Send Unix errors without wrapping them or casting them.

While trying to import MountedFast into k8s to detect mount points
using openat2, several tests failed because they rely on
os.IsErrNotExist / os.IsPermission which do not actually test the
underlying error.

For a non-existent path, if we return the bare error, this problem
is solved. I tested for the following cases to ensure that
backward compatibility is not broken:

fmt.Println(os.IsNotExist(err)) returns false
fmt.Println(os.IsNotExist(&os.PathError{
		Op:   "lstat",
		Path: "somepath",
		Err:  err,
})) returns false

fmt.Println(os.IsNotExist(fmt.Errorf("unable: %w", err))) returns false
fmt.Println(errors.Is(fmt.Errorf("unable: %w", err), os.ErrNotExist))
returns true.

mountinfo/mounted_linux.go

@@ -44,7 +44,7 @@ func mountedByOpenat2(path string) (bool, error) {
 		Flags: unix.O_PATH | unix.O_CLOEXEC,
 	})
 	if err != nil {
-		return false, &os.PathError{Op: "openat2", Path: dir, Err: err}
+		return false, err
 	}
 	fd, err := unix.Openat2(dirfd, last, &unix.OpenHow{
 		Flags:   unix.O_PATH | unix.O_CLOEXEC | unix.O_NOFOLLOW,
@@ -59,7 +59,7 @@ func mountedByOpenat2(path string) (bool, error) {
 		return true, nil
 	}
 	// not sure
-	return false, &os.PathError{Op: "openat2", Path: path, Err: err}
+	return false, err
 }
 
 // mountedFast is similar to MountedFast, except it expects a normalized path.

mountinfo/mounted_unix.go

@@ -4,7 +4,6 @@
 package mountinfo
 
 import (
-	"fmt"
 	"os"
 	"path/filepath"
 
@@ -15,12 +14,12 @@ func mountedByStat(path string) (bool, error) {
 	var st unix.Stat_t
 
 	if err := unix.Lstat(path, &st); err != nil {
-		return false, &os.PathError{Op: "stat", Path: path, Err: err}
+		return false, err
 	}
 	dev := st.Dev
 	parent := filepath.Dir(path)
 	if err := unix.Lstat(parent, &st); err != nil {
-		return false, &os.PathError{Op: "stat", Path: parent, Err: err}
+		return false, err
 	}
 	if dev != st.Dev {
 		// Device differs from that of parent,
@@ -33,13 +32,13 @@ func mountedByStat(path string) (bool, error) {
 
 func normalizePath(path string) (realPath string, err error) {
 	if realPath, err = filepath.Abs(path); err != nil {
-		return "", fmt.Errorf("unable to get absolute path for %q: %w", path, err)
+		return "", err
 	}
 	if realPath, err = filepath.EvalSymlinks(realPath); err != nil {
-		return "", fmt.Errorf("failed to canonicalise path for %q: %w", path, err)
+		return "", err
 	}
 	if _, err := os.Stat(realPath); err != nil {
-		return "", fmt.Errorf("failed to stat target of %q: %w", path, err)
+		return "", err
 	}
 	return realPath, nil
 }