cmd/go: avoid race on test environment

Fixes #14337.

Change-Id: I58aef7e08d936b0712da577dd1ce5c9ed5d8bfd2
Reviewed-on: https://go-review.googlesource.com/19513Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/test.go

@@ -947,6 +947,11 @@ func (t *tester) raceTest(dt *distTest) error {
 	t.addCmd(dt, "src", "go", "test", "-race", "-i", "runtime/race", "flag", "os/exec")
 	t.addCmd(dt, "src", "go", "test", "-race", "-run=Output", "runtime/race")
 	t.addCmd(dt, "src", "go", "test", "-race", "-short", "-run=TestParse|TestEcho", "flag", "os/exec")
+	// We don't want the following line, because it
+	// slows down all.bash (by 10 seconds on my laptop).
+	// The race builder should catch any error here, but doesn't.
+	// TODO(iant): Figure out how to catch this.
+	// t.addCmd(dt, "src", "go", "test", "-race", "-run=TestParallelTest", "cmd/go")
 	if t.cgoEnabled {
 		env := mergeEnvLists([]string{"GOTRACEBACK=2"}, os.Environ())
 		cmd := t.addCmd(dt, "misc/cgo/test", "go", "test", "-race", "-short")

src/cmd/go/go_test.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"go/build"
 	"go/format"
+	"internal/race"
 	"internal/testenv"
 	"io"
 	"io/ioutil"
@@ -69,7 +70,11 @@ func TestMain(m *testing.M) {
 	flag.Parse()
 
 	if canRun {
-		out, err := exec.Command("go", "build", "-tags", "testgo", "-o", "testgo"+exeSuffix).CombinedOutput()
+		args := []string{"build", "-tags", "testgo", "-o", "testgo" + exeSuffix}
+		if race.Enabled {
+			args = append(args, "-race")
+		}
+		out, err := exec.Command("go", args...).CombinedOutput()
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "building testgo failed: %v\n%s", err, out)
 			os.Exit(2)
@@ -2735,3 +2740,22 @@ func TestIssue13655(t *testing.T) {
 		tg.grepStdout("runtime/internal/sys", "did not find required dependency of "+pkg+" on runtime/internal/sys")
 	}
 }
+
+// For issue 14337.
+func TestParallelTest(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.makeTempdir()
+	const testSrc = `package package_test
+		import (
+			"testing"
+		)
+		func TestTest(t *testing.T) {
+		}`
+	tg.tempFile("src/p1/p1_test.go", strings.Replace(testSrc, "package_test", "p1_test", 1))
+	tg.tempFile("src/p2/p2_test.go", strings.Replace(testSrc, "package_test", "p2_test", 1))
+	tg.tempFile("src/p3/p3_test.go", strings.Replace(testSrc, "package_test", "p3_test", 1))
+	tg.tempFile("src/p4/p4_test.go", strings.Replace(testSrc, "package_test", "p4_test", 1))
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("test", "-p=4", "p1", "p2", "p3", "p4")
+}

src/cmd/go/main.go

@@ -454,7 +454,9 @@ func envForDir(dir string, base []string) []string {
 
 // mergeEnvLists merges the two environment lists such that
 // variables with the same name in "in" replace those in "out".
+// This always returns a newly allocated slice.
 func mergeEnvLists(in, out []string) []string {
+	out = append([]string(nil), out...)
 NextVar:
 	for _, inkv := range in {
 		k := strings.SplitAfterN(inkv, "=", 2)[0]