cmd/go: don't let "go test -c -o /dev/null" overwrite /dev/null

Fixes #28549

Change-Id: Iba71bb2edd0759004e0c7df92b2b8f1197bd62d3
Reviewed-on: https://go-review.googlesource.com/c/146901
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -1462,8 +1462,38 @@ func TestInstallIntoGOPATH(t *testing.T) {
 func TestBuildOutputToDevNull(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()
+	fi1, err1 := os.Lstat(os.DevNull)
 	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
 	tg.run("build", "-o", os.DevNull, "go-cmd-test")
+	fi2, err2 := os.Lstat(os.DevNull)
+	if err1 == nil {
+		if err2 != nil {
+			t.Errorf("second stat of /dev/null failed: %v", err2)
+		} else if !os.SameFile(fi1, fi2) {
+			t.Errorf("/dev/null changed: now %v was %v", fi1, fi2)
+		}
+	}
+}
+
+// Issue 28549.
+func TestTestOutputToDevNull(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	fi1, err1 := os.Lstat(os.DevNull)
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+	tg.tempFile("src/p/p.go", "package p\n")
+	tg.tempFile("src/p/p_test.go", "package p\nimport \"testing\"\nfunc TestX(t *testing.T) {}\n")
+	tg.run("test", "-o", os.DevNull, "-c", "p")
+	tg.mustNotExist("p.test")
+	fi2, err2 := os.Lstat(os.DevNull)
+	if err1 == nil {
+		if err2 != nil {
+			t.Errorf("second stat of /dev/null failed: %v", err2)
+		} else if !os.SameFile(fi1, fi2) {
+			t.Errorf("/dev/null changed: now %v was %v", fi1, fi2)
+		}
+	}
 }
 
 func TestPackageMainTestImportsArchiveNotBinary(t *testing.T) {

src/cmd/go/internal/test/test.go

@@ -887,15 +887,19 @@ func builderTest(b *work.Builder, p *load.Package) (buildAction, runAction, prin
 				target = filepath.Join(base.Cwd, target)
 			}
 		}
-		pmain.Target = target
-		installAction = &work.Action{
-			Mode:    "test build",
-			Func:    work.BuildInstallFunc,
-			Deps:    []*work.Action{buildAction},
-			Package: pmain,
-			Target:  target,
+		if target == os.DevNull {
+			runAction = buildAction
+		} else {
+			pmain.Target = target
+			installAction = &work.Action{
+				Mode:    "test build",
+				Func:    work.BuildInstallFunc,
+				Deps:    []*work.Action{buildAction},
+				Package: pmain,
+				Target:  target,
+			}
+			runAction = installAction // make sure runAction != nil even if not running test
 		}
-		runAction = installAction // make sure runAction != nil even if not running test
 	}
 	var vetRunAction *work.Action
 	if testC {