fix: release camera capture handle on failed or repeated open

- Fix critical resource leak when cv2.VideoCapture.isOpened() returns False:
  create capture in local variable, verify it opened, call .release() before
  raising RuntimeError, only assign to self._capture after confirming success
- Fix potential leak on repeated open(): release any existing self._capture
  before creating a new one
- Add regression test: test_open_raises_when_capture_not_opened now verifies
  .release() is called on failed capture
- Add new test: test_open_twice_releases_first_capture verifies first capture's
  .release() is called before second is assigned

All 28 tests passing (5 camera + 23 existing).

bookmark/camera.py

@@ -16,9 +16,16 @@ class CameraSource:
         self._capture: cv2.VideoCapture | None = None
 
     def open(self) -> None:
-        self._capture = cv2.VideoCapture(self.device_index)
-        if not self._capture.isOpened():
+        # Release any existing capture before creating a new one
+        if self._capture is not None:
+            self._capture.release()
+
+        # Create capture in local variable and verify it opened before assignment
+        capture = cv2.VideoCapture(self.device_index)
+        if not capture.isOpened():
+            capture.release()
             raise RuntimeError(f"Could not open camera at index {self.device_index}")
+        self._capture = capture
 
     def read_frame(self) -> np.ndarray:
         if self._capture is None:

tests/test_camera.py

@@ -16,6 +16,9 @@ def test_open_raises_when_capture_not_opened():
         with pytest.raises(RuntimeError, match="Could not open camera"):
             source.open()
 
+        # Verify that release() was called on the failed capture
+        mock_instance.release.assert_called_once()
+
 
 def test_read_frame_returns_frame_from_capture():
     with patch("bookmark.camera.cv2.VideoCapture") as mock_video_capture:
@@ -49,3 +52,26 @@ def test_read_frame_raises_if_not_opened():
     source = CameraSource(device_index=0)
     with pytest.raises(RuntimeError, match="not opened"):
         source.read_frame()
+
+
+def test_open_twice_releases_first_capture():
+    with patch("bookmark.camera.cv2.VideoCapture") as mock_video_capture:
+        # Setup two distinct mock instances
+        first_mock = MagicMock()
+        first_mock.isOpened.return_value = True
+        second_mock = MagicMock()
+        second_mock.isOpened.return_value = True
+        mock_video_capture.side_effect = [first_mock, second_mock]
+
+        source = CameraSource(device_index=0)
+
+        # First open
+        source.open()
+        assert source._capture is first_mock
+
+        # Second open should release the first capture
+        source.open()
+        assert source._capture is second_mock
+
+        # Verify the first mock's release() was called before the second was assigned
+        first_mock.release.assert_called_once()