Better management of blacklisted playlists

Added an expiration time field to playlists blacklisted to allow
Exoplayer to continue playback when playlists that failed were
recovered from a bad state.

In live environments, some times occur that primary encoder stop
working for a while. In that cases, HLS failover mechanism in the
player should detect the situation and “switch” to playlists served by
the backup encoder (in case a backup encoder exists). This was well
managed before these changes.

However, and to ensure a playback experience that can recover itself
from temporary issues, we cannot blacklist a playlist forever. When
streaming live events using HLS, it is quite typical that the player
needs to switch from primary to backup playlists, and from backup to
primary ones, from time to time to have playback working when temporary
issues in the network/encoder are happening. Most of the issues are
recoverable, so what I have implemented is a mechanism that makes
blacklisted playlist to be available again after a while (60 seconds).

Evaluation of this algorithm should happen just when something fails.
If player is working with a backup playlist, it shouldn’t switch to the
primary one at least something fail.

library/src/main/java/com/google/android/exoplayer/hls/HlsChunkSource.java

@@ -106,6 +106,12 @@ public class HlsChunkSource {
    */
   public static final long DEFAULT_MAX_BUFFER_TO_SWITCH_DOWN_MS = 20000;
 
+  /**
+   * The default maximum time a media playlist is blacklisted without
+   * rechecking if it is alive again (because an encoder reset, for example)
+   */
+  public static final long DEFAULT_MAX_TIME_MEDIA_PLAYLIST_BLACKLISTED_MS = 60000;
+
   private static final String TAG = "HlsChunkSource";
   private static final String AAC_FILE_EXTENSION = ".aac";
   private static final float BANDWIDTH_FRACTION = 0.8f;
@@ -127,6 +133,7 @@ public class HlsChunkSource {
   /* package */ byte[] scratchSpace;
   /* package */ final HlsMediaPlaylist[] mediaPlaylists;
   /* package */ final boolean[] mediaPlaylistBlacklistFlags;
+  /* package */ final long[] mediaPlaylistBlacklistedTimeMs;
   /* package */ final long[] lastMediaPlaylistLoadTimesMs;
   /* package */ boolean live;
   /* package */ long durationUs;
@@ -182,6 +189,7 @@ public class HlsChunkSource {
       enabledVariants = new Variant[] {new Variant(0, playlistUrl, 0, null, -1, -1)};
       mediaPlaylists = new HlsMediaPlaylist[1];
       mediaPlaylistBlacklistFlags = new boolean[1];
+      mediaPlaylistBlacklistedTimeMs = new long[1];
       lastMediaPlaylistLoadTimesMs = new long[1];
       setMediaPlaylist(0, (HlsMediaPlaylist) playlist);
     } else {
@@ -189,6 +197,7 @@ public class HlsChunkSource {
       enabledVariants = filterVariants((HlsMasterPlaylist) playlist, variantIndices);
       mediaPlaylists = new HlsMediaPlaylist[enabledVariants.length];
       mediaPlaylistBlacklistFlags = new boolean[enabledVariants.length];
+      mediaPlaylistBlacklistedTimeMs = new long[enabledVariants.length];
       lastMediaPlaylistLoadTimesMs = new long[enabledVariants.length];
     }
 
@@ -362,6 +371,8 @@ public class HlsChunkSource {
       if (responseCode == 404 || responseCode == 410) {
         MediaPlaylistChunk playlistChunk = (MediaPlaylistChunk) chunk;
         mediaPlaylistBlacklistFlags[playlistChunk.variantIndex] = true;
+        mediaPlaylistBlacklistedTimeMs[playlistChunk.variantIndex] = SystemClock.elapsedRealtime();
+        evaluatePlaylistBlacklistedTimestamps();
         if (!allPlaylistsBlacklisted()) {
           // We've handled the 404/410 by blacklisting the playlist.
           Log.w(TAG, "Blacklisted playlist (" + responseCode + "): "
@@ -372,6 +383,7 @@ public class HlsChunkSource {
           Log.w(TAG, "Final playlist not blacklisted (" + responseCode + "): "
               + playlistChunk.dataSpec.uri);
           mediaPlaylistBlacklistFlags[playlistChunk.variantIndex] = false;
+          mediaPlaylistBlacklistedTimeMs[playlistChunk.variantIndex] = 0;
           return false;
         }
       }
@@ -541,6 +553,17 @@ public class HlsChunkSource {
     return true;
   }
 
+  private void evaluatePlaylistBlacklistedTimestamps()
+  {
+    long currentTime = SystemClock.elapsedRealtime();
+    for (int i = 0; i < mediaPlaylistBlacklistFlags.length; i++) {
+      if (mediaPlaylistBlacklistFlags[i] && currentTime - mediaPlaylistBlacklistedTimeMs[i] > DEFAULT_MAX_TIME_MEDIA_PLAYLIST_BLACKLISTED_MS) {
+        mediaPlaylistBlacklistFlags[i] = false;
+        mediaPlaylistBlacklistedTimeMs[i] = 0;
+      }
+    }
+  }
+
   private class MediaPlaylistChunk extends DataChunk {
 
     @SuppressWarnings("hiding")