feat: graceful shutdown and systemd socket activation in adapter-node

.changeset/early-peas-listen.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/adapter-node": minor
+---
+
+feat: add systemd socket activation

.changeset/stale-donkeys-mix.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/adapter-node": major
+---
+
+breaking: add graceful shutdown

documentation/docs/25-build-and-deploy/40-adapter-node.md

@@ -118,6 +118,14 @@ We instead read from the _right_, accounting for the number of trusted proxies.
 
 The maximum request body size to accept in bytes including while streaming. Defaults to 512kb. You can disable this option with a value of 0 and implement a custom check in [`handle`](hooks#server-hooks-handle) if you need something more advanced.
 
+### `SHUTDOWN_TIMEOUT`
+
+The number of seconds to wait before forcefully closing any remaining connections after receiving a `SIGTERM` or `SIGINT` signal. Defaults to `30`. Internally the adapter calls [`closeAllConnections`](https://nodejs.org/api/http.html#servercloseallconnections). See [Graceful shutdown](#graceful-shutdown) for more details.
+
+### `IDLE_TIMEOUT`
+
+When using systemd socket activation, `IDLE_TIMEOUT` specifies the number of seconds after which the app is automatically put to sleep when receiving no requests. If not set, the app runs continuously. See [Socket activation](#socket-activation) for more details.
+
 ## Options
 
 The adapter can be configured with various options:
@@ -162,6 +170,46 @@ MY_CUSTOM_ORIGIN=https://my.site \
 node build
 ```
 
+## Graceful shutdown
+
+By default `adapter-node` gracefully shuts down the HTTP server when a `SIGTERM` or `SIGINT` signal is received. It will:
+
+1. reject new requests ([`server.close`](https://nodejs.org/api/http.html#serverclosecallback))
+2. wait for requests that have already been made but not received a response yet to finish and close connections once they become idle ([`server.closeIdleConnections`](https://nodejs.org/api/http.html#servercloseidleconnections))
+3. and finally, close any remaining connections that are still active after [`SHUTDOWN_TIMEOUT`](#shutdown_timeout) seconds. ([`server.closeAllConnections`](https://nodejs.org/api/http.html#servercloseallconnections))
+
+> If you want to customize this behaviour you can use a [custom server](#custom-server).
+
+## Socket activation
+
+Most Linux operating systems today use a modern process manager called systemd to start the server and run and manage services. You can configure your server to allocate a socket and start and scale your app on demand. This is called [socket activation](http://0pointer.de/blog/projects/socket-activated-containers.html). In this case, the OS will pass two environment variables to your app — `LISTEN_PID` and `LISTEN_FDS`. The adapter will then listen on file descriptor 3 which refers to a systemd socket unit that you will have to create.
+
+> You can still use [`envPrefix`](#envprefix) with systemd socket activation. `LISTEN_PID` and `LISTEN_FDS` are always passed to the app without a prefix.
+
+To take advantage of socket activation follow these steps.
+
+1. Run your app as a [systemd service](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html). It can either run directly on the host system or inside a container (using Docker or a systemd portable service for example). If you additionally pass an [`IDLE_TIMEOUT`](#idle_timeout) environment variable to your app it will gracefully shutdown if there are no requests for `IDLE_TIMEOUT` seconds. systemd will automatically start your app again when new requests are coming in.
+
+```ini
+/// file: /etc/systemd/system/myapp.service
+[Service]
+Environment=NODE_ENV=production IDLE_TIMEOUT=60
+ExecStart=/usr/bin/node /usr/bin/myapp/build
+```
+
+2. Create an accompanying [socket unit](https://www.freedesktop.org/software/systemd/man/latest/systemd.socket.html). The adapter only accepts a single socket.
+
+```ini
+/// file: /etc/systemd/system/myapp.socket
+[Socket]
+ListenStream=3000
+
+[Install]
+WantedBy=sockets.target
+```
+
+3. Make sure systemd has recognised both units by running `sudo systemctl daemon-reload`. Then enable the socket on boot and start it immediately using `sudo systemctl enable --now myapp.socket`. The app will then automatically start once the first request is made to `localhost:3000`.
+
 ## Custom server
 
 The adapter creates two files in your build directory — `index.js` and `handler.js`. Running `index.js` — e.g. `node build`, if you use the default build directory — will start a server on the configured port.
@@ -191,9 +239,9 @@ app.listen(3000, () => {
 
 ## Troubleshooting
 
-### Is there a hook for cleaning up before the server exits?
+### Is there a hook for cleaning up before the app exits?
 
-There's nothing built-in to SvelteKit for this, because such a cleanup hook depends highly on the execution environment you're on. For Node, you can use its built-in `process.on(...)` to implement a callback that runs before the server exits:
+There's nothing built-in to SvelteKit for this, because such a cleanup hook depends highly on the execution environment you're on. For Node, you can use its built-in `process.on(...)` to implement a callback that runs before the app exits:
 
 ```js
 // @errors: 2304 2580
@@ -202,6 +250,5 @@ function shutdownGracefully() {
 	db.shutdown();
 }
 
-process.on('SIGINT', shutdownGracefully);
-process.on('SIGTERM', shutdownGracefully);
+process.on('exit', shutdownGracefully);
 ```

packages/adapter-node/src/env.js

@@ -10,9 +10,13 @@ const expected = new Set([
 	'PROTOCOL_HEADER',
 	'HOST_HEADER',
 	'PORT_HEADER',
-	'BODY_SIZE_LIMIT'
+	'BODY_SIZE_LIMIT',
+	'SHUTDOWN_TIMEOUT',
+	'IDLE_TIMEOUT'
 ]);
 
+const expected_unprefixed = new Set(['LISTEN_PID', 'LISTEN_FDS']);
+
 if (ENV_PREFIX) {
 	for (const name in process.env) {
 		if (name.startsWith(ENV_PREFIX)) {
@@ -31,6 +35,7 @@ if (ENV_PREFIX) {
  * @param {any} fallback
  */
 export function env(name, fallback) {
-	const prefixed = ENV_PREFIX + name;
+	const prefix = expected_unprefixed.has(name) ? '' : ENV_PREFIX;
+	const prefixed = prefix + name;
 	return prefixed in process.env ? process.env[prefixed] : fallback;
 }

packages/adapter-node/src/index.js

@@ -6,10 +6,90 @@ export const path = env('SOCKET_PATH', false);
 export const host = env('HOST', '0.0.0.0');
 export const port = env('PORT', !path && '3000');
 
+const shutdown_timeout = parseInt(env('SHUTDOWN_TIMEOUT', '30'));
+const idle_timeout = parseInt(env('IDLE_TIMEOUT', '0'));
+const listen_pid = parseInt(env('LISTEN_PID', '0'));
+const listen_fds = parseInt(env('LISTEN_FDS', '0'));
+// https://www.freedesktop.org/software/systemd/man/latest/sd_listen_fds.html
+const SD_LISTEN_FDS_START = 3;
+
+if (listen_pid !== 0 && listen_pid !== process.pid) {
+	throw new Error(`received LISTEN_PID ${listen_pid} but current process id is ${process.pid}`);
+}
+if (listen_fds > 1) {
+	throw new Error(
+		`only one socket is allowed for socket activation, but LISTEN_FDS was set to ${listen_fds}`
+	);
+}
+
+const socket_activation = listen_pid === process.pid && listen_fds === 1;
+
+let requests = 0;
+/** @type {NodeJS.Timeout | void} */
+let shutdown_timeout_id;
+/** @type {NodeJS.Timeout | void} */
+let idle_timeout_id;
+
 const server = polka().use(handler);
 
-server.listen({ path, host, port }, () => {
-	console.log(`Listening on ${path ? path : host + ':' + port}`);
-});
+function shutdown() {
+	if (shutdown_timeout_id) return;
+
+	// @ts-expect-error this was added in 18.2.0 but is not reflected in the types
+	server.server.closeIdleConnections();
+
+	server.server.close(() => {
+		if (shutdown_timeout_id) {
+			shutdown_timeout_id = clearTimeout(shutdown_timeout_id);
+		}
+		if (idle_timeout_id) {
+			idle_timeout_id = clearTimeout(idle_timeout_id);
+		}
+	});
+
+	shutdown_timeout_id = setTimeout(
+		// @ts-expect-error this was added in 18.2.0 but is not reflected in the types
+		() => server.server.closeAllConnections(),
+		shutdown_timeout * 1000
+	);
+}
+
+server.server.on(
+	'request',
+	/** @param {import('node:http').IncomingMessage} req */
+	(req) => {
+		requests++;
+
+		if (socket_activation && idle_timeout_id) {
+			idle_timeout_id = clearTimeout(idle_timeout_id);
+		}
+
+		req.on('close', () => {
+			requests--;
+
+			if (requests === 0 && shutdown_timeout_id) {
+				// when all requests are done, close the connections, so the app shuts down without delay
+				// @ts-expect-error this was added in 18.2.0 but is not reflected in the types
+				server.server.closeIdleConnections();
+			}
+			if (requests === 0 && socket_activation && idle_timeout) {
+				idle_timeout_id = setTimeout(shutdown, idle_timeout * 1000);
+			}
+		});
+	}
+);
+
+if (socket_activation) {
+	server.listen({ fd: SD_LISTEN_FDS_START }, () => {
+		console.log(`Listening on file descriptor ${SD_LISTEN_FDS_START}`);
+	});
+} else {
+	server.listen({ path, host, port }, () => {
+		console.log(`Listening on ${path ? path : host + ':' + port}`);
+	});
+}
+
+process.on('SIGTERM', shutdown);
+process.on('SIGINT', shutdown);
 
 export { server };