Repair Compression & Content Encoding Bugs

I have analyzed the middleware snippet. The issue stems from the proxy attempting to calculate Content-Length on the compressed stream before the flush is complete, leading to premature socket closure.

Streaming Compression Repair

const zlib = require('zlib');
const { pipeline } = require('stream');

// REPAIR: Remove manual Content-Length setting for dynamic streams.
// Use Transfer-Encoding: chunked to avoid truncation risks.
function proxyMiddleware(req, res, upstreamResponse) {
  const acceptEncoding = req.headers['accept-encoding'] || '';
  
  // Ensure we handle multiple encodings or lack thereof
  if (acceptEncoding.includes('gzip')) {
    res.setHeader('Content-Encoding', 'gzip');
    res.setHeader('Transfer-Encoding', 'chunked');
    res.removeHeader('Content-Length');

    const gzip = zlib.createGzip({
      flush: zlib.constants.Z_SYNC_FLUSH,
      finishFlush: zlib.constants.Z_STREAM_END
    });

    pipeline(upstreamResponse, gzip, res, (err) => {
      if (err) {
        console.error('Pipeline failed:', err);
        if (!res.headersSent) {
          res.statusCode = 500;
          res.end();
        } else {
          res.destroy(); // Terminate corrupted stream
        }
      }
    });
  } else {
    upstreamResponse.pipe(res);
  }
}

Invariants Applied

Next steps

1. Deploy to staging and verify the Transfer-Encoding: chunked header is present in browser dev tools.
2. Run a load test with payloads larger than 64KB to ensure the buffer doesn't overflow.
3. Check upstream logs to confirm the connection is not being reset by the proxy's timeout.