gzhttp: Handle informational headers

[rtribotte]

Hello,

This PR introduces the support of 1xx status codes forwarding.
One unit test has been added and is an adapted version of the go standard library test for 1xx status code responses.

Fixes #814

[klauspost]

@rtribotte Thanks!

Didn't see #814 - I will check it out.

[klauspost]

Very good. Thanks for the details in #814

[klauspost]

@rtribotte Seems like you need to gate this for Go 1.20+

[rtribotte]

Hey @klauspost,

Thanks for the prompt review!
I pushed a review commit to gate the test for go1.20.

[klauspost]

@rtribotte Shouldn't the functionality also be gated?

As far as I can tell, this could break on older Go versions, where if you send a 100, it will be sent upstream, which will send all headers.

For older versions this would make the headers immutable, meaning that Content-Encoding can no longer be set - and the content will be sent compressed, but without the appropriate header.

This is all speculation, so feel free to correct me.

[rtribotte]

@klauspost Thanks for the feedback, I think you are right regarding the immutability of headers.

For package net/http the 1xx codes are correctly handled starting with v1.19:
go1.18 :https://github.com/golang/go/blob/581603cb7d02019bbf4ff508014038f3120a3dcb/src/net/http/server.go#L1146
go1.19: https://github.com/golang/go/blob/484330d038d060c6e4db3dc8e6ea2b811b2a44d8/src/net/http/server.go#L1168-L1171

For package x/net/http2 the fix (golang/net@9564170) is shipped since v0.9.0, which seems to be targeted for go1.17+.

So the specific concern of headers immutability after the first write, I would say that the handling of informational headers should gated to go1.19+.

Nonetheless, for package net/http/httputil the fix to forward 1xx responses has been shipped with go1.20: golang/go@972870d

Considering the latter I pushed a new commit to gate the feature for go1.20+.

[klauspost]

@rtribotte Thanks. I prefer the conservative choice. Merging when tests have run.