Function isPortAvailableOnHost

packages/cli/src/server/portUtils.ts:42–61

* Test whether a port is free on a specific host. * * Attempts an ephemeral bind-and-release with `net.createServer()`. Only * `EADDRINUSE` means "genuinely occupied" — other errnos (EADDRNOTAVAIL when * IPv6 is disabled, EACCES for privileged ports, EAFNOSUPPORT for missing * address families)

(port: number, host: string)

 * the port as free for this host rather than poisoning the whole scan.
 */
async function isPortAvailableOnHost(port: number, host: string): Promise<boolean> {
  const probe = net.createServer();
  probe.unref();

  const bindError = await new Promise<NodeJS.ErrnoException | null>((settle) => {
    const handleError = (err: NodeJS.ErrnoException): void => settle(err);
    probe.once("error", handleError);
    probe.listen({ port, host }, () => {
      probe.removeListener("error", handleError);
      settle(null);
    });
  });

  if (bindError !== null) {
    return bindError.code !== "EADDRINUSE";
  }

  await new Promise<void>((done) => probe.close(() => done()));
  return true;
}

export const PORT_PROBE_HOSTS = ["127.0.0.1", "0.0.0.0", "::1", "::"] as const;