Refuse to leave a truncated concat file when cleanup fails

src/docgen/concat.py

@@ -117,8 +117,11 @@ def _build_one(self, out_name: str, seg_ids: list[str]) -> None:
 
 def _unlink_incomplete(path: Path) -> None:
     """Remove a truncated concat output so later stages cannot treat it as finished."""
-    if path.exists():
-        try:
-            path.unlink()
-        except OSError:
-            pass
+    if not path.exists():
+        return
+    try:
+        path.unlink()
+    except OSError as exc:
+        raise ConcatError(
+            f"[concat] incomplete {path.name} could not be removed"
+        ) from exc

tests/test_concat.py

@@ -157,6 +157,23 @@ def fake_run(cmd, **_kwargs):
     assert not list((tmp_path / "recordings").glob(".concat-*.txt"))
 
 
+def test_concat_timeout_refuses_when_incomplete_output_remains(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    cfg = _cfg(tmp_path, {"full": ["01", "02"]})
+    _seed_recordings(tmp_path)
+    out = tmp_path / "recordings" / "full.mp4"
+    out.mkdir()
+
+    def fake_run(cmd, **_kwargs):
+        raise subprocess.TimeoutExpired(cmd=cmd, timeout=300)
+
+    monkeypatch.setattr(subprocess, "run", fake_run)
+    with pytest.raises(ConcatError, match="could not be removed"):
+        ConcatBuilder(cfg).build(name="full")
+    assert out.is_dir()
+
+
 def test_concat_ffmpeg_failure_removes_incomplete_output(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: