fix: propagate batchFn sync throws to the loader instead of crashing

[boopathi]

If the batch load function throws synchronously, the error is not caught anywhere in the load pipeline. A synchronous error in the batchLoadFn cannot be caught. It crashes the server as an uncaught exception. Since the batchLoadFn is invoked as a result of .load the errors happening inside should also be available at the .load call.

This PR fixes this issue by wrapping a try catch around the batchLoadFn call and returning a failed dispatch.

const loader = new Dataloader( keys => {
  const mergedKeys = functionThatCanThrowSynchronously(keys);
  // correctly returns a Promise as expected by the Dataloader
  return fetch(mergedKeys).then(mergedValues => split(mergedValues));
});

For example, consider the following minimal repro -

const loader = new DataLoader(() => {
  throw new Error("mock error");
});

try {
  loader.load(1).catch(e => {
    // mock error will not be caught here
  })
} catch (e) {
  // mock error will also not be caught here
}

With this PR, the "mock error" would be caught in the load.catch handler.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: boopathi / name: Boopathi Rajaa (6381616)

[oporkka]

We had a past experience that this bug caused our application crash in a hard to discover way because of the uncaught exception it allows. What would be needed to get this merged?