fix(test_scanner.py): Add test for unopenable file condition (#4834)

* Fixes #762

Author: JigyasuRajput

test/test_scanner.py

@@ -358,7 +358,10 @@ def test_does_not_scan_symlinks(self):
                 non_existant_link.unlink()
 
     def test_cannot_open_file(self, caplog):
-        """Test behaviour when file cannot be opened"""
+        """
+        Test behavior when the file does not exist.
+        This covers the case where scan_file immediately detects that the file is missing.
+        """
         self.scanner.logger.setLevel(logging.DEBUG)
         with pytest.raises(StopIteration):
             next(
@@ -368,6 +371,31 @@ def test_cannot_open_file(self, caplog):
             )
         assert str.find("Invalid file", caplog.text)
 
+    def test_unopenable_file(self):
+        """
+        Test behavior when the file exists but cannot be opened.
+        Simulates a permission error by patching open to raise an OSError.
+        This is significant because it tests the error handling beyond just checking for file existence.
+        """
+        # Create a temporary file that exists
+        tmp = tempfile.NamedTemporaryFile(
+            "w+b",
+            suffix="-test-unopenable.out",
+            dir=self.mapping_test_dir,
+            delete=False,
+        )
+        tmp_filename = tmp.name
+        tmp.close()
+        try:
+            # Patch open to force OSError signaling that the file is unopenable
+            with unittest.mock.patch(
+                "builtins.open", side_effect=OSError("Permission denied")
+            ):
+                with pytest.raises(StopIteration):
+                    next(self.scanner.scan_file(tmp_filename))
+        finally:
+            os.remove(tmp_filename)
+
     def test_clean_file_path(self):
         filepath = "/tmp/cve-bin-tool/dhtei34fd/file_name.extracted/usr/bin/vulnerable_file"  # nosec
         # temp path is hardcoded for testing, not for usage