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+state/
+admin.sock
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+# toybox
+
+Vats whose code is installed at runtime: each vat is a Durable Object whose app runs as a [facet](https://developers.cloudflare.com/dynamic-workers/usage/durable-object-facets/) on code loaded through workerd's Worker Loader. Think `vatAdmin.createVat(bundle)`, with a supervisor per vat.
+
+- [supervisor.js](supervisor.js): `Vat` stores code by SHA-256 and runs the app on it; `Public` and `Admin` are the two entrypoints.
+- [config.capnp](config.capnp): the sockets have no addresses; whoever starts workerd hands them in, so holding a socket is the authority it grants.
+
+| socket | reaches | what its holder can do |
+|---|---|---|
+| `http` | `Public` | `GET /<vat>/…`: talk to a vat's app |
+| `admin` | `Admin` | `PUT /<vat>` with an ES module body: install it and (re)start the app; its storage carries over |
+
+An app must export `class App extends DurableObject` (see [apps/](apps/)). It gets an empty `env` and no `fetch`.
+
+## Run it
+
+```sh
+pnpm install
+pnpm test     # a real workerd: install, upgrade, isolation, no fetch, crash and restart
+pnpm start    # systemd-socket-activate on 127.0.0.1:8787 and ./admin.sock; state in ./state
+```
+
+Then, in another terminal:
+
+```sh
+curl -X PUT --unix-socket admin.sock --data-binary @apps/counter-v1.js http://toybox/alice
+curl localhost:8787/alice                # v1 count 1
+curl -X PUT --unix-socket admin.sock --data-binary @apps/counter-v2.js http://toybox/alice
+curl localhost:8787/alice                # v2 count 2
+```
+
+## Under systemd
+
+[systemd/](systemd/) has a socket and service pair, assuming this directory is installed at `/opt/toybox`. The service gets no network of its own (`PrivateNetwork=yes`): only the two sockets it is handed.
+
+`PrivateNetwork=` needs the system manager; as a `systemd-run --user` unit, systemd logs "lack privileges for network namespace, proceeding without".
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+import { DurableObject } from "cloudflare:workers";
+export class App extends DurableObject {
+  fetch() {
+    const n = (this.ctx.storage.kv.get("n") ?? 0) + 1;
+    this.ctx.storage.kv.put("n", n);
+    return new Response(`v1 count ${n}\n`);
+  }
+}
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+import { DurableObject } from "cloudflare:workers";
+export class App extends DurableObject {
+  fetch() {
+    const n = (this.ctx.storage.kv.get("n") ?? 0) + 1;
+    this.ctx.storage.kv.put("n", n);
+    return new Response(`v2 count ${n}\n`);
+  }
+}
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+import { DurableObject } from "cloudflare:workers";
+export class App extends DurableObject {
+  async fetch() {
+    try { await fetch("https://example.com/"); return new Response("fetch ALLOWED\n"); }
+    catch (e) { return new Response(`fetch refused: ${e.message}\n`); }
+  }
+}
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+# toybox: vats whose code is installed at runtime, each a Durable Object facet.
+#
+# Sockets have no address here: whoever starts workerd hands them in
+# (-S name=fd from systemd, or -s name=addr), so holding a socket is the
+# authority it grants. `state` is likewise overridden with -d state=<dir>.
+using Workerd = import "/workerd/workerd.capnp";
+
+const config :Workerd.Config = (
+  services = [
+    (name = "main", worker = .supervisor),
+    (name = "nowhere", network = (allow = [])),
+    (name = "state", disk = (path = "state", writable = true)),
+  ],
+  sockets = [
+    # reach a running vat by name
+    (name = "http", http = (), service = (name = "main", entrypoint = "Public")),
+    # install code into a vat and (re)start it
+    (name = "admin", http = (), service = (name = "main", entrypoint = "Admin")),
+  ],
+);
+
+const supervisor :Workerd.Worker = (
+  compatibilityDate = "2026-10-01",
+  modules = [(name = "supervisor.js", esModule = embed "supervisor.js")],
+  durableObjectNamespaces = [(className = "Vat", uniqueKey = "toybox-vat", enableSql = true)],
+  durableObjectStorage = (localDisk = "state"),
+  bindings = [
+    (name = "VATS", durableObjectNamespace = "Vat"),
+    (name = "LOADER", workerLoader = ()),
+  ],
+  globalOutbound = "nowhere",
+);
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+{
+	"name": "toybox",
+	"version": "0.0.1",
+	"private": true,
+	"type": "module",
+	"scripts": {
+		"start": "mkdir -p state && rm -f admin.sock && systemd-socket-activate -l 127.0.0.1:8787 -l ./admin.sock -- node_modules/workerd/bin/workerd serve --experimental config.capnp -S http=3 -S admin=4 -d state=state",
+		"test": "node --test"
+	},
+	"devDependencies": {
+		"workerd": "1.20261006.1"
+	}
+}
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+lockfileVersion: '9.0'
+
+settings:
+  autoInstallPeers: true
+  excludeLinksFromLockfile: false
+
+importers:
+
+  .:
+    devDependencies:
+      workerd:
+        specifier: 1.20261006.1
+        version: 1.20261006.1
+
+packages:
+
+  '@cloudflare/workerd-darwin-64@1.20261006.1':
+    resolution: {integrity: sha512-lCX6avJO2It9AyDEYitjmOtJRVp/xmaW0TziyoLAnfxlzozyyRbSFfIIMMCiDx/sTP1++E4VLbV+7BHSD33f0Q==}
+    engines: {node: '>=16'}
+    cpu: [x64]
+    os: [darwin]
+
+  '@cloudflare/workerd-darwin-arm64@1.20261006.1':
+    resolution: {integrity: sha512-PyFGjOaldtppnLuKsdkoYWXfLVpDjuf8u+uoykX+1Xvxo2EPmCVViq0iDKm+U7c7PRno85ehS3/MPVXyk2HwHw==}
+    engines: {node: '>=16'}
+    cpu: [arm64]
+    os: [darwin]
+
+  '@cloudflare/workerd-linux-64@1.20261006.1':
+    resolution: {integrity: sha512-Ntf9S8eesmVufdjaT2kzWlz6Ng9XGYTc6cHQVRKAg59lFufwsIabGeMT6U4KkrpEeSIxgHq8aPtuLbZXE19tCA==}
+    engines: {node: '>=16'}
+    cpu: [x64]
+    os: [linux]
+
+  '@cloudflare/workerd-linux-arm64@1.20261006.1':
+    resolution: {integrity: sha512-JyeDBymVWw5tVRebSsVE8ojDYxd6UlZrAseHU8G+NpIyd5wF61tXlF/axLeCUqu0pOhBFHb8I5QKi0jwrCK41Q==}
+    engines: {node: '>=16'}
+    cpu: [arm64]
+    os: [linux]
+
+  '@cloudflare/workerd-windows-64@1.20261006.1':
+    resolution: {integrity: sha512-nWP0URNqOBN9fa/gZ7eh/w051DwjOXN8j4lDZDYVXyQPsux6sV3DfyYZT+CjR/rt9nKBSmJT9QE9/bWT18+84w==}
+    engines: {node: '>=16'}
+    cpu: [x64]
+    os: [win32]
+
+  workerd@1.20261006.1:
+    resolution: {integrity: sha512-8MadBuffZ5NdpLwZjUmsqT5QlaKXq4gjgT6wzcKeynZTICNXyePhc+5QVoKiK7+MMhFTLNrRO2KXLlpk8Q6BkA==}
+    engines: {node: '>=16'}
+    hasBin: true
+
+snapshots:
+
+  '@cloudflare/workerd-darwin-64@1.20261006.1':
+    optional: true
+
+  '@cloudflare/workerd-darwin-arm64@1.20261006.1':
+    optional: true
+
+  '@cloudflare/workerd-linux-64@1.20261006.1':
+    optional: true
+
+  '@cloudflare/workerd-linux-arm64@1.20261006.1':
+    optional: true
+
+  '@cloudflare/workerd-windows-64@1.20261006.1':
+    optional: true
+
+  workerd@1.20261006.1:
+    optionalDependencies:
+      '@cloudflare/workerd-darwin-64': 1.20261006.1
+      '@cloudflare/workerd-darwin-arm64': 1.20261006.1
+      '@cloudflare/workerd-linux-64': 1.20261006.1
+      '@cloudflare/workerd-linux-arm64': 1.20261006.1
+      '@cloudflare/workerd-windows-64': 1.20261006.1
@@ -0,0 +1,72 @@
+/** @file toybox supervisor: one Vat per name; its app runs as a facet on installed code. */
+import { DurableObject, WorkerEntrypoint } from "cloudflare:workers";
+
+const hex = (buf) => [...new Uint8Array(buf)].map((b) => b.toString(16).padStart(2, "0")).join("");
+
+/**
+ * Holds the vat's installed code; the app facet holds the app's state.
+ *
+ * Code is stored by hash so the facet can be rebuilt after hibernation or a
+ * restart. The app gets an empty `env` and no `fetch`.
+ */
+export class Vat extends DurableObject {
+	constructor(ctx, env) {
+		super(ctx, env);
+		ctx.storage.sql.exec("CREATE TABLE IF NOT EXISTS bundle (hash TEXT PRIMARY KEY, src TEXT)");
+		ctx.storage.sql.exec("CREATE TABLE IF NOT EXISTS current (k INTEGER PRIMARY KEY CHECK (k = 0), hash TEXT)");
+	}
+
+	/**
+	 * Install `src` and run the app on it; the app's storage carries over.
+	 *
+	 * @param {string} src an ES module exporting `class App extends DurableObject`
+	 * @returns {Promise<string>} the code's SHA-256, hex
+	 */
+	async start(src) {
+		const hash = hex(await crypto.subtle.digest("SHA-256", new TextEncoder().encode(src)));
+		this.ctx.storage.sql.exec("INSERT OR IGNORE INTO bundle VALUES (?, ?)", hash, src);
+		this.ctx.storage.sql.exec("INSERT OR REPLACE INTO current VALUES (0, ?)", hash);
+		this.ctx.facets.abort("app", new Error("upgraded"));
+		return hash;
+	}
+
+	#app(hash, src) {
+		return this.ctx.facets.get("app", () => {
+			const worker = this.env.LOADER.get(hash, () => ({
+				compatibilityDate: "2026-10-01",
+				mainModule: "app.js",
+				modules: { "app.js": src },
+				env: {},
+				globalOutbound: null,
+			}));
+			return { class: worker.getDurableObjectClass("App") };
+		});
+	}
+
+	fetch(request) {
+		const [row] = this.ctx.storage.sql
+			.exec("SELECT b.hash, b.src FROM current c JOIN bundle b USING (hash)")
+			.toArray();
+		if (!row) return new Response("no code installed\n", { status: 404 });
+		return this.#app(row.hash, row.src).fetch(request);
+	}
+}
+
+const vatOf = (env, url) => env.VATS.getByName(new URL(url).pathname.split("/")[1]);
+
+/** The http socket: `/<vat>/...` goes to that vat's app. */
+export class Public extends WorkerEntrypoint {
+	fetch(request) {
+		return vatOf(this.env, request.url).fetch(request);
+	}
+}
+
+/** The admin socket: `PUT /<vat>` with an ES module body installs and starts it. */
+export class Admin extends WorkerEntrypoint {
+	async fetch(request) {
+		if (request.method !== "PUT") return new Response("PUT code to /<vat>\n", { status: 405 });
+		return new Response((await vatOf(this.env, request.url).start(await request.text())) + "\n");
+	}
+}
+
+export default Public;
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+[Unit]
+Description=toybox vats (workerd)
+Requires=toybox.socket
+
+[Service]
+ExecStart=/opt/toybox/node_modules/workerd/bin/workerd serve --experimental /opt/toybox/config.capnp -S http=3 -S admin=4 -d state=/var/lib/toybox
+DynamicUser=yes
+StateDirectory=toybox
+# no network but the sockets handed in
+PrivateNetwork=yes
+RestrictAddressFamilies=AF_UNIX
+ProtectSystem=strict
+ProtectHome=yes
+PrivateTmp=yes
+SystemCallFilter=@system-service
+# not MemoryDenyWriteExecute: V8's JIT needs W+X
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+[Unit]
+Description=toybox vats: public port and admin socket
+
+[Socket]
+# order sets the fds: http=3, admin=4 (see toybox.service)
+ListenStream=127.0.0.1:8787
+ListenStream=/run/toybox/admin.sock
+# whoever can open admin.sock can install code
+SocketUser=root
+SocketMode=0600
+
+[Install]
+WantedBy=sockets.target
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+/** @file toybox end to end: a real workerd, an admin socket, a public port. */
+import { spawn } from "node:child_process";
+import { mkdir, mkdtemp, readFile, rm } from "node:fs/promises";
+import { request } from "node:http";
+import { createServer } from "node:net";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { after, before, test } from "node:test";
+import assert from "node:assert/strict";
+
+const here = new URL("..", import.meta.url).pathname;
+// the package binary, not the .bin shim: the shim forks it, so a kill would orphan workerd
+const workerd = join(here, "node_modules/workerd/bin/workerd");
+const app = (name) => readFile(join(here, "apps", name), "utf8");
+
+const freePort = () =>
+	new Promise((resolve) => {
+		const s = createServer().listen(0, "127.0.0.1", () => {
+			const { port } = s.address();
+			s.close(() => resolve(port));
+		});
+	});
+
+/** @param {string} socketPath the admin socket; holding it is the authority to install */
+const makeAdmin = (socketPath) => ({
+	install: (vat, src) =>
+		new Promise((resolve, reject) => {
+			const req = request({ socketPath, path: `/${vat}`, method: "PUT", agent: false }, (res) => {
+				let body = "";
+				res.on("data", (c) => (body += c));
+				res.on("end", () => resolve(body.trim()));
+			});
+			req.on("error", reject);
+			req.end(src);
+		}),
+});
+
+/** @param {string} base the public origin */
+const makePublic = (base) => ({
+	get: async (vat) => (await fetch(`${base}/${vat}`)).text(),
+	put: async (vat, src) => (await fetch(`${base}/${vat}`, { method: "PUT", body: src })).status,
+});
+
+/** Run workerd on `dir`'s state until `stop()`; resolves once the port answers. */
+const startWorkerd = async (dir, port) => {
+	await rm(join(dir, "admin.sock"), { force: true }); // left behind by a crash; bind() would fail
+	const child = spawn(
+		workerd,
+		["serve", "--experimental", join(here, "config.capnp"),
+			"-s", `http=127.0.0.1:${port}`, "-s", `admin=unix:${join(dir, "admin.sock")}`,
+			"-d", `state=${join(dir, "state")}`],
+		{ stdio: ["ignore", "ignore", "pipe"] },
+	);
+	let log = "";
+	child.stderr.on("data", (c) => (log += c));
+	const exited = new Promise((resolve) => child.on("exit", resolve));
+	for (let i = 0; i < 100; i++) {
+		try {
+			await fetch(`http://127.0.0.1:${port}/`);
+			// SIGKILL: a crash is the harder durability test
+			return { stop: () => (child.kill("SIGKILL"), exited) };
+		} catch {
+			await new Promise((r) => setTimeout(r, 50));
+		}
+	}
+	child.kill("SIGKILL");
+	throw new Error(`workerd did not start: ${log}`);
+};
+
+let dir, port, server, admin, pub;
+
+before(async () => {
+	dir = await mkdtemp(join(tmpdir(), "toybox-"));
+	await mkdir(join(dir, "state"));
+	port = await freePort();
+	server = await startWorkerd(dir, port);
+	admin = makeAdmin(join(dir, "admin.sock"));
+	pub = makePublic(`http://127.0.0.1:${port}`);
+});
+
+after(async () => {
+	await server?.stop();
+	await rm(dir, { recursive: true, force: true });
+});
+
+test("a vat with no code is 404", async () => {
+	assert.equal(await pub.get("nobody"), "no code installed\n");
+});
+
+test("install, run, upgrade: the app's state carries over", async () => {
+	assert.match(await admin.install("alice", await app("counter-v1.js")), /^[0-9a-f]{64}$/);
+	assert.equal(await pub.get("alice"), "v1 count 1\n");
+	assert.equal(await pub.get("alice"), "v1 count 2\n");
+	await admin.install("alice", await app("counter-v2.js"));
+	assert.equal(await pub.get("alice"), "v2 count 3\n");
+});
+
+test("vats don't share state", async () => {
+	await admin.install("carol", await app("counter-v1.js"));
+	assert.equal(await pub.get("carol"), "v1 count 1\n");
+});
+
+test("an app gets no fetch", async () => {
+	await admin.install("bob", await app("try-fetch.js"));
+	assert.match(await pub.get("bob"), /^fetch refused: This worker is not permitted to access the internet/);
+});
+
+test("the public port can't install code", async () => {
+	assert.equal(await pub.put("mallory", "export class App {}"), 404);
+	assert.equal(await pub.get("mallory"), "no code installed\n");
+});
+
+test("code and state survive a restart", async () => {
+	await admin.install("dave", await app("counter-v1.js"));
+	assert.equal(await pub.get("dave"), "v1 count 1\n");
+	await server.stop();
+	server = await startWorkerd(dir, port);
+	assert.equal(await pub.get("dave"), "v1 count 2\n");
+});