net/http: expose standard transport for js

Implement a separate type jsTransport that implements RoundTripper using
the WHATWG Fetch API, and make it the DefaultTransport on js/wasm. This
allows using the standard Transport implementation on js/wasm by
specifying a DialContext for the Transport. Note that the default would
use net.Dial, that can not be used in the browser.

Before this change the same Transport type was used also on js, but with
a different implementation. This implementation disregarded any of the
documented Transport settings and just used the browser Fetch API. This
made it impossible to use the Transport implementation, e.g., by
supplying a DialContext that works in the browser.

Fixes golang#27495

Author: antong

src/net/http/default_transport.go

@@ -0,0 +1,14 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !js
+
+package http
+
+// DefaultTransport is the default implementation of Transport and is
+// used by DefaultClient. It establishes network connections as needed
+// and caches them for reuse by subsequent calls. It uses HTTP proxies
+// as directed by the $HTTP_PROXY and $NO_PROXY (or $http_proxy and
+// $no_proxy) environment variables.
+var DefaultTransport RoundTripper = defaultTransport

src/net/http/default_transport_js.go

@@ -0,0 +1,12 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js,wasm
+
+package http
+
+// DefaultTransport is the default implementation of Transport and is
+// used by DefaultClient. It implements the RoundTripper interface
+// using the WHATWG Fetch API.
+var DefaultTransport RoundTripper = (*jsTransport)(nil)

src/net/http/roundtrip.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !js !wasm
-
 package http
 
 // RoundTrip implements the RoundTripper interface.

src/net/http/roundtrip_js.go

@@ -41,10 +41,12 @@ const jsFetchCreds = "js.fetch:credentials"
 // Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
 const jsFetchRedirect = "js.fetch:redirect"
 
+type jsTransport struct{}
+
 // RoundTrip implements the RoundTripper interface using the WHATWG Fetch API.
-func (t *Transport) RoundTrip(req *Request) (*Response, error) {
+func (_ *jsTransport) RoundTrip(req *Request) (*Response, error) {
 	if useFakeNetwork() {
-		return t.roundTrip(req)
+		return defaultTransport.roundTrip(req)
 	}
 
 	ac := js.Global().Get("AbortController")

src/net/http/transport.go

@@ -34,12 +34,7 @@ import (
 	"internal/x/net/http/httpproxy"
 )
 
-// DefaultTransport is the default implementation of Transport and is
-// used by DefaultClient. It establishes network connections as needed
-// and caches them for reuse by subsequent calls. It uses HTTP proxies
-// as directed by the $HTTP_PROXY and $NO_PROXY (or $http_proxy and
-// $no_proxy) environment variables.
-var DefaultTransport RoundTripper = &Transport{
+var defaultTransport = &Transport{
 	Proxy: ProxyFromEnvironment,
 	DialContext: (&net.Dialer{
 		Timeout:   30 * time.Second,