os/exec: avoid calling LookPath in cmd.Start for resolved paths

Follow up on CL 511458, see https://go-review.googlesource.com/c/go/+/511458/2..4/src/cmd/go/main.go#b270 .

For #36768.

Change-Id: Icc2a4dbb1219b1d69dd10a900478957b0e975847

Change-Id: Icc2a4dbb1219b1d69dd10a900478957b0e975847
GitHub-Last-Rev: bac7e66496806d505270c5b90d53672d80a1ca29
GitHub-Pull-Request: golang/go#61517
Reviewed-on: https://go-review.googlesource.com/c/go/+/512155
Auto-Submit: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>

src/os/exec/exec.go

@@ -590,32 +590,6 @@ func (c *Cmd) Run() error {
  return c.Wait()
 }
 
-// lookExtensions finds windows executable by its dir and path.
-// It uses LookPath to try appropriate extensions.
-// lookExtensions does not search PATH, instead it converts `prog` into `.\prog`.
-func lookExtensions(path, dir string) (string, error) {
- if filepath.Base(path) == path {
- path = "." + string(filepath.Separator) + path
- }
- if dir == "" {
- return LookPath(path)
- }
- if filepath.VolumeName(path) != "" {
- return LookPath(path)
- }
- if len(path) > 1 && os.IsPathSeparator(path[0]) {
- return LookPath(path)
- }
- dirandpath := filepath.Join(dir, path)
- // We assume that LookPath will only add file extension.
- lp, err := LookPath(dirandpath)
- if err != nil {
- return "", err
- }
- ext := strings.TrimPrefix(lp, dirandpath)
- return path + ext, nil
-}
-
 // Start starts the specified command but does not wait for it to complete.
 //
 // If Start returns successfully, the c.Process field will be set.
@@ -649,13 +623,11 @@ func (c *Cmd) Start() error {
  }
  return c.Err
  }
- if runtime.GOOS == "windows" {
- lp, err := lookExtensions(c.Path, c.Dir)
- if err != nil {
- return err
- }
- c.Path = lp
+ lp, err := lookExtensions(c.Path, c.Dir)
+ if err != nil {
+ return err
  }
+ c.Path = lp
  if c.Cancel != nil && c.ctx == nil {
  return errors.New("exec: command with a non-nil Cancel was not created with CommandContext")
  }

src/os/exec/lp_plan9.go

@@ -64,3 +64,9 @@ func LookPath(file string) (string, error) {
  }
  return "", &Error{file, ErrNotFound}
 }
+
+// lookExtensions is a no-op on non-Windows platforms, since
+// they do not restrict executables to specific extensions.
+func lookExtensions(path, dir string) (string, error) {
+ return path, nil
+}

src/os/exec/lp_unix.go

@@ -80,3 +80,9 @@ func LookPath(file string) (string, error) {
  }
  return "", &Error{file, ErrNotFound}
 }
+
+// lookExtensions is a no-op on non-Windows platforms, since
+// they do not restrict executables to specific extensions.
+func lookExtensions(path, dir string) (string, error) {
+ return path, nil
+}

src/os/exec/lp_wasm.go

@@ -21,3 +21,9 @@ func LookPath(file string) (string, error) {
  // Wasm can not execute processes, so act as if there are no executables at all.
  return "", &Error{file, ErrNotFound}
 }
+
+// lookExtensions is a no-op on non-Windows platforms, since
+// they do not restrict executables to specific extensions.
+func lookExtensions(path, dir string) (string, error) {
+ return path, nil
+}

src/os/exec/lp_windows.go

@@ -63,6 +63,45 @@ func findExecutable(file string, exts []string) (string, error) {
 // As of Go 1.19, LookPath will instead return that path along with an error satisfying
 // errors.Is(err, ErrDot). See the package documentation for more details.
 func LookPath(file string) (string, error) {
+ return lookPath(file, pathExt())
+}
+
+// lookExtensions finds windows executable by its dir and path.
+// It uses LookPath to try appropriate extensions.
+// lookExtensions does not search PATH, instead it converts `prog` into `.\prog`.
+func lookExtensions(path, dir string) (string, error) {
+ if filepath.Base(path) == path {
+ path = "." + string(filepath.Separator) + path
+ }
+ exts := pathExt()
+ if ext := filepath.Ext(path); ext != "" {
+ for _, e := range exts {
+ if strings.EqualFold(ext, e) {
+ // Assume that path has already been resolved.
+ return path, nil
+ }
+ }
+ }
+ if dir == "" {
+ return lookPath(path, exts)
+ }
+ if filepath.VolumeName(path) != "" {
+ return lookPath(path, exts)
+ }
+ if len(path) > 1 && os.IsPathSeparator(path[0]) {
+ return lookPath(path, exts)
+ }
+ dirandpath := filepath.Join(dir, path)
+ // We assume that LookPath will only add file extension.
+ lp, err := lookPath(dirandpath, exts)
+ if err != nil {
+ return "", err
+ }
+ ext := strings.TrimPrefix(lp, dirandpath)
+ return path + ext, nil
+}
+
+func pathExt() []string {
  var exts []string
  x := os.Getenv(`PATHEXT`)
  if x != "" {
@@ -78,7 +117,11 @@ func LookPath(file string) (string, error) {
  } else {
  exts = []string{".com", ".exe", ".bat", ".cmd"}
  }
+ return exts
+}
 
+// lookPath implements LookPath for the given PATHEXT list.
+func lookPath(file string, exts []string) (string, error) {
  if strings.ContainsAny(file, `:\/`) {
  f, err := findExecutable(file, exts)
  if err == nil {