net/http: guarantee that the Transport dial functions are respected in js/wasm

AUTHORS

@@ -1015,6 +1015,7 @@ Nathan Youngman <[email protected]>
 Nathaniel Cook <[email protected]>
 Naveen Kumar Sangi <[email protected]>
 Neelesh Chandola <[email protected]>
+Neil Alexander <[email protected]>
 Neil Lyons <[email protected]>
 Netflix, Inc.
 Neuman Vong <[email protected]>

CONTRIBUTORS

@@ -1812,6 +1812,7 @@ Naveen Kumar Sangi <[email protected]>
 Neeilan Selvalingam <[email protected]>
 Neelesh Chandola <[email protected]>
 Nehal J Wani <[email protected]>
+Neil Alexander <[email protected]>
 Neil Lyons <[email protected]>
 Neuman Vong <[email protected]>
 Neven Sajko <[email protected]>

src/net/http/roundtrip_js.go

@@ -41,11 +41,19 @@ const jsFetchCreds = "js.fetch:credentials"
 // Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
 const jsFetchRedirect = "js.fetch:redirect"
 
-var useFakeNetwork = js.Global().Get("fetch").IsUndefined()
+// jsFetchMissing will be true if the Fetch API is not present in
+// the browser globals.
+var jsFetchMissing = js.Global().Get("fetch").IsUndefined()
 
 // RoundTrip implements the RoundTripper interface using the WHATWG Fetch API.
 func (t *Transport) RoundTrip(req *Request) (*Response, error) {
-	if useFakeNetwork {
+	// The Transport has a documented contract that states that if the DialContext or
+	// DialTLSContext functions are set, they will be used to set up the connections.
+	// If they aren't set then the documented contract is to use Dial or DialTLS, even
+	// though they are deprecated. Therefore, if any of these are set, we should obey
+	// the contract and dial using the regular round-trip instead. Otherwise, we'll try
+	// to fall back on the Fetch API, unless it's not available.
+	if t.Dial != nil || t.DialContext != nil || t.DialTLS != nil || t.DialTLSContext != nil || jsFetchMissing {
 		return t.roundTrip(req)
 	}
 

src/net/http/transport.go

@@ -42,10 +42,10 @@ import (
 // $no_proxy) environment variables.
 var DefaultTransport RoundTripper = &Transport{
 	Proxy: ProxyFromEnvironment,
-	DialContext: (&net.Dialer{
+	DialContext: defaultTransportDialContext(&net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,
-	}).DialContext,
+	}),
 	ForceAttemptHTTP2:     true,
 	MaxIdleConns:          100,
 	IdleConnTimeout:       90 * time.Second,

src/net/http/transport_default_js.go

@@ -0,0 +1,17 @@
+// Copyright 2021 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.
+
+//go:build js && wasm
+// +build js,wasm
+
+package http
+
+import (
+	"context"
+	"net"
+)
+
+func defaultTransportDialContext(dialer *net.Dialer) func(context.Context, string, string) (net.Conn, error) {
+	return nil
+}

src/net/http/transport_default_other.go

@@ -0,0 +1,17 @@
+// Copyright 2021 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.
+
+//go:build !(js && wasm)
+// +build !js !wasm
+
+package http
+
+import (
+	"context"
+	"net"
+)
+
+func defaultTransportDialContext(dialer *net.Dialer) func(context.Context, string, string) (net.Conn, error) {
+	return dialer.DialContext
+}