net/http: no way to set Fetch options when making outgoing http calls with js/wasm

[eginez]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version devel +c814ac44c0 Thu Jul 19 21:30:27 2018 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="wasm"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="js"
GOPATH="/Users/eginez/repos/goland"
GOPROXY=""
GORACE=""
GOROOT="/Users/eginez/repos/go"
GOTMPDIR=""
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/pm/tlg5p5nx4vj3s05h4rfsv3t40000gn/T/go-build136489492=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

The native http client, maps to the fetch() api. With the current client it is impossible to set additional options as supported by the fetch call, see: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch.

What did you expect to see?

A way to specify options for the fetch call

[agnivade]

Yes, this is known. Currently, there is no good way to expose fetch parameters from user level to the http.RoundTripper which is what we are using to make the fetch call. It is not very straightforward, as we have to make some decisions on how best to integrate browser options into a Go API.

See a very small discussion here - https://go-review.googlesource.com/c/go/+/114515/8/src/net/http/roundtrip_js.go#34

The main tracking issue for a better wasm http client is #25695. I would suggest that you comment on that issue with your suggestions on how best we can take this forward.

[bradfitz]

@agnivade, things aren't that doomed. We can use http.Request.WithContext or HTTP headers to something to set options for Fetch.

/cc @neelance @dmitshur

[bradfitz]

(That is, we don't need to wait for a new HTTP client package design to make progress on this)

[agnivade]

Ah, using WithContext sounds like a good idea. I was overcomplicating things.

Also @johanbrandhorst

[johanbrandhorst]

That's a cool idea, but how do we make it obvious to the user? Is using the context as a transport for arbitrary downstream method invocations something well established? I'd be happy to implement this if there's a proposal.

[bradfitz]

Using contexts for options is actually generally considered a gross practice. But we can do so if needed.

But before designing anything, what are the specific option(s) we're talking about here? Maybe they already map to something we have in Go.

[agnivade]

To me, mode and credentials are the 2 most important options that we should expose.

• mode can take cors, no-cors, same-origin.
• credentials can take omit, same-origin, include.

These are settings which arise only in a browser context. So as I see, there is no clear way to map them in Go.

We also have redirect which somehow maps with client.CheckRedirect.

See here for remaining options - https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters