How we found a bug in the hyper HTTP library

Cloudflare engineers investigated intermittent truncation of large Images binding responses after replacing an intermediary; kernel-level tracing revealed a race in hyper where poll_flush() could return Poll::Pending but its result was discarded, allowing hyper to prematurely call shutdown and drop most of the response body. The team built a deterministic test that simulated a constrained socket, implemented a targeted fix to flush remaining buffered bytes before shutdown, merged the change upstream (PR #4018), and ran an internal fork until the patched hyper release is available.