fix(utils): stabilize segmented video reversal

CHANGELOG.md

@@ -35,6 +35,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   [@lapotist](https://github.com/lapotist) [#664](https://github.com/jeertmans/manim-slides/pull/664)
 - Fixed presenter looping when reversing multiple slides.
   [@wuerfelfreak](https://github.com/wuerfelfreak) [#665](https://github.com/jeertmans/manim-slides/pull/665)
+- Fixed segmented video reversal stalls and made segment ordering deterministic.
+  [@rusetiq](https://github.com/rusetiq) [#668](https://github.com/jeertmans/manim-slides/pull/668)
 
 (v5.6.0)=
 ## [v5.6.0](https://github.com/jeertmans/manim-slides/compare/v5.5.4...v5.6.0)

manim_slides/convert.py

@@ -52,7 +52,7 @@ def open_with_default(file: Path) -> None:
     if system == "Darwin":
         subprocess.call(("open", str(file)))
     elif system == "Windows":
-        os.startfile(str(file))  # type: ignore[attr-defined]
+        os.startfile(str(file))  # nosec B606  # ty: ignore[unresolved-attribute]
     else:
         subprocess.call(("xdg-open", str(file)))
 

manim_slides/utils.py

@@ -4,7 +4,7 @@
 import tempfile
 from collections.abc import Iterator
 from itertools import pairwise
-from multiprocessing import Pool
+from multiprocessing import get_context
 from pathlib import Path
 from typing import Any, TypeVar, cast
 
@@ -197,7 +197,7 @@ def reverse_video_file(
         with tempfile.TemporaryDirectory() as tmpdirname:
             tmpdir = Path(tmpdirname)
             with av.open(
-                str(tmpdir / f"%04d.{src.suffix}"),
+                str(tmpdir / f"%04d{src.suffix}"),
                 "w",
                 format="segment",
                 options={"segment_time": str(max_segment_duration)},
@@ -215,12 +215,15 @@ def reverse_video_file(
                     packet.stream = output_stream
                     output_container.mux(packet)
 
-            src_files = list(tmpdir.iterdir())
+            src_files = sorted(tmpdir.iterdir())
             rev_files = [
                 src_file.with_stem("rev_" + src_file.stem) for src_file in src_files
             ]
 
-            with Pool(num_processes, maxtasksperchild=1) as pool:
+            # Forking after the renderer and PyAV have initialized can inherit
+            # locked native state and stall the worker pool.  Fresh interpreters
+            # avoid that state while preserving parallel segment processing.
+            with get_context("spawn").Pool(num_processes, maxtasksperchild=1) as pool:
                 for _ in tqdm(
                     pool.imap_unordered(
                         reverse_video_file_in_one_chunk,

tests/test_convert.py

@@ -179,7 +179,7 @@ def test_quoted_enum(enum_type: type[Enum]) -> None:
 )
 def test_quoted_enum_survives_pydantic_validation(enum_type: type[Enum]) -> None:
     class Model(BaseModel):
-        value: enum_type  # type: ignore[valid-type]
+        value: enum_type  # ty: ignore[invalid-type-form]
 
     for enum in enum_type:
         if enum in ["true", "false", "null"]:

tests/test_utils.py

@@ -1,6 +1,11 @@
+from collections.abc import Iterator
 from pathlib import Path
 
-from manim_slides.utils import merge_basenames
+import av
+import numpy as np
+import pytest
+
+from manim_slides.utils import merge_basenames, reverse_video_file
 
 
 def test_merge_basenames(paths: list[Path]) -> None:
@@ -20,3 +25,49 @@ def test_merge_basenames_same_with_different_parent_directories(
     p4 = d2 / "d/e/f/two.txt"
 
     assert merge_basenames([p1, p2]).name == merge_basenames([p3, p4]).name
+
+
+def test_reverse_video_file_with_unordered_segments(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    src = tmp_path / "src.mp4"
+    dest = tmp_path / "dest.mp4"
+
+    with av.open(str(src), mode="w") as container:
+        stream = container.add_stream("libx264", rate=2)
+        stream.width = 16
+        stream.height = 16
+        stream.pix_fmt = "yuv420p"
+        stream.codec_context.gop_size = 1
+
+        for value in range(6):
+            array = np.full((16, 16, 3), value * 40, dtype=np.uint8)
+            frame = av.VideoFrame.from_ndarray(array, format="rgb24")
+            for packet in stream.encode(frame):
+                container.mux(packet)
+
+        for packet in stream.encode():
+            container.mux(packet)
+
+    original_iterdir = Path.iterdir
+
+    def reversed_iterdir(path: Path) -> Iterator[Path]:
+        return iter(reversed(list(original_iterdir(path))))
+
+    monkeypatch.setattr(Path, "iterdir", reversed_iterdir)
+
+    reverse_video_file(
+        src,
+        dest,
+        max_segment_duration=1.0,
+        num_processes=1,
+        disable=True,
+    )
+
+    with av.open(str(dest)) as container:
+        values = [
+            float(frame.to_ndarray(format="gray").mean())
+            for frame in container.decode(video=0)
+        ]
+
+    np.testing.assert_array_equal(values, sorted(values, reverse=True))