os/exec: simplify Windows-specific tests

- Use the test binary itself for printing paths instead of building a
  separate binary and running it through additional subprocesses.

- Factor out a common chdir helper.

- Use t.Setenv where appropriate.

- Reduce indirection in test helpers.

- Set NoDefaultCurrentDirectoryInExePath consistently in the
  environment.

Also add a test case demonstrating an interesting behavior for
relative paths that may interact with #62596.

Fixes #62594.
For #62596.

Change-Id: I19b9325034edf78cd0ca747594476cd7432bb451
Reviewed-on: https://go-review.googlesource.com/c/go/+/528035
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/os/exec/dot_test.go

@@ -24,7 +24,7 @@ var pathVar string = func() string {
 
 func TestLookPath(t *testing.T) {
 	testenv.MustHaveExec(t)
-	// Not parallel: uses os.Chdir and t.Setenv.
+	// Not parallel: uses Chdir and Setenv.
 
 	tmpDir := filepath.Join(t.TempDir(), "testdir")
 	if err := os.Mkdir(tmpDir, 0777); err != nil {
@@ -38,18 +38,7 @@ func TestLookPath(t *testing.T) {
 	if err := os.WriteFile(filepath.Join(tmpDir, executable), []byte{1, 2, 3}, 0777); err != nil {
 		t.Fatal(err)
 	}
-	cwd, err := os.Getwd()
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer func() {
-		if err := os.Chdir(cwd); err != nil {
-			panic(err)
-		}
-	}()
-	if err = os.Chdir(tmpDir); err != nil {
-		t.Fatal(err)
-	}
+	chdir(t, tmpDir)
 	t.Setenv("PWD", tmpDir)
 	t.Logf(". is %#q", tmpDir)
 

src/os/exec/exec_test.go

@@ -77,6 +77,21 @@ func TestMain(m *testing.M) {
 	if os.Getenv("GO_EXEC_TEST_PID") == "" {
 		os.Setenv("GO_EXEC_TEST_PID", strconv.Itoa(pid))
 
+		if runtime.GOOS == "windows" {
+			// Normalize environment so that test behavior is consistent.
+			// (The behavior of LookPath varies depending on this variable.)
+			//
+			// Ideally we would test both with the variable set and with it cleared,
+			// but I (bcmills) am not sure that that's feasible: it may already be set
+			// in the Windows registry, and I'm not sure if it is possible to remove
+			// a registry variable in a program's environment.
+			//
+			// Per https://learn.microsoft.com/en-us/windows/win32/api/processenv/nf-processenv-needcurrentdirectoryforexepathw#remarks,
+			// “the existence of the NoDefaultCurrentDirectoryInExePath environment
+			// variable is checked, and not its value.”
+			os.Setenv("NoDefaultCurrentDirectoryInExePath", "TRUE")
+		}
+
 		code := m.Run()
 		if code == 0 && flag.Lookup("test.run").Value.String() == "" && flag.Lookup("test.list").Value.String() == "" {
 			for cmd := range helperCommands {
@@ -180,6 +195,28 @@ var exeOnce struct {
 	sync.Once
 }
 
+func chdir(t *testing.T, dir string) {
+	t.Helper()
+
+	prev, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chdir(dir); err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Chdir(%#q)", dir)
+
+	t.Cleanup(func() {
+		if err := os.Chdir(prev); err != nil {
+			// Couldn't chdir back to the original working directory.
+			// panic instead of t.Fatal so that we don't run other tests
+			// in an unexpected location.
+			panic("couldn't restore working directory: " + err.Error())
+		}
+	})
+}
+
 var helperCommandUsed sync.Map
 
 var helperCommands = map[string]func(...string){

src/os/exec/lp_unix_test.go

@@ -4,30 +4,19 @@
 
 //go:build unix
 
-package exec
+package exec_test
 
 import (
 	"os"
+	"os/exec"
 	"testing"
 )
 
 func TestLookPathUnixEmptyPath(t *testing.T) {
-	// Not parallel: uses os.Chdir.
+	// Not parallel: uses Chdir and Setenv.
 
-	tmp, err := os.MkdirTemp("", "TestLookPathUnixEmptyPath")
-	if err != nil {
-		t.Fatal("TempDir failed: ", err)
-	}
-	defer os.RemoveAll(tmp)
-	wd, err := os.Getwd()
-	if err != nil {
-		t.Fatal("Getwd failed: ", err)
-	}
-	err = os.Chdir(tmp)
-	if err != nil {
-		t.Fatal("Chdir failed: ", err)
-	}
-	defer os.Chdir(wd)
+	tmp := t.TempDir()
+	chdir(t, tmp)
 
 	f, err := os.OpenFile("exec_me", os.O_CREATE|os.O_EXCL, 0700)
 	if err != nil {
@@ -40,7 +29,7 @@ func TestLookPathUnixEmptyPath(t *testing.T) {
 
 	t.Setenv("PATH", "")
 
-	path, err := LookPath("exec_me")
+	path, err := exec.LookPath("exec_me")
 	if err == nil {
 		t.Fatal("LookPath found exec_me in empty $PATH")
 	}