Backpressure propagation

[sigJoe]

Hi I recently worked on an app using Node 22 busboy 1.6.0 and found that the app sometimes hitting OOM when handling uploads for large files (e.g. 1 GB). Heap dumps showed all the bloat is coming from busboy's FileStream._readableState.buffer, where it is trying to store almost the entire file upload.

The app pipes busboy's filestream (file.pipe(writeStream)), and I saw backpressure caused by a slow writestream was (as expected) pausing the filestream, but then busboy in turn was not pausing the original request. Where the app flow is request -> busboy filestream -> writestream, the request was continuing to push chunks to the filestream.

A quick fix was to add this snippet to my app:

file.on('pause', () => {
    if (req.readable && !req.isPaused()) {
        req.pause();
    }
});
file.on('resume', () => {
    if (req.readable && req.isPaused()) {
        req.resume();
    }
});

It took me a while to figure out where the problem was, so I thought I'd ask is this expected behaviour or not?

If this is expected, then I suggest adding a warning or similar code snippet to examples in the readme.