hlsdemux2: Improve live playlist update intervals

The live playlists should be updated at a defined interval. The problem is that
this interval was used *after* the playlist was finally received and processed,
which resulted in a gradual shift happening in playlist updates.

Instead store and use the time at which playlists were requested to determine
when the next one should be downloaded.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3883>

subprojects/gst-plugins-good/ext/adaptivedemux2/hls/gsthlsdemux-playlist-loader.c

@@ -179,7 +179,31 @@ static void
 schedule_next_playlist_load (GstHLSDemuxPlaylistLoader * pl,
     GstHLSDemuxPlaylistLoaderPrivate * priv, GstClockTime next_load_interval)
 {
-  GST_LOG_OBJECT (pl, "Scheduling next playlist reload");
+  /* If we have a valid request time, compute a more accurate download time for
+   * the playlist.
+   *
+   * This better takes into account the time it took to actually get and process
+   * the current playlist.
+   */
+  if (priv->current_playlist
+      && GST_CLOCK_TIME_IS_VALID (priv->current_playlist->request_time)) {
+    GstClockTime now = gst_adaptive_demux2_get_monotonic_time (priv->demux);
+    GstClockTimeDiff load_diff = GST_CLOCK_DIFF (now,
+        priv->current_playlist->request_time + next_load_interval);
+    GST_LOG_OBJECT (pl,
+        "now %" GST_TIME_FORMAT " request_time %" GST_TIME_FORMAT
+        " next_load_interval %" GST_TIME_FORMAT, GST_TIME_ARGS (now),
+        GST_TIME_ARGS (priv->current_playlist->request_time),
+        GST_TIME_ARGS (next_load_interval));
+    if (load_diff < 0) {
+      GST_LOG_OBJECT (pl, "Playlist update already late by %" GST_STIME_FORMAT,
+          GST_STIME_ARGS (load_diff));
+    };
+    next_load_interval = MAX (0, load_diff);
+  }
+
+  GST_LOG_OBJECT (pl, "Scheduling next playlist reload in %" GST_TIME_FORMAT,
+      GST_TIME_ARGS (next_load_interval));
   g_assert (priv->pending_cb_id == 0);
   priv->state = PLAYLIST_LOADER_STATE_WAITING;
   priv->pending_cb_id =
@@ -557,6 +581,7 @@ on_download_complete (DownloadRequest * download, DownloadRequestState state,
     GST_DEBUG_OBJECT (pl, "playlist data was unchanged");
     playlist = gst_hls_media_playlist_ref (current_playlist);
     playlist->reloaded = TRUE;
+    playlist->request_time = GST_CLOCK_TIME_NONE;
     g_free (playlist_data);
   } else {
     playlist =
@@ -566,6 +591,7 @@ on_download_complete (DownloadRequest * download, DownloadRequestState state,
       GST_WARNING_OBJECT (pl, "Couldn't parse playlist");
       goto error_retry_out;
     }
+    playlist->request_time = download->download_request_time;
   }
 
   /* Transfer over any skipped segments from the current playlist if

subprojects/gst-plugins-good/ext/adaptivedemux2/hls/m3u8.c

@@ -410,6 +410,7 @@ gst_hls_media_playlist_new (const gchar * uri, const gchar * base_uri)
   m3u8->playlist_ts = GST_CLOCK_TIME_NONE;
   m3u8->uri = g_strdup (uri);
   m3u8->base_uri = g_strdup (base_uri);
+  m3u8->request_time = GST_CLOCK_TIME_NONE;
 
   m3u8->version = 1;
   m3u8->type = GST_HLS_PLAYLIST_TYPE_UNDEFINED;

subprojects/gst-plugins-good/ext/adaptivedemux2/hls/m3u8.h

@@ -98,6 +98,7 @@ struct _GstHLSMediaPlaylist
   gchar *base_uri;              /* URI to use as base for resolving relative URIs.
                                  * This will be different to uri in case of redirects */
   GstClockTime playlist_ts;     /* Monotonic clock time estimate for this playlist's validity from download time and cached Age */
+  GstClockTime request_time;	/* Time at which this playlist was requested in monotonic clock time. */
 
   /* Base Tag */
   gint version;                 /* EXT-X-VERSION (default 1) */