paseo/packages/cli/tests/helpers/network.ts

import { createServer } from "node:net";

/**
 * Reserve and release an ephemeral TCP port, returning the port number.
 * Tests can use this to reduce port collision flakiness when spawning daemons.
 */
export async function getAvailablePort(): Promise<number> {
  return new Promise<number>((resolve, reject) => {
    const server = createServer();
    server.unref();

    server.once("error", reject);
    server.listen(0, "127.0.0.1", () => {
      const address = server.address();
      if (!address || typeof address === "string") {
        server.close(() => {
          reject(new Error("Failed to resolve an available TCP port"));
        });
        return;
      }

      const { port } = address;
      server.close((error) => {
        if (error) {
          reject(error);
          return;
        }
        resolve(port);
      });
    });
  });
}