[release-branch.go1.12] cmd/go/internal/{modconv,modfetch,modload}: set modfetch proxy URL in tests

Updates #30571
Fixes #34789

Change-Id: Id4c74e83ee58a080d1c2894ae5ebdbf4aeb1ce42
Reviewed-on: https://go-review.googlesource.com/c/go/+/167084
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit bd680d94)
Reviewed-on: https://go-review.googlesource.com/c/go/+/200037

src/cmd/go/internal/modconv/convert_test.go

@@ -28,6 +28,8 @@ func TestMain(m *testing.M) {
 }
 
 func testMain(m *testing.M) int {
+	modfetch.SetProxy("direct")
+
 	if _, err := exec.LookPath("git"); err != nil {
 		fmt.Fprintln(os.Stderr, "skipping because git binary not found")
 		fmt.Println("PASS")

src/cmd/go/internal/modfetch/coderepo_test.go

@@ -24,6 +24,8 @@ func TestMain(m *testing.M) {
 }
 
 func testMain(m *testing.M) int {
+	SetProxy("direct")
+
 	dir, err := ioutil.TempDir("", "gitrepo-test-")
 	if err != nil {
 		log.Fatal(err)

src/cmd/go/internal/modfetch/proxy.go

@@ -87,6 +87,14 @@ cached module versions with GOPROXY=https://example.com/proxy.
 
 var proxyURL = os.Getenv("GOPROXY")
 
+// SetProxy sets the proxy to use when fetching modules.
+// It accepts the same syntax as the GOPROXY environment variable,
+// which also provides its default configuration.
+// SetProxy must not be called after the first module fetch has begun.
+func SetProxy(url string) {
+	proxyURL = url
+}
+
 func lookupProxy(path string) (Repo, error) {
 	if strings.Contains(proxyURL, ",") {
 		return nil, fmt.Errorf("invalid $GOPROXY setting: cannot have comma")

src/cmd/go/internal/modload/query_test.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 	"testing"
 
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modfetch/codehost"
 	"cmd/go/internal/module"
@@ -24,11 +25,16 @@ func TestMain(m *testing.M) {
 }
 
 func testMain(m *testing.M) int {
+	modfetch.SetProxy("direct")
+
 	dir, err := ioutil.TempDir("", "modload-test-")
 	if err != nil {
 		log.Fatal(err)
 	}
 	defer os.RemoveAll(dir)
+
+	os.Setenv("GOPATH", dir)
+	cfg.BuildContext.GOPATH = dir
 	modfetch.PkgMod = filepath.Join(dir, "pkg/mod")
 	codehost.WorkRoot = filepath.Join(dir, "codework")
 	return m.Run()