Function probe

test/server/proxy-option.test.js:902–931

(path)

    // handshake (`opened`) and whether the upgrade was forwarded to the backend
    // proxy (`forwarded`).
    const probe = async (path) => {
      const before = backendUpgradeCount;

      const ws = new WebSocket(`ws://localhost:${port3}${path}`);

      // Resolve as soon as the socket reaches a terminal state instead of
      // waiting a fixed delay: `open` means the handshake completed, `error`
      // means it was rejected. The timeout is only a fallback in case neither
      // event ever fires, so it can be generous without slowing the happy path.
      const opened = await new Promise((resolve) => {
        const timer = setTimeout(() => resolve(false), 2000);

        ws.once("open", () => {
          clearTimeout(timer);
          resolve(true);
        });
        ws.once("error", () => {
          clearTimeout(timer);
          resolve(false);
        });
      });

      try {
        ws.close();
      } catch {
        // ignore close errors on already-failed sockets
      }

      return { opened, forwarded: backendUpgradeCount > before };
    };

    // Behavior shared by every WebSocket server implementation: the HMR socket
    // is served locally and never forwarded, while any path the HMR server does