src/sandbox.js

const BOOT_FILES = {
  "package.json": {
    file: {
      contents: JSON.stringify(
        {
          type: "module",
          scripts: {
            demo: "node hello.js",
          },
          dependencies: {},
          devDependencies: {},
        },
        null,
        2,
      ),
    },
  },
  "README.md": {
    file: {
      contents:
        "# Pi web sandbox\n\nThis filesystem and every spawned process run inside WebContainers in this browser tab.\n",
    },
  },
  "hello.js": {
    file: {
      contents: 'console.log("hello from the browser sandbox");\nconsole.log(2 + 2);\n',
    },
  },
};

const MAX_OUTPUT_CHARS = 16000;
const DEFAULT_TIMEOUT_MS = 10000;

export function createSandbox({ onLog = () => {}, onStatus = () => {} } = {}) {
  let instance = null;
  let booting = null;
  let root = "";

  async function boot() {
    if (instance) return instance;
    if (booting) return booting;

    booting = (async () => {
      if (!globalThis.crossOriginIsolated) {
        throw new Error("WebContainers need cross-origin isolation. Start this app through Vite or another server that sends COOP/COEP headers.");
      }

      onStatus("Booting WebContainer");
      const { WebContainer } = await import("@webcontainer/api");
      instance = await WebContainer.boot({
        coep: "credentialless",
        workdirName: "workspace",
      });
      root = instance.workdir;
      await instance.mount(BOOT_FILES);
      onLog(`sandbox booted at ${root}`);
      onStatus("Sandbox ready");
      return instance;
    })();

    try {
      return await booting;
    } finally {
      booting = null;
    }
  }

  function assertRelativePath(path) {
    const value = String(path || "").trim().replace(/^\/+/, "");
    if (!value || value.includes("\0")) {
      throw new Error("Path is required.");
    }
    const segments = value.split("/").filter(Boolean);
    if (segments.some((segment) => segment === "." || segment === "..")) {
      throw new Error("Paths must stay inside the sandbox workspace.");
    }
    return segments.join("/");
  }

  function toWorkspacePath(path) {
    return assertRelativePath(path);
  }

  async function reset() {
    const wc = await boot();
    const entries = await wc.fs.readdir(".");
    await Promise.all(
      entries.map((entry) =>
        wc.fs.rm(entry, {
          force: true,
          recursive: true,
        }),
      ),
    );
    await wc.mount(BOOT_FILES);
    onLog("sandbox reset");
    return "Sandbox reset to the starter project.";
  }

  async function listFiles(path = ".") {
    const wc = await boot();
    const target = path === "." || path === "" ? "." : toWorkspacePath(path);
    const entries = await wc.fs.readdir(target, { withFileTypes: true });
    return entries.map((entry) => `${entry.isDirectory() ? "dir " : "file"} ${entry.name}`).join("\n") || "(empty)";
  }

  async function readFile(path) {
    const wc = await boot();
    return await wc.fs.readFile(toWorkspacePath(path), "utf-8");
  }

  async function writeFile(path, content) {
    const wc = await boot();
    const relative = assertRelativePath(path);
    const parent = relative.split("/").slice(0, -1).join("/");
    if (parent) {
      await wc.fs.mkdir(parent, { recursive: true });
    }
    await wc.fs.writeFile(relative, String(content ?? ""));
    onLog(`wrote ${relative}`);
    return `Wrote ${relative}`;
  }

  async function runCommand(command, args = [], timeoutMs = DEFAULT_TIMEOUT_MS) {
    const wc = await boot();
    const cmd = String(command || "").trim();
    if (!cmd) throw new Error("Command is required.");
    const cleanArgs = Array.isArray(args) ? args.map((arg) => String(arg)) : [];

    onLog(`$ ${[cmd, ...cleanArgs].join(" ")}`);
    const process = await wc.spawn(cmd, cleanArgs, {
      terminal: {
        cols: 96,
        rows: 28,
      },
    });

    let output = "";
    const reader = process.output.getReader();
    const pump = (async () => {
      while (true) {
        const { done, value } = await reader.read();
        if (done) break;
        output += value;
        if (output.length > MAX_OUTPUT_CHARS) {
          output = `${output.slice(0, MAX_OUTPUT_CHARS)}\n[output truncated]`;
          process.kill();
          break;
        }
      }
    })();

    const timeout = new Promise((resolve) => {
      setTimeout(() => {
        process.kill();
        resolve("timeout");
      }, Number(timeoutMs) || DEFAULT_TIMEOUT_MS);
    });

    const exitCode = await Promise.race([process.exit, timeout]);
    await pump.catch(() => {});
    const normalizedExitCode = exitCode === "timeout" ? 124 : exitCode;
    const result = {
      command: [cmd, ...cleanArgs].join(" "),
      exitCode: normalizedExitCode,
      output: output.trimEnd(),
    };
    onLog(`exit ${normalizedExitCode}`);
    return result;
  }

  return {
    boot,
    reset,
    listFiles,
    readFile,
    writeFile,
    runCommand,
    get isReady() {
      return Boolean(instance);
    },
  };
}