fix(mp4): serialize pymp4 parses of bit-field boxes across threads

tests/core/test_get_boxes.py

@@ -0,0 +1,44 @@
+"""get_boxes must give each thread its own box when tracks read their init segments at the same time."""
+
+from __future__ import annotations
+
+import struct
+import threading
+from uuid import UUID, uuid4
+
+from unshackle.core.utilities import get_boxes
+
+
+def tenc_init(kid: UUID, version: int, crypt_skip: int) -> bytes:
+    body = struct.pack(">B3s", version, b"\0\0\0") + bytes([0, crypt_skip if version else 0]) + b"\x01\x10" + kid.bytes
+    return b"\x00" * 8 + struct.pack(">I", 8 + len(body)) + b"tenc" + body
+
+
+def test_get_boxes_finds_the_tenc() -> None:
+    kid = uuid4()
+    (box,) = get_boxes(tenc_init(kid, 1, 0x19), b"tenc")
+    assert box.key_ID == kid
+
+
+def test_get_boxes_is_right_when_tracks_parse_at_the_same_time() -> None:
+    cases = [(uuid4(), version, crypt_skip) for version, crypt_skip in ((0, 0), (1, 0x19), (1, 0x91), (1, 0x55))] * 2
+    failures: list[str] = []
+
+    def parse(kid: UUID, version: int, crypt_skip: int) -> None:
+        data = tenc_init(kid, version, crypt_skip)
+        for _ in range(1500):
+            try:
+                boxes = list(get_boxes(data, b"tenc"))
+            except Exception as e:
+                failures.append(repr(e))
+                continue
+            if [box.key_ID for box in boxes] != [kid]:
+                failures.append("wrong or missing tenc")
+
+    threads = [threading.Thread(target=parse, args=case) for case in cases]
+    for thread in threads:
+        thread.start()
+    for thread in threads:
+        thread.join(timeout=120)
+    assert not failures, failures[:3]
+    assert not any(thread.is_alive() for thread in threads)