[youtube:channel] Improve channel id extraction and detect unavailable channels (Closes #10009)

youtube_dl/extractor/youtube.py

@@ -1995,9 +1995,13 @@ class YoutubeChannelIE(YoutubePlaylistBaseInfoExtractor):
             channel_playlist_id = self._html_search_meta(
                 'channelId', channel_page, 'channel id', default=None)
             if not channel_playlist_id:
+                channel_url = self._html_search_meta(
+                    ('al:ios:url', 'twitter:app:url:iphone', 'twitter:app:url:ipad'),
+                    channel_page, 'channel url', default=None)
+                if channel_url:
                     channel_playlist_id = self._search_regex(
-                    r'data-(?:channel-external-|yt)id="([^"]+)"',
-                    channel_page, 'channel id', default=None)
+                        r'vnd\.youtube://user/([0-9A-Za-z_-]+)',
+                        channel_url, 'channel id', default=None)
         if channel_playlist_id and channel_playlist_id.startswith('UC'):
             playlist_id = 'UU' + channel_playlist_id[2:]
             return self.url_result(
@@ -2020,6 +2024,15 @@ class YoutubeChannelIE(YoutubePlaylistBaseInfoExtractor):
                 for video_id, video_title in self.extract_videos_from_page(channel_page)]
             return self.playlist_result(entries, channel_id)
 
+        try:
+            next(self._entries(channel_page, channel_id))
+        except StopIteration:
+            alert_message = self._html_search_regex(
+                r'(?s)<div[^>]+class=(["\']).*?\byt-alert-message\b.*?\1[^>]*>(?P<alert>[^<]+)</div>',
+                channel_page, 'alert', default=None, group='alert')
+            if alert_message:
+                raise ExtractorError('Youtube said: %s' % alert_message, expected=True)
+
         return self.playlist_result(self._entries(channel_page, channel_id), channel_id)
 
 
@@ -2033,7 +2046,8 @@ class YoutubeUserIE(YoutubeChannelIE):
         'url': 'https://www.youtube.com/user/TheLinuxFoundation',
         'playlist_mincount': 320,
         'info_dict': {
-            'title': 'TheLinuxFoundation',
+            'id': 'UUfX55Sx5hEFjoC3cNs6mCUQ',
+            'title': 'Uploads from The Linux Foundation',
         }
     }, {
         'url': 'ytuser:phihag',
@@ -2041,6 +2055,10 @@ class YoutubeUserIE(YoutubeChannelIE):
     }, {
         'url': 'https://www.youtube.com/c/gametrailers',
         'only_matching': True,
+    }, {
+        # This channel is not available.
+        'url': 'https://www.youtube.com/user/kananishinoSMEJ/videos',
+        'only_matching': True,
     }]
 
     @classmethod