[fd/dash] Force native downloader for `--live-from-start` (#8339)

Closes #8212
Authored by: bashonly

yt_dlp/downloader/dash.py

@@ -15,12 +15,15 @@ class DashSegmentsFD(FragmentFD):
     FD_NAME = 'dashsegments'
 
     def real_download(self, filename, info_dict):
-        if info_dict.get('is_live') and set(info_dict['protocol'].split('+')) != {'http_dash_segments_generator'}:
-            self.report_error('Live DASH videos are not supported')
+        if 'http_dash_segments_generator' in info_dict['protocol'].split('+'):
+            real_downloader = None  # No external FD can support --live-from-start
+        else:
+            if info_dict.get('is_live'):
+                self.report_error('Live DASH videos are not supported')
+            real_downloader = get_suitable_downloader(
+                info_dict, self.params, None, protocol='dash_frag_urls', to_stdout=(filename == '-'))
 
         real_start = time.time()
-        real_downloader = get_suitable_downloader(
-            info_dict, self.params, None, protocol='dash_frag_urls', to_stdout=(filename == '-'))
 
         requested_formats = [{**info_dict, **fmt} for fmt in info_dict.get('requested_formats', [])]
         args = []