Tighten disk safety checks

internal/disk/disk.go

@@ -26,7 +26,7 @@ type DiskInfo struct {
 	MountPath  string    `json:"mount_path"`
 }
 
-const markerDir = ".jukebox"
+const MarkerDir = ".jukebox"
 const idFile = "disk.id"
 
 func Probe(mountPath string) (DiskInfo, error) {
@@ -43,7 +43,7 @@ func Probe(mountPath string) (DiskInfo, error) {
 	info.TotalBytes = total
 	info.FreeBytes = free
 
-	idPath := filepath.Join(mountPath, markerDir, idFile)
+	idPath := filepath.Join(mountPath, MarkerDir, idFile)
 	data, err := os.ReadFile(idPath)
 	if errors.Is(err, os.ErrNotExist) {
 		info.State = DiskForeign
@@ -59,8 +59,45 @@ func Probe(mountPath string) (DiskInfo, error) {
 	return info, nil
 }
 
+func IsMountPoint(path string) bool {
+	pathInfo, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+
+	parent := filepath.Dir(filepath.Clean(path))
+	parentInfo, err := os.Stat(parent)
+	if err != nil {
+		return false
+	}
+
+	pathStat, ok := pathInfo.Sys().(*syscall.Stat_t)
+	if !ok {
+		return false
+	}
+	parentStat, ok := parentInfo.Sys().(*syscall.Stat_t)
+	if !ok {
+		return false
+	}
+
+	return pathStat.Dev != parentStat.Dev
+}
+
+func CheckWritable(path string) error {
+	f, err := os.CreateTemp(path, ".jukebox-writecheck-*")
+	if err != nil {
+		return err
+	}
+	name := f.Name()
+	if err := f.Close(); err != nil {
+		_ = os.Remove(name)
+		return err
+	}
+	return os.Remove(name)
+}
+
 func InitDisk(mountPath string) (string, error) {
-	dir := filepath.Join(mountPath, markerDir)
+	dir := filepath.Join(mountPath, MarkerDir)
 	if err := os.MkdirAll(dir, 0o755); err != nil {
 		return "", err
 	}
@@ -73,7 +110,7 @@ func InitDisk(mountPath string) (string, error) {
 }
 
 func DBPath(mountPath string) string {
-	return filepath.Join(mountPath, markerDir, "history.db")
+	return filepath.Join(mountPath, MarkerDir, "history.db")
 }
 
 func DiskUsage(mountPath string) (total, free int64, err error) {