[downloader/ism] Prevent writing the header again when resuming an interrupted download

yt_dlp/downloader/ism.py

@@ -221,10 +221,13 @@ class IsmFD(FragmentFD):
 
         self._prepare_and_start_frag_download(ctx)
 
+        extra_state = ctx.setdefault('extra_state', {
+            'ism_track_written': False,
+        })
+
         fragment_retries = self.params.get('fragment_retries', 0)
         skip_unavailable_fragments = self.params.get('skip_unavailable_fragments', True)
 
-        track_written = False
         frag_index = 0
         for i, segment in enumerate(segments):
             frag_index += 1
@@ -236,11 +239,11 @@ class IsmFD(FragmentFD):
                     success, frag_content = self._download_fragment(ctx, segment['url'], info_dict)
                     if not success:
                         return False
-                    if not track_written:
+                    if not extra_state['ism_track_written']:
                         tfhd_data = extract_box_data(frag_content, [b'moof', b'traf', b'tfhd'])
                         info_dict['_download_params']['track_id'] = u32.unpack(tfhd_data[4:8])[0]
                         write_piff_header(ctx['dest_stream'], info_dict['_download_params'])
-                        track_written = True
+                        extra_state['ism_track_written'] = True
                     self._append_fragment(ctx, frag_content)
                     break
                 except compat_urllib_error.HTTPError as err: