waiProxyTo hangs when the server responds with HTTP 103 Early Hints

[thomasjm]

HTTP 103 Early Hints is a newish HTTP feature where the server can indicate assets that the client can begin downloading immediately, before the server finishes its response.

It does this by sending 103 Early Hint followed by a Link: header.

I made a full repro here: https://github.com/codedownio/http-reverse-proxy/tree/http-103-early-hints-repro

The repro starts up a Node.js server which serves two routes, /simple and /early_hints. It also contains a tiny Haskell app which uses Warp to proxy to the Node.js server.

The /early_hints route sends early hints, and causes waiProxyTo to fail. The repro README describes how to run it and shows the output of curl -v against the different endpoints.