os/exec: on Windows, suppress ErrDot if the implicit path matches the explicit one

If the current directory is also listed explicitly in %PATH%,
this changes the behavior of LookPath to prefer the explicit name for it
(and thereby avoid ErrDot).

However, in order to avoid running a different executable from what
would have been run by previous Go versions, we still return the
implicit path (and ErrDot) if it refers to a different file entirely.

Fixes #53536.
Updates #43724.

Change-Id: I7ab01074e21a0e8b07a176e3bc6d3b8cf0c873cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/414054
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 116 insertions, 7 deletions

diff --git a/src/os/exec/dot_test.go b/src/os/exec/dot_test.go
index 932d907c9e..e2d2dba7a5 100644
--- a/src/os/exec/dot_test.go
+++ b/src/os/exec/dot_test.go
@@ -15,6 +15,13 @@ import (
 	"testing"
 )
 
+var pathVar string = func() string {
+	if runtime.GOOS == "plan9" {
+		return "path"
+	}
+	return "PATH"
+}()
+
 func TestLookPath(t *testing.T) {
 	testenv.MustHaveExec(t)
 
@@ -42,22 +49,24 @@ func TestLookPath(t *testing.T) {
 	if err = os.Chdir(tmpDir); err != nil {
 		t.Fatal(err)
 	}
-	origPath := os.Getenv("PATH")
-	defer os.Setenv("PATH", origPath)
+	t.Setenv("PWD", tmpDir)
+	t.Logf(". is %#q", tmpDir)
+
+	origPath := os.Getenv(pathVar)
 
 	// Add "." to PATH so that exec.LookPath looks in the current directory on all systems.
 	// And try to trick it with "../testdir" too.
 	for _, dir := range []string{".", "../testdir"} {
-		os.Setenv("PATH", dir+string(filepath.ListSeparator)+origPath)
-		t.Run("PATH="+dir, func(t *testing.T) {
+		t.Run(pathVar+"="+dir, func(t *testing.T) {
+			t.Setenv(pathVar, dir+string(filepath.ListSeparator)+origPath)
 			good := dir + "/execabs-test"
 			if found, err := LookPath(good); err != nil || !strings.HasPrefix(found, good) {
-				t.Fatalf("LookPath(%q) = %q, %v, want \"%s...\", nil", good, found, err, good)
+				t.Fatalf(`LookPath(%#q) = %#q, %v, want "%s...", nil`, good, found, err, good)
 			}
 			if runtime.GOOS == "windows" {
 				good = dir + `\execabs-test`
 				if found, err := LookPath(good); err != nil || !strings.HasPrefix(found, good) {
-					t.Fatalf("LookPath(%q) = %q, %v, want \"%s...\", nil", good, found, err, good)
+					t.Fatalf(`LookPath(%#q) = %#q, %v, want "%s...", nil`, good, found, err, good)
 				}
 			}
 
@@ -84,4 +93,81 @@ func TestLookPath(t *testing.T) {
 			}
 		})
 	}
+
+	// Test the behavior when the first entry in PATH is an absolute name for the
+	// current directory.
+	//
+	// On Windows, "." may or may not be implicitly included before the explicit
+	// %PATH%, depending on the process environment;
+	// see https://go.dev/issue/4394.
+	//
+	// If the relative entry from "." resolves to the same executable as what
+	// would be resolved from an absolute entry in %PATH% alone, LookPath should
+	// return the absolute version of the path instead of ErrDot.
+	// (See https://go.dev/issue/53536.)
+	//
+	// If PATH does not implicitly include "." (such as on Unix platforms, or on
+	// Windows configured with NoDefaultCurrentDirectoryInExePath), then this
+	// lookup should succeed regardless of the behavior for ".", so it may be
+	// useful to run as a control case even on those platforms.
+	t.Run(pathVar+"=$PWD", func(t *testing.T) {
+		t.Setenv(pathVar, tmpDir+string(filepath.ListSeparator)+origPath)
+		good := filepath.Join(tmpDir, "execabs-test")
+		if found, err := LookPath(good); err != nil || !strings.HasPrefix(found, good) {
+			t.Fatalf(`LookPath(%#q) = %#q, %v, want \"%s...\", nil`, good, found, err, good)
+		}
+
+		if found, err := LookPath("execabs-test"); err != nil || !strings.HasPrefix(found, good) {
+			t.Fatalf(`LookPath(%#q) = %#q, %v, want \"%s...\", nil`, "execabs-test", found, err, good)
+		}
+
+		cmd := Command("execabs-test")
+		if cmd.Err != nil {
+			t.Fatalf("Command(%#q).Err = %v; want nil", "execabs-test", cmd.Err)
+		}
+	})
+
+	t.Run(pathVar+"=$OTHER", func(t *testing.T) {
+		// Control case: if the lookup returns ErrDot when PATH is empty, then we
+		// know that PATH implicitly includes ".". If it does not, then we don't
+		// expect to see ErrDot at all in this test (because the path will be
+		// unambiguously absolute).
+		wantErrDot := false
+		t.Setenv(pathVar, "")
+		if found, err := LookPath("execabs-test"); errors.Is(err, ErrDot) {
+			wantErrDot = true
+		} else if err == nil {
+			t.Fatalf(`with PATH='', LookPath(%#q) = %#q; want non-nil error`, "execabs-test", found)
+		}
+
+		// Set PATH to include an explicit directory that contains a completely
+		// independent executable that happens to have the same name as an
+		// executable in ".". If "." is included implicitly, looking up the
+		// (unqualified) executable name will return ErrDot; otherwise, the
+		// executable in "." should have no effect and the lookup should
+		// unambiguously resolve to the directory in PATH.
+
+		dir := t.TempDir()
+		executable := "execabs-test"
+		if runtime.GOOS == "windows" {
+			executable += ".exe"
+		}
+		if err := os.WriteFile(filepath.Join(dir, executable), []byte{1, 2, 3}, 0777); err != nil {
+			t.Fatal(err)
+		}
+		t.Setenv(pathVar, dir+string(filepath.ListSeparator)+origPath)
+
+		found, err := LookPath("execabs-test")
+		if wantErrDot {
+			wantFound := filepath.Join(".", executable)
+			if found != wantFound || !errors.Is(err, ErrDot) {
+				t.Fatalf(`LookPath(%#q) = %#q, %v, want %#q, Is ErrDot`, "execabs-test", found, err, wantFound)
+			}
+		} else {
+			wantFound := filepath.Join(dir, executable)
+			if found != wantFound || err != nil {
+				t.Fatalf(`LookPath(%#q) = %#q, %v, want %#q, nil`, "execabs-test", found, err, wantFound)
+			}
+		}
+	})
 }
diff --git a/src/os/exec/lp_windows.go b/src/os/exec/lp_windows.go
index dab5770298..da047585eb 100644
--- a/src/os/exec/lp_windows.go
+++ b/src/os/exec/lp_windows.go
@@ -96,20 +96,43 @@ func LookPath(file string) (string, error) {
 	// have configured their environment this way!
 	// https://docs.microsoft.com/en-us/windows/win32/api/processenv/nf-processenv-needcurrentdirectoryforexepathw
 	// See also go.dev/issue/43947.
+	var (
+		dotf   string
+		dotErr error
+	)
 	if _, found := syscall.Getenv("NoDefaultCurrentDirectoryInExePath"); !found {
 		if f, err := findExecutable(filepath.Join(".", file), exts); err == nil {
-			return f, &Error{file, ErrDot}
+			dotf, dotErr = f, &Error{file, ErrDot}
 		}
 	}
 
 	path := os.Getenv("path")
 	for _, dir := range filepath.SplitList(path) {
 		if f, err := findExecutable(filepath.Join(dir, file), exts); err == nil {
+			if dotErr != nil {
+				// https://go.dev/issue/53536: if we resolved a relative path implicitly,
+				// and it is the same executable that would be resolved from the explicit %PATH%,
+				// prefer the explicit name for the executable (and, likely, no error) instead
+				// of the equivalent implicit name with ErrDot.
+				//
+				// Otherwise, return the ErrDot for the implicit path as soon as we find
+				// out that the explicit one doesn't match.
+				dotfi, dotfiErr := os.Lstat(dotf)
+				fi, fiErr := os.Lstat(f)
+				if dotfiErr != nil || fiErr != nil || !os.SameFile(dotfi, fi) {
+					return dotf, dotErr
+				}
+			}
+
 			if !filepath.IsAbs(f) {
 				return f, &Error{file, ErrDot}
 			}
 			return f, nil
 		}
 	}
+
+	if dotErr != nil {
+		return dotf, dotErr
+	}
 	return "", &Error{file, ErrNotFound}
 }