net/http: support configuring fetch options

The default WASM RoundTripper is implemented using
the browser Fetch API. Some options don't readily map to
existing http.Request options, so we use the precedent
set by the TrailerPrefix constant to allow a user to configure
the "mode" and "credentials" options by supplying them
as headers in the http.Request.

Updates #26769

Change-Id: If42d24418c4ffb17211f57e36708cf460fb4c579
GitHub-Last-Rev: b230502084d628938cd50818d3d336f9f911d48d
GitHub-Pull-Request: golang/go#26784
Reviewed-on: https://go-review.googlesource.com/127718Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/roundtrip_js.go

@@ -17,17 +17,27 @@ import (
 	"syscall/js"
 )
 
+// jsFetchMode is a Request.Header map key that, if present,
+// signals that the map entry is actually an option to the Fetch API mode setting.
+// Valid values are: "cors", "no-cors", "same-origin", "navigate"
+// The default is "same-origin".
+//
+// Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
+const jsFetchMode = "js.fetch:mode"
+
+// jsFetchCreds is a Request.Header map key that, if present,
+// signals that the map entry is actually an option to the Fetch API credentials setting.
+// Valid values are: "omit", "same-origin", "include"
+// The default is "same-origin".
+//
+// Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
+const jsFetchCreds = "js.fetch:credentials"
+
 // RoundTrip implements the RoundTripper interface using the WHATWG Fetch API.
 func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	if useFakeNetwork() {
 		return t.roundTrip(req)
 	}
-	headers := js.Global().Get("Headers").New()
-	for key, values := range req.Header {
-		for _, value := range values {
-			headers.Call("append", key, value)
-		}
-	}
 
 	ac := js.Global().Get("AbortController")
 	if ac != js.Undefined() {
@@ -40,12 +50,26 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	opt := js.Global().Get("Object").New()
 	// See https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch
 	// for options available.
-	opt.Set("headers", headers)
 	opt.Set("method", req.Method)
 	opt.Set("credentials", "same-origin")
+	if h := req.Header.Get(jsFetchCreds); h != "" {
+		opt.Set("credentials", h)
+		req.Header.Del(jsFetchCreds)
+	}
+	if h := req.Header.Get(jsFetchMode); h != "" {
+		opt.Set("mode", h)
+		req.Header.Del(jsFetchMode)
+	}
 	if ac != js.Undefined() {
 		opt.Set("signal", ac.Get("signal"))
 	}
+	headers := js.Global().Get("Headers").New()
+	for key, values := range req.Header {
+		for _, value := range values {
+			headers.Call("append", key, value)
+		}
+	}
+	opt.Set("headers", headers)
 
 	if req.Body != nil {
 		// TODO(johanbrandhorst): Stream request body when possible.