Refuse a non-positive OCR sample interval

src/docgen/ocr.py

@@ -40,6 +40,21 @@ def _require_positive_fps(cap: Any, raw: float) -> float:
     return fps
 
 
+def _require_positive_sample_interval(raw: object) -> float:
+    """A zero or negative interval makes the sample loop run forever."""
+    try:
+        interval = float(raw)  # type: ignore[arg-type]
+    except (TypeError, ValueError) as exc:
+        raise ValueError(
+            "OCR sample interval must be a positive finite number of seconds"
+        ) from exc
+    if not math.isfinite(interval) or interval <= 0:
+        raise ValueError(
+            "OCR sample interval must be a positive finite number of seconds"
+        )
+    return interval
+
+
 class OCRScanner:
     def __init__(self, config: Config) -> None:
         self.config = config
@@ -59,9 +74,11 @@ def scan_video(self, video_path: str | Path) -> OCRReport:
             return report
 
         fps = _require_positive_fps(cap, cap.get(cv2.CAP_PROP_FPS))
+        interval = _require_positive_sample_interval(
+            self.ocr_cfg.get("sample_interval_sec", 2)
+        )
         total_frames = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
         duration = total_frames / fps if fps > 0 else 0
-        interval = self.ocr_cfg.get("sample_interval_sec", 2)
         error_patterns = self.ocr_cfg.get("error_patterns", [])
         min_conf = self.ocr_cfg.get("min_confidence", 40)
 

tests/test_ocr.py

@@ -52,6 +52,24 @@ def test_zero_fps_is_refused(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) ->
     assert cap.sampled is False
 
 
+def test_zero_sample_interval_is_refused(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    raw = {
+        "segments": {"all": ["01"], "default": ["01"]},
+        "validation": {"ocr": {"sample_interval_sec": 0}},
+    }
+    path = tmp_path / "docgen.yaml"
+    path.write_text(yaml.dump(raw), encoding="utf-8")
+    cap = _Cap()
+    cap.fps = 30.0
+    cap.frames = 0.0
+    monkeypatch.setattr(cv2, "VideoCapture", lambda *_a, **_k: cap)
+    with pytest.raises(ValueError, match="sample interval"):
+        OCRScanner(Config.from_yaml(path)).scan_video(tmp_path / "clip.mp4")
+    assert cap.sampled is False
+
+
 def test_positive_fps_still_samples(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
     cap = _Cap()
     cap.fps = 30.0