From 43fcb030f89b33c0debd9f361726d6187c2f548d Mon Sep 17 00:00:00 2001
From: imSp4rky <george+slight@slight.me>
Date: Sun, 25 Oct 2026 13:22:08 -0600
Subject: [PATCH] fix(tracks): read sidx boxes with struct so concurrent tracks
 parse correctly

---
 tests/test_sample.py           | 18 ++++++++++++++++++
 unshackle/core/tracks/track.py | 32 ++++++++++++++++----------------
 2 files changed, 34 insertions(+), 16 deletions(-)

diff --git a/tests/test_sample.py b/tests/test_sample.py
index 50a2e97c..8fb6fda7 100644
--- a/tests/test_sample.py
+++ b/tests/test_sample.py
@@ -345,6 +345,24 @@ def test_parse_sidx_refuses_what_it_cannot_cut() -> None:
     assert parse_sidx(sidx_box(count=3)[:-4], 0) == (0, [])
 
 
+def test_parse_sidx_is_right_when_tracks_parse_at_the_same_time() -> None:
+    boxes = {758: sidx_box(count=758), 1408: sidx_box(count=1408, size=500)}
+    wrong: list[int] = []
+
+    def parse(count: int) -> None:
+        for _ in range(100):
+            if len(parse_sidx(boxes[count], 0)[1]) != count:
+                wrong.append(count)
+
+    threads = [threading.Thread(target=parse, args=(count,)) for count in (758, 1408, 758, 1408)]
+    for thread in threads:
+        thread.start()
+    for thread in threads:
+        thread.join(timeout=30)
+    assert not wrong
+    assert not any(thread.is_alive() for thread in threads)
+
+
 def test_parse_sidx_refuses_a_box_that_counts_more_references_than_it_holds() -> None:
     box = bytearray(sidx_box(count=3))
     box[30:32] = (5).to_bytes(2, "big")
diff --git a/unshackle/core/tracks/track.py b/unshackle/core/tracks/track.py
index 02ef7562..942eef42 100644
--- a/unshackle/core/tracks/track.py
+++ b/unshackle/core/tracks/track.py
@@ -18,9 +18,7 @@ from uuid import UUID
 from zlib import crc32
 
 import click
-from construct import ConstructError
 from langcodes import Language
-from pymp4.parser import Box
 from requests import Session
 from requests.adapters import HTTPAdapter, Retry
 
@@ -360,23 +358,25 @@ def parse_sidx(data: bytes, offset: int) -> tuple[int, list[tuple[int, int, int]
     Returns the timescale and one (start time, first byte, last byte) for each subsegment. The list
     is empty when the box is not a ``sidx`` or when it points at more indexes and not at media.
     """
+    # read by hand: pymp4's Box.parse gives wrong results when two threads parse at once
     size, header = int.from_bytes(data[:4], "big"), 32 if data[8:9] == b"\x00" else 40
-    # pymp4 loops forever on a box that counts more references than its size holds
-    if data[4:8] != b"sidx" or size < header + 12 * int.from_bytes(data[header - 2 : header], "big"):
+    count = int.from_bytes(data[header - 2 : header], "big")
+    if data[4:8] != b"sidx" or len(data) < header or min(size, len(data)) < header + 12 * count:
         return 0, []
-    try:
-        box = Box.parse(data)
-    except ConstructError:
-        return 0, []
-    if any(reference.reference_type != "MEDIA" for reference in box.references):
-        return 0, []
-    start, byte = box.earliest_presentation_time, offset + box.end + box.first_offset
+    if header == 32:
+        timescale, start, first_offset = struct.unpack_from(">III", data, 16)
+    else:
+        timescale, start, first_offset = struct.unpack_from(">IQQ", data, 16)
+    byte = offset + size + first_offset
     subsegments: list[tuple[int, int, int]] = []
-    for reference in box.references:
-        subsegments.append((start, byte, byte + reference.referenced_size - 1))
-        start += reference.segment_duration
-        byte += reference.referenced_size
-    return box.timescale, subsegments
+    for i in range(count):
+        word, duration = struct.unpack_from(">II", data, header + 12 * i)
+        if word >> 31:
+            return 0, []
+        subsegments.append((start, byte, byte + (word & 0x7FFFFFFF) - 1))
+        start += duration
+        byte += word & 0x7FFFFFFF
+    return timescale, subsegments
 
 
 @dataclass