[youtube] Support --download-sections for YT Livestream from start

yt_dlp/YoutubeDL.py

@@ -26,7 +26,12 @@
 from .cache import Cache
 from .compat import compat_os_name, compat_shlex_quote
 from .cookies import load_cookies
-from .downloader import FFmpegFD, get_suitable_downloader, shorten_protocol_name
+from .downloader import (
+    DashSegmentsFD,
+    FFmpegFD,
+    get_suitable_downloader,
+    shorten_protocol_name,
+)
 from .downloader.rtmp import rtmpdump_version
 from .extractor import gen_extractor_classes, get_info_extractor
 from .extractor.common import UnsupportedURLIE
@@ -3143,8 +3148,8 @@ def existing_video_file(*filepaths):
                 fd, success = None, True
                 if info_dict.get('protocol') or info_dict.get('url'):
                     fd = get_suitable_downloader(info_dict, self.params, to_stdout=temp_filename == '-')
-                    if fd is not FFmpegFD and 'no-direct-merge' not in self.params['compat_opts'] and (
-                            info_dict.get('section_start') or info_dict.get('section_end')):
+                    if not(fd is FFmpegFD or fd is DashSegmentsFD) and 'no-direct-merge' not in self.params['compat_opts'] and (
+                        info_dict.get('section_start') or info_dict.get('section_end')):
                         msg = ('This format cannot be partially downloaded' if FFmpegFD.available()
                                else 'You have requested downloading the video partially, but ffmpeg is not installed')
                         self.report_error(f'{msg}. Aborting')

yt_dlp/downloader/dash.py

@@ -33,6 +33,8 @@ def real_download(self, filename, info_dict):
                 'filename': fmt.get('filepath') or filename,
                 'live': 'is_from_start' if fmt.get('is_from_start') else fmt.get('is_live'),
                 'total_frags': fragment_count,
+                'section_start': info_dict.get('section_start'),
+                'section_end': info_dict.get('section_end'),
             }
 
             if real_downloader:

yt_dlp/extractor/youtube.py

@@ -2757,6 +2757,8 @@ def _live_dash_fragments(self, video_id, format_id, live_start_time, mpd_feed, m
 
         begin_index = 0
         download_start_time = ctx.get('start') or time.time()
+        section_start = ctx.get('section_start') or 0
+        section_end = ctx.get('section_end') or math.inf
 
         lack_early_segments = download_start_time - (live_start_time or download_start_time) > MAX_DURATION
         if lack_early_segments:
@@ -2797,8 +2799,7 @@ def _extract_sequence_from_mpd(refresh_sequence, immediate):
             fragment_base_url = fmt_info['fragment_base_url']
             assert fragment_base_url
 
-            _last_seq = int(re.search(r'(?:/|^)sq/(\d+)', fragments[-1]['path']).group(1))
-            return True, _last_seq
+            return True
 
         self.write_debug(f'[{video_id}] Generating fragments for format {format_id}')
         while is_live:
@@ -2818,11 +2819,19 @@ def _extract_sequence_from_mpd(refresh_sequence, immediate):
                     last_segment_url = None
                     continue
             else:
-                should_continue, last_seq = _extract_sequence_from_mpd(True, no_fragment_score > 15)
+                should_continue = _extract_sequence_from_mpd(True, no_fragment_score > 15)
                 no_fragment_score += 2
                 if not should_continue:
                     continue
 
+            last_fragment = fragments[-1]
+            last_seq = int(re.search(r'(?:/|^)sq/(\d+)', last_fragment['path']).group(1))
+
+            known_fragment = next(
+                (fragment for fragment in fragments if f'sq/{known_idx}' in fragment['path']), None)
+            if known_fragment and known_fragment['end'] > section_end:
+                break
+
             if known_idx > last_seq:
                 last_segment_url = None
                 continue
@@ -2832,20 +2841,31 @@ def _extract_sequence_from_mpd(refresh_sequence, immediate):
             if begin_index < 0 and known_idx < 0:
                 # skip from the start when it's negative value
                 known_idx = last_seq + begin_index
+
             if lack_early_segments:
-                known_idx = max(known_idx, last_seq - int(MAX_DURATION // fragments[-1]['duration']))
+                known_idx = max(known_idx, last_seq - int(MAX_DURATION // last_fragment['duration']))
+
             try:
                 for idx in range(known_idx, last_seq):
                     # do not update sequence here or you'll get skipped some part of it
-                    should_continue, _ = _extract_sequence_from_mpd(False, False)
+                    should_continue = _extract_sequence_from_mpd(False, False)
                     if not should_continue:
                         known_idx = idx - 1
                         raise ExtractorError('breaking out of outer loop')
+
                     last_segment_url = urljoin(fragment_base_url, 'sq/%d' % idx)
-                    yield {
-                        'url': last_segment_url,
-                        'fragment_count': last_seq,
-                    }
+                    frag_duration = last_fragment['duration']
+                    frag_start = last_fragment['start'] - (last_seq - idx) * frag_duration
+                    frag_end = frag_start + frag_duration
+
+                    if frag_start >= section_start and frag_end <= section_end:
+                        yield {
+                            'url': last_segment_url,
+                            'duration': frag_duration,
+                            'start': frag_start,
+                            'end': frag_end,
+                        }
+
                 if known_idx == last_seq:
                     no_fragment_score += 5
                 else: