Island: Raise distinct errors when openning a file

IFileRepository now distincts between file not found and a file that
could not be retrieved

monkey/monkey_island/cc/repository/file_storage/i_file_repository.py

@@ -30,7 +30,8 @@ class IFileRepository(metaclass=abc.ABCMeta):
 
         :param unsafe_file_name: An unsanitized file name that identifies the file to be opened
         :return: A file-like object providing access to the file's contents
-        :raises FileRetrievalError: if the file cannot be opened
+        :raises FileRetrievalError: if the file cannot be retrieved
+        :raises FileNotFoundError: if the file cannot be found
         """
         pass
 

monkey/monkey_island/cc/repository/file_storage/local_storage_file_repository.py

@@ -1,3 +1,4 @@
+import errno
 import logging
 import shutil
 from pathlib import Path
@@ -44,11 +45,12 @@ class LocalStorageFileRepository(IFileRepository):
             logger.debug(f"Opening {safe_file_path}")
             return open(safe_file_path, "rb")
         except OSError as err:
-            # TODO: The interface should make a destinction between file not found and an error when
-            #       retrieving a file that should exist. The built-in `FileNotFoundError` is not
-            #       sufficient because it inherits from `OSError` and the interface does not
-            #       guarantee that the file is stored on the local file system.
-            raise FileRetrievalError(f"Failed to retrieve file {safe_file_path}: {err}") from err
+            if err.errno == errno.ENOENT:
+                raise FileNotFoundError(f"No file {safe_file_path} found: {err}") from err
+            else:
+                raise FileRetrievalError(
+                    f"Failed to retrieve file {safe_file_path}: {err}"
+                ) from err
 
     def delete_file(self, unsafe_file_name: str):
         safe_file_path = self._get_safe_file_path(unsafe_file_name)

monkey/tests/unit_tests/monkey_island/cc/repository/test_local_storage_file_repository.py

@@ -1,5 +1,7 @@
+import errno
 import io
 from pathlib import Path
+from unittest.mock import Mock, patch
 
 import pytest
 from tests.monkey_island.utils import assert_linux_permissions, assert_windows_permissions
@@ -131,5 +133,15 @@ def test_remove_nonexistant_file(tmp_path):
 def test_open_nonexistant_file(tmp_path):
     fss = LocalStorageFileRepository(tmp_path)
 
-    with pytest.raises(FileRetrievalError):
+    with pytest.raises(FileNotFoundError):
         fss.open_file("nonexistant_file.txt")
+
+
+def test_open_locked_file(tmp_path, monkeypatch):
+    fss = LocalStorageFileRepository(tmp_path)
+
+    with patch(
+        "builtins.open", Mock(side_effect=OSError(errno.EIO, "File could not be retrieved"))
+    ):
+        with pytest.raises(FileRetrievalError):
+            fss.open_file("locked_file.txt")