Apply threading lock to HLS DRM preparation

Without this, if two threads started at the same time there was a very good chance they would run the code and license twice, which is unnecessary.

devine/core/manifests/hls.py

@@ -11,7 +11,7 @@ from functools import partial
 from hashlib import md5
 from pathlib import Path
 from queue import Queue
-from threading import Event
+from threading import Event, Lock
 from typing import Any, Callable, Optional, Union
 
 import m3u8
@@ -214,6 +214,8 @@ class HLS:
             log.error("Track's HLS playlist has no segments, expecting an invariant M3U8 playlist.")
             sys.exit(1)
 
+        drm_lock = Lock()
+
         def download_segment(filename: str, segment: m3u8.Segment, init_data: Queue, segment_key: Queue) -> int:
             if stop_event.is_set():
                 # the track already started downloading, but another failed or was stopped
@@ -221,6 +223,7 @@ class HLS:
 
             segment_save_path = (save_dir / filename).with_suffix(".mp4")
 
+            with drm_lock:
                 newest_segment_key = segment_key.get()
                 try:
                     if segment.key and newest_segment_key[1] != segment.key: