interp: determine access to a directory via unix.Access

Rip out the body of interp.hasPermissionToDir and replace it with just a
call to unix.Access(path, unix.X_OK).

Update the function signature of hasPermissionToDir in os_notunix.go,
too.

Fixes #1033.

interp/builtin.go

@@ -963,7 +963,7 @@ func (r *Runner) changeDir(ctx context.Context, path string) int {
 	if err != nil || !info.IsDir() {
 		return 1
 	}
-	if !hasPermissionToDir(info) {
+	if !hasPermissionToDir(path) {
 		return 1
 	}
 	r.Dir = path

interp/os_notunix.go

@@ -7,14 +7,13 @@ package interp
 
 import (
 	"fmt"
-	"os"
 )
 
 func mkfifo(path string, mode uint32) error {
 	return fmt.Errorf("unsupported")
 }
 
 // hasPermissionToDir is a no-op on Windows.
-func hasPermissionToDir(info os.FileInfo) bool {
+func hasPermissionToDir(string) bool {
 	return true
 }

interp/os_unix.go

@@ -6,52 +6,15 @@
 package interp
 
 import (
-	"os"
-	"os/user"
-	"strconv"
-	"syscall"
-
 	"golang.org/x/sys/unix"
 )
 
 func mkfifo(path string, mode uint32) error {
 	return unix.Mkfifo(path, mode)
 }
 
-// hasPermissionToDir returns if the OS current user has execute permission
-// to the given directory
-func hasPermissionToDir(info os.FileInfo) bool {
-	user, err := user.Current()
-	if err != nil {
-		return false // unknown user; assume no permissions
-	}
-	uid, err := strconv.Atoi(user.Uid)
-	if err != nil {
-		return false // on POSIX systems, Uid should always be a decimal number
-	}
-	if uid == 0 {
-		return true // super-user
-	}
-
-	st, _ := info.Sys().(*syscall.Stat_t)
-	if st == nil {
-		panic("unexpected info.Sys type")
-	}
-	perm := info.Mode().Perm()
-	// user (u)
-	if perm&0o100 != 0 && st.Uid == uint32(uid) {
-		return true
-	}
-
-	gid, _ := strconv.Atoi(user.Gid)
-	// other users in group (g)
-	if perm&0o010 != 0 && st.Uid != uint32(uid) && st.Gid == uint32(gid) {
-		return true
-	}
-	// remaining users (o)
-	if perm&0o001 != 0 && st.Uid != uint32(uid) && st.Gid != uint32(gid) {
-		return true
-	}
-
-	return false
+// hasPermissionToDir returns true if the OS current user has execute
+// permission to the given directory
+func hasPermissionToDir(path string) bool {
+	return unix.Access(path, unix.X_OK) == nil
 }