Implement watch command

Add command that watches the source files for changes and (re)spawns the
serve local server whenever a change is detected.  This skips having to
manually restart the serve server every time a change is made.

src/cli.ts

@@ -1,3 +1,4 @@
+import { type ChildProcess, fork } from "node:child_process";
 import * as fs from "node:fs";
 import * as posix from "node:path/posix";
 import { prettify, htmlDocument } from "antihtml";
@@ -6,6 +7,9 @@ import type { Page } from "./types.js";
 import { resolveRefs } from "./utils/resolve-refs.js";
 import { createServer } from "./utils/http-server.js";
 
+const srcDir = "build/node";
+const webDir = "web";
+
 function pageToHtml(page: Page) {
 	if (!page.ref.startsWith("/")) {
 		throw new Error(`ref "${page.ref}" for "${page.title}" is not absolute.`);
@@ -21,7 +25,6 @@ function pageToHtml(page: Page) {
 }
 
 function assembleResources() {
-	const webDir = "web";
 	const resources = new Map<string, string | Page>();
 	for (const entry of fs.readdirSync(webDir, { recursive: true, withFileTypes: true })) {
 		if (!entry.isFile())
@@ -77,10 +80,34 @@ function serve() {
 	console.log("Listening on http://localhost:8080");
 }
 
+function watch(script: string) {
+	let child: ChildProcess;
+	function start() {
+		child = fork(script, ["serve"]);
+	}
+	function restart() {
+		if (child.exitCode === null) {
+			child.kill();
+			child.once("close", start);
+		} else {
+			start();
+		}
+	}
+	let restartTimeout: ReturnType<typeof setTimeout>;
+	function onChange() {
+		clearTimeout(restartTimeout);
+		restartTimeout = setTimeout(restart, 100)
+	}
+	fs.watch(webDir, { recursive: true }, onChange);
+	fs.watch(srcDir, { recursive: true }, onChange);
+	start();
+}
+
 function printUsage() {
 	console.log("Usage: cli.js <cmd>");
 	console.log("    build - Copy resources and generated pages to build directory.");
 	console.log("    serve - Host website on localhost:8080 for development purposes.");
+	console.log("    watch - Run serve and automatically restart it on changes.");
 }
 
 function main(runtime: string, script: string, args: string[]) {
@@ -95,6 +122,8 @@ function main(runtime: string, script: string, args: string[]) {
 		build();
 	} else if (command === "serve") {
 		serve();
+	} else if (command === "watch") {
+		watch(script);
 	} else {
 		console.log(`Error: Unkown sub-command ${command}`);
 		printUsage();