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

The net/http package has a documented contract that if DialContext, DialDLSContext, Dial or DialTLS are specified in an instance of Transport, that they will be used to set up the connection. If they are not specified, then a reasonable fallback is made (e.g. using the net package).

This is ordinarily true, except for when compiling for the js/wasm target, where the browser's Fetch API is preferred in all cases (except for when it is undefined/unavailable) and therefore the dial functions are all ignored. As a result, the http.Transport implementation under js/wasm doesn't meet that contract.

This PR updates the RoundTrip behaviour of http.Transport so that if DialContext, DialTLSContext, Dial or DialTLS are specified, they are used as expected. The Fetch API will be used as a fallback if they are not specified.

Fixes #27495

Change-Id: I88c6eb6ffdd077827b421d606f3e60ebdafd538f
GitHub-Last-Rev: 948a0ed97e
GitHub-Pull-Request: golang/go#46923
Reviewed-on: https://go-review.googlesource.com/c/go/+/330852
Reviewed-by: Damien Neil <dneil@google.com>
Trust: Damien Neil <dneil@google.com>
Trust: Carlos Amedee <carlos@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

AUTHORS

@@ -1015,6 +1015,7 @@ Nathan Youngman <git@nathany.com>
 Nathaniel Cook <nvcook42@gmail.com>
 Naveen Kumar Sangi <naveenkumarsangi@protonmail.com>
 Neelesh Chandola <neelesh.c98@gmail.com>
+Neil Alexander <neilalexander@neilalexander.dev>
 Neil Lyons <nwjlyons@googlemail.com>
 Netflix, Inc.
 Neuman Vong <neuman.vong@gmail.com>

CONTRIBUTORS

@@ -1901,6 +1901,7 @@ Naveen Kumar Sangi <naveenkumarsangi@protonmail.com>
 Neeilan Selvalingam <neeilan96@gmail.com>
 Neelesh Chandola <neelesh.c98@gmail.com>
 Nehal J Wani <nehaljw.kkd1@gmail.com>
+Neil Alexander <neilalexander@neilalexander.dev>
 Neil Lyons <nwjlyons@googlemail.com>
 Neuman Vong <neuman.vong@gmail.com>
 Neven Sajko <nsajko@gmail.com>

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
+}