net/http: Server/Handler/ResponseWriter doesn't allowing sending arbitrary 1xx responses

[bradfitz]

As part of #17739 (properly handling 1xx responses per the HTTP specs), we need to be able to forward 1xx responses in ReverseProxy (#26088)

To forward such responses, we need a way for for an http.Handler in a Server to use the ResponseWriter (or something) to send a 1xx response.

[bradfitz]

(Notably, to send any number of 1xx responses and then a final non-1xx response)

Maybe we just allow multiple ResponseWriter.WriteHeader calls and say that 1xx writes clear the header for future calls to Header?

[gopherbot]

Change https://golang.org/cl/269997 mentions this issue: net/http: allow sending 1xx responses

[gopherbot]

Change https://golang.org/cl/270157 mentions this issue: http2: allow sending 1xx responses

[gopherbot]

Change https://golang.org/cl/291029 mentions this issue: http2: allow sending 1xx responses