Prevent ffmpeg stderr backpressure

tests/test_video.py

@@ -1950,30 +1950,6 @@ class TestVideoVisibleScan:
 
 
 class TestVideoVisibleEncoding:
-    @pytest.mark.parametrize("suffix", [".mp4", ".webm"])
-    def test_encoder_bounds_codec_threads(
-        self,
-        tmp_path: Path,
-        monkeypatch: pytest.MonkeyPatch,
-        suffix: str,
-    ):
-        from remove_ai_watermarks import video_encoding
-
-        monkeypatch.setattr(video_encoding.shutil, "which", lambda _name: "/usr/bin/ffmpeg")
-        command = video_encoding.raw_video_command(
-            tmp_path / "source.mp4",
-            tmp_path / f"clean{suffix}",
-            width=12,
-            height=8,
-            fps=24.0,
-            strip_metadata=True,
-            crf=14,
-            profile=video_encoding.VideoEncodeProfile(),
-        )
-
-        assert command[1:3] == ["-filter_threads", "1"]
-        assert command[command.index("-threads:v") + 1] == "2"
-
     @staticmethod
     def _patch_single_frame_encode(
         monkeypatch: pytest.MonkeyPatch,
@@ -2008,6 +1984,12 @@ class TestVideoVisibleEncoding:
             def poll(self) -> int:
                 return 1
 
+            def wait(self) -> int:
+                return 1
+
+            def discard_stderr(self) -> None:
+                pass
+
         targets: list[Path] = []
 
         def fake_command(_source: Path, target: Path, **_kwargs: object) -> list[str]:

tests/test_video_invisible.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import sys
+import threading
 from types import SimpleNamespace
 from typing import TYPE_CHECKING
 
@@ -14,6 +16,55 @@ if TYPE_CHECKING:
     from pathlib import Path
 
 
+def test_encoder_redirects_large_stderr_while_frames_are_streaming(
+    tmp_path: Path,
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    diagnostic = "synthetic ffmpeg diagnostic"
+    tail_diagnostic = "synthetic ffmpeg diagnostic tail"
+    caplog.set_level("INFO", logger=video_encoding.__name__)
+    command = [
+        sys.executable,
+        "-c",
+        (
+            "import sys; "
+            f"sys.stderr.buffer.write({diagnostic.encode()!r} + b'x' * 262144 + {tail_diagnostic.encode()!r}); "
+            "sys.stderr.buffer.flush(); "
+            "sys.stdin.buffer.read(); "
+            "raise SystemExit(7)"
+        ),
+    ]
+    encoder = video_encoding.start_raw_video_encoder(command)
+    write_finished = threading.Event()
+    write_errors: list[Exception] = []
+
+    def write_frames() -> None:
+        try:
+            encoder.stdin.write(b"f" * 262144)
+            encoder.stdin.flush()
+        except Exception as exc:  # pragma: no cover - mutation cleanup path
+            write_errors.append(exc)
+        finally:
+            write_finished.set()
+
+    writer = threading.Thread(target=write_frames)
+    writer.start()
+    try:
+        assert write_finished.wait(5), "stderr backpressure blocked the frame producer"
+        assert write_errors == []
+        with pytest.raises(RuntimeError, match=diagnostic):
+            video_encoding.finish_raw_video_encoder(
+                encoder,
+                tmp_path / "unused.mp4",
+                operation="synthetic encode",
+            )
+        assert "ffmpeg stderr truncated" in caplog.text
+        assert tail_diagnostic in caplog.text
+    finally:
+        video_encoding.abort_raw_video_encoder(encoder)
+        writer.join(timeout=5)
+
+
 def test_availability_requires_both_optional_packages(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setattr(
         video_invisible,