`fetchEventSource` doesn't abort when passing in an already aborted signal

[kevin-dp]

Javascript's default fetch rejects with an AbortError when passing an already aborted signal:

const controller = new AbortController()
controller.abort()
await fetch("https://google.com", { signal: controller.signal })
Uncaught:
DOMException [AbortError]: This operation was aborted
    at node:internal/deps/undici/undici:13392:13
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async REPL17:1:33

However, when using fetch-event-source it fails to detect that the signal is already aborted:

const controller = new AbortController()
controller.abort()
await fetchEventSource("https://any-website-supporting-sse.com", { signal: controller.signal })
// the call above is never aborted

This doesn't abort the fetch.

The reason is that fetchEventSource creates a new aborter and subscribes to the original signal's abort event to then abort its own controller, but the abort event does not trigger retroactively (i.e. if the signal is already aborted). Here's the relevant snippet from the fetchEventSource implementation:

let curRequestController: AbortController;

function dispose() {
  // ...
  curRequestController.abort();
}

inputSignal?.addEventListener('abort', () => {
  // this callback is not called if the inputSignal is already aborted!
  // hence, curRequestController is not aborted
  dispose();
  resolve();
});

async function create() {
  curRequestController = new AbortController();
    try {
      const response = await fetch(input, {
        ...rest,
        headers,
        signal: curRequestController.signal,
      });
      // ...
    } catch (err) {
      if (!curRequestController.signal.aborted) {
        // ...
      }
    }
};

I patched it locally as follows:

inputSignal?.addEventListener('abort', () => {
  dispose();
  // no longer resolving the promise here
  // because 1) we should reject instead of resolve when aborted
  // and 2) dispose() will abort the curRequestController.signal
  // which will cause the ongoing fetch request to throw an AbortError
  // which we will catch and then we will reject the promise with the error
});

async function create() {
  curRequestController = new AbortController();
  // use the input signal if it is already aborted
  const sig = inputSignal.aborted ? inputSignal : curRequestController.signal
    try {
      const response = await fetch(input, {
        ...rest,
        headers,
        signal: sig,
      });
      // ...
    } catch (err) {
      if (sig.aborted) {
        dispose();
        reject(err);
      } else if (!curRequestController.signal.aborted) {
        // ...
      }
    }
};