Emitting 'error' from Multipart is undefined behaviour

[davidje13]

Manually emitting 'error' events from Writable._write causes undefined behaviour due to this event name being used internally.

This happens in busboy if multipart headers are malformed or too long:

https://github.com/mscdex/busboy/blob/master/lib/types/multipart.js#L398

(the route is a bit circuitous but ultimately that code is invoked from within the _write handler)

In practice, the impact is that if this error is emitted, the writable never emits end / close, meaning that code like this never completes:

const myInput = Readable.from(Buffer.from('--foo\r\n: oops\r\n\r\ncontent\r\n--foo--'));
const handler = busboy({ headers: { 'content-type': 'multipart/form-data; boundary=foo' } });
handler.on('close', () => {
  console.log('done'); // this is never printed even though busboy sets emitClose: true
});
myInput.pipe(handler);

Changing the event name (e.g. to 'warning') fixes the issue, and the close event fires as expected.

Note that other failure modes (e.g. for the input --foo\r\nx:) emit the error and also the close event (as expected), because the error is sent via the callback of _final rather than emitted directly.