feat(dash): add merge_segments to append segments during the download

Opt-in rolling merge for DASH video and audio: each contiguous segment is appended to the output as it lands, so the post-download "Merging" stage goes away. Decrypts the init segment eagerly when decrypt_segments is on, disables resume for the track, and skips subtitle tracks.

tests/core/test_dash_collapse_integration.py

@@ -24,6 +24,7 @@ from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
 
 import pytest
 
+from unshackle.core.config import config
 from unshackle.core.constants import DOWNLOAD_CANCELLED, DownloadCancelled
 from unshackle.core.manifests import DASH
 from unshackle.core.tracks.track import DownloadContext
@@ -235,3 +236,33 @@ def test_cancelled_sibling_does_not_report_missing_segments(server, tmp_path):
     # missing segments here would mask the sibling failure that set the flag
     assert track.path is None
     assert not ctx.save_path.exists()
+
+
+def test_rolling_merge_appends_in_order_and_leaves_no_segments(server, tmp_path, monkeypatch):
+    monkeypatch.setattr(config, "merge_segments", True)
+    server.routes["/init.bin"] = INIT2
+    server.routes["/part0.bin"] = PART0
+    server.routes["/part1.bin"] = PART1
+    track = make_track(mixed_mpd(server), server)
+    ctx = make_ctx(tmp_path, "rolling.mp4")
+
+    DASH.download_track(track, ctx)
+
+    assert ctx.save_path.read_bytes() == INIT2 + PART0 + PART1
+    assert not ctx.save_dir.exists() or not list(ctx.save_dir.glob("*"))
+
+
+def test_rolling_merge_cancel_leaves_no_partial_output(server, tmp_path, monkeypatch):
+    monkeypatch.setattr(config, "merge_segments", True)
+    server.routes["/init.bin"] = INIT2
+    server.routes["/part0.bin"] = PART0
+    server.routes["/part1.bin"] = PART1
+    track = make_track(mixed_mpd(server), server)
+    ctx = make_ctx(tmp_path, "rolling-cancelled.mp4")
+
+    DOWNLOAD_CANCELLED.set()
+    with pytest.raises(DownloadCancelled):
+        DASH.download_track(track, ctx)
+
+    assert track.path is None
+    assert not ctx.save_path.exists()

tests/test_rolling_merge.py

@@ -0,0 +1,51 @@
+import io
+import random
+from concurrent.futures import Future
+from pathlib import Path
+
+from unshackle.core.manifests.dash import RollingMerge, append_segment
+
+
+def write_segments(save_dir: Path, payloads: list[bytes]) -> int:
+    save_dir.mkdir()
+    width = len(str(len(payloads)))
+    for i, payload in enumerate(payloads):
+        (save_dir / f"{i:0{width}d}.mp4").write_bytes(payload)
+    return width
+
+
+def expected(init: bytes, payloads: list[bytes], is_text_subtitle: bool, tmp_path: Path) -> bytes:
+    scratch = tmp_path / "scratch"
+    width = write_segments(scratch, payloads)
+    out = io.BytesIO()
+    out.write(init)
+    for i in range(len(payloads)):
+        append_segment(out, scratch / f"{i:0{width}d}.mp4", is_text_subtitle)
+    return out.getvalue()
+
+
+def test_out_of_order_segments_match_whole_merge(tmp_path: Path) -> None:
+    rng = random.Random(7)
+    payloads = [rng.randbytes(rng.randint(1, 4096)) for _ in range(23)]
+    init = b"ftyp-moov"
+    want = expected(init, payloads, False, tmp_path)
+
+    save_dir = tmp_path / "segments"
+    width = write_segments(save_dir, payloads)
+    dst = io.BytesIO()
+    dst.write(init)
+    merger = RollingMerge(dst, save_dir, width)
+
+    order = list(range(len(payloads)))
+    rng.shuffle(order)
+    for index in order:
+        future: Future = Future()
+        future.set_result(None)
+        merger.add(index, future if index % 2 else None)
+        # the cursor never rests on a segment that is ready: every contiguous run is drained
+        assert merger.merged not in merger.ready
+
+    assert merger.merged == len(payloads)
+    assert dst.getvalue() == want
+    assert dst.getvalue().startswith(init)
+    assert list(save_dir.iterdir()) == []