, Windows can host updates on S3 or any other static file host.
-You can read the documents of [Squirrel.Windows][squirrel-windows] to get more details
-about how Squirrel.Windows works.
-
 ## Events
 
 The `autoUpdater` object emits the following events:
@@ -137,9 +143,7 @@ application starts.
 
 [squirrel-mac]: https://github.com/Squirrel/Squirrel.Mac
 [server-support]: https://github.com/Squirrel/Squirrel.Mac#server-support
-[squirrel-windows]: https://github.com/Squirrel/Squirrel.Windows
-[installer]: https://github.com/electron-archive/grunt-electron-installer
 [installer-lib]: https://github.com/electron/windows-installer
-[electron-forge-lib]: https://github.com/electron/forge
+[electron-forge-lib]: https://www.electronforge.io/config/makers/squirrel.windows
 [app-user-model-id]: https://learn.microsoft.com/en-us/windows/win32/shell/appids
 [event-emitter]: https://nodejs.org/api/events.html#events_class_eventemitter

docs/tutorial/updates.md

@@ -261,7 +261,59 @@ server-communication aspect of the process by loading your update from a local d
 
 ## Update server specification
 
-A Squirrel-compatible update server has different
+For advanced deployment needs, you can also roll out your own Squirrel-compatible update server.
+For example, you may want to have percentage-based rollouts, distribute your app through separate
+release channels, or put your update server behind an authentication check.
+
+Squirrel.Windows and Squirrel.Mac clients require different response formats,
+but you can use a single server for both platforms by sending requests to
+different endpoints depending on the value of `process.platform`.
+
+```js title='main.js'
+const { app, autoUpdater } = require('electron')
+
+const server = 'https://your-deployment-url.com'
+// e.g. for Windows and app version 1.2.3
+// https://your-deployment-url.com/update/win32/1.2.3
+const url = `${server}/update/${process.platform}/${app.getVersion()}`
+
+autoUpdater.setFeedURL({ url })
+```
+
+### Windows
+
+A Squirrel.Windows client expects the update server to return the `RELEASES` artifact
+of the latest available build at the `/RELEASES` subpath of your endpoint.
+
+For example, if your feed URL is `https://your-deployment-url.com/update/win32/1.2.3`,
+then the `https://your-deployment-url.com/update/win32/1.2.3/RELEASES` endpoint
+should return the contents of the `RELEASES` artifact of the version you want to serve.
+
+```plaintext title='https://your-deployment-url.com/update/win32/1.2.3/RELEASES'
+B0892F3C7AC91D72A6271FF36905FEF8FE993520 https://your-static.storage/your-app-1.2.3-full.nupkg 103298365
+```
+
+Squirrel.Windows does the comparison check to see if the current app should update to
+the version returned in `RELEASES`, so you should return a response even when no update
+is available.
+
+### macOS
+
+When an update is available, the Squirrel.Mac client expects a JSON response at the feed URL's endpoint.
+This object has a mandatory `url` property that maps to a ZIP archive of the
+app update. All other properties in the object are optional.
+
+```json title='https://your-deployment-url.com/update/darwin/0.31.0'
+{
+    "url": "https://your-static.storage/your-app-1.2.3-darwin.zip",
+    "name": "1.2.3",
+    "notes": "Theses are some release notes innit",
+    "pub_date": "2024-09-18T12:29:53+01:00"
+}
+```
+
+If no update is available, the server should return a [`204 No Content`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/204)
+HTTP response.
 
 [vercel]: https://vercel.com
 [hazel]: https://github.com/vercel/hazel