prepare for unit tests

internal/sysutil/common.go

@@ -49,10 +49,14 @@ type File interface {
 	Name() string
 	Close() error
 	Stat() (os.FileInfo, error)
+
+	Read(p []byte) (n int, err error)
 }
 
 type osOperator interface {
 	getpwnam(username string) (*User, error)
 	mkdir(dir string, user *User, perm os.FileMode) error
 	createFileForWrite(file string, user *User, perm os.FileMode) (io.WriteCloser, error)
+	createTempFile(dir, pattern string, user *User) (File, error)
+	openFile(name string, flag int, perm os.FileMode) (File, error)
 }

internal/sysutil/os_operations_unix.go

@@ -31,6 +31,10 @@ type osOperatorImpl struct {
 	osOpHelper
 }
 
+func (o *osOperatorImpl) openFile(name string, flag int, perm os.FileMode) (File, error) {
+	return os.OpenFile(name, flag, perm)
+}
+
 func (o *osOperatorImpl) getpwnam(username string) (*User, error) {
 	content, err := o.ReadFile("/etc/passwd")
 	if err != nil {

internal/sysutil/sysmanager_linux.go

@@ -31,7 +31,7 @@ func (s *SysManager) CopyFileAttribute(from, to string) error {
 // ReadFileOfUser reads a file of the given user
 // either the user is root (i.e. uid=0), or the file has to be owned by the user
 func (s *SysManager) ReadFileOfUser(filename string, user *User) ([]byte, error) {
-	file, err := os.OpenFile(filename, os.O_RDONLY, 0)
+	file, err := s.openFile(filename, os.O_RDONLY, 0)
 	if err != nil {
 		return nil, err
 	}