cmd/go: derive TestExecutableGOROOT environment from tg.env instead of os.Environ()

TestExecutableGOROOT, unlike most other tests in go_test.go, was
running subcommands in a process with an environment derived directly
from os.Environ(), rather than using tg.env on its testgoData object.

Since tg.env is what sets GO111MODULE=off for GOPATH-mode tests, that
caused TestExecutableGOROOT to unexpectedly run in module mode instead
of GOPATH mode. If the user's environment included 'GOFLAGS=-mod=mod',
that would cause the test to spuriously fail due to the inability to
download modules to $HOME (which in this test binary is hard-coded to
"/test-go-home-does-not-exist").

Updates #33848

Change-Id: I2f343008dd9e38cd76b9919eafd5a3181d0cbd6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/205064
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -4687,23 +4687,19 @@ func copyFile(src, dst string, perm os.FileMode) error {
 	return err2
 }
 
+// TestExecutableGOROOT verifies that the cmd/go binary itself uses
+// os.Executable (when available) to locate GOROOT.
 func TestExecutableGOROOT(t *testing.T) {
 	skipIfGccgo(t, "gccgo has no GOROOT")
-	if runtime.GOOS == "openbsd" {
-		t.Skipf("test case does not work on %s, missing os.Executable", runtime.GOOS)
-	}
 
-	// Env with no GOROOT.
-	var env []string
-	for _, e := range os.Environ() {
-		if !strings.HasPrefix(e, "GOROOT=") {
-			env = append(env, e)
-		}
-	}
+	// Note: Must not call tg methods inside subtests: tg is attached to outer t.
+	tg := testgo(t)
+	tg.unsetenv("GOROOT")
+	defer tg.cleanup()
 
 	check := func(t *testing.T, exe, want string) {
 		cmd := exec.Command(exe, "env", "GOROOT")
-		cmd.Env = env
+		cmd.Env = tg.env
 		out, err := cmd.CombinedOutput()
 		if err != nil {
 			t.Fatalf("%s env GOROOT: %v, %s", exe, err, out)
@@ -4723,10 +4719,6 @@ func TestExecutableGOROOT(t *testing.T) {
 		}
 	}
 
-	// Note: Must not call tg methods inside subtests: tg is attached to outer t.
-	tg := testgo(t)
-	defer tg.cleanup()
-
 	tg.makeTempdir()
 	tg.tempDir("new/bin")
 	newGoTool := tg.path("new/bin/go" + exeSuffix)
@@ -4773,8 +4765,9 @@ func TestExecutableGOROOT(t *testing.T) {
 		}
 
 		cmd := exec.Command(newGoTool, "run", "testdata/print_goroot.go")
-		cmd.Env = env
-		out, err := cmd.CombinedOutput()
+		cmd.Env = tg.env
+		cmd.Stderr = os.Stderr
+		out, err := cmd.Output()
 		if err != nil {
 			t.Fatalf("%s run testdata/print_goroot.go: %v, %s", newGoTool, err, out)
 		}

src/cmd/go/testdata/print_goroot.go

@@ -4,8 +4,11 @@
 
 package main
 
-import "runtime"
+import (
+	"fmt"
+	"runtime"
+)
 
 func main() {
-	println(runtime.GOROOT())
+	fmt.Println(runtime.GOROOT())
 }