Prevent old playlist snapshots from being used on live HLS streams

This aims to replace InvalidCodeResponse's with BLWE's caused by trying to
load chunks that have been removed from the server.

Issue:#2344

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=145679171

library/src/main/java/com/google/android/exoplayer2/source/hls/HlsChunkSource.java

@@ -194,15 +194,16 @@ import java.util.Locale;
 
 
     // Select the variant.
     // Select the variant.
     trackSelection.updateSelectedTrack(bufferedDurationUs);
     trackSelection.updateSelectedTrack(bufferedDurationUs);
-    int newVariantIndex = trackSelection.getSelectedIndexInTrackGroup();
+    int selectedVariantIndex = trackSelection.getSelectedIndexInTrackGroup();
 
 
-    boolean switchingVariant = oldVariantIndex != newVariantIndex;
-    HlsMediaPlaylist mediaPlaylist = playlistTracker.getPlaylistSnapshot(variants[newVariantIndex]);
-    if (mediaPlaylist == null) {
-      out.playlist = variants[newVariantIndex];
+    boolean switchingVariant = oldVariantIndex != selectedVariantIndex;
+    HlsUrl selectedUrl = variants[selectedVariantIndex];
+    if (!playlistTracker.isSnapshotValid(selectedUrl)) {
+      out.playlist = selectedUrl;
       // Retry when playlist is refreshed.
       // Retry when playlist is refreshed.
       return;
       return;
     }
     }
+    HlsMediaPlaylist mediaPlaylist = playlistTracker.getPlaylistSnapshot(selectedUrl);
 
 
     // Select the chunk.
     // Select the chunk.
     int chunkMediaSequence;
     int chunkMediaSequence;
@@ -218,8 +219,9 @@ import java.util.Locale;
         if (chunkMediaSequence < mediaPlaylist.mediaSequence && previous != null) {
         if (chunkMediaSequence < mediaPlaylist.mediaSequence && previous != null) {
           // We try getting the next chunk without adapting in case that's the reason for falling
           // We try getting the next chunk without adapting in case that's the reason for falling
           // behind the live window.
           // behind the live window.
-          newVariantIndex = oldVariantIndex;
-          mediaPlaylist = playlistTracker.getPlaylistSnapshot(variants[newVariantIndex]);
+          selectedVariantIndex = oldVariantIndex;
+          selectedUrl = variants[selectedVariantIndex];
+          mediaPlaylist = playlistTracker.getPlaylistSnapshot(selectedUrl);
           chunkMediaSequence = previous.getNextChunkIndex();
           chunkMediaSequence = previous.getNextChunkIndex();
         }
         }
       }
       }
@@ -236,7 +238,7 @@ import java.util.Locale;
       if (mediaPlaylist.hasEndTag) {
       if (mediaPlaylist.hasEndTag) {
         out.endOfStream = true;
         out.endOfStream = true;
       } else /* Live */ {
       } else /* Live */ {
-        out.playlist = variants[newVariantIndex];
+        out.playlist = selectedUrl;
       }
       }
       return;
       return;
     }
     }
@@ -249,7 +251,7 @@ import java.util.Locale;
       Uri keyUri = UriUtil.resolveToUri(mediaPlaylist.baseUri, segment.encryptionKeyUri);
       Uri keyUri = UriUtil.resolveToUri(mediaPlaylist.baseUri, segment.encryptionKeyUri);
       if (!keyUri.equals(encryptionKeyUri)) {
       if (!keyUri.equals(encryptionKeyUri)) {
         // Encryption is specified and the key has changed.
         // Encryption is specified and the key has changed.
-        out.chunk = newEncryptionKeyChunk(keyUri, segment.encryptionIV, newVariantIndex,
+        out.chunk = newEncryptionKeyChunk(keyUri, segment.encryptionIV, selectedVariantIndex,
             trackSelection.getSelectionReason(), trackSelection.getSelectionData());
             trackSelection.getSelectionReason(), trackSelection.getSelectionData());
         return;
         return;
       }
       }
@@ -279,7 +281,7 @@ import java.util.Locale;
     Uri chunkUri = UriUtil.resolveToUri(mediaPlaylist.baseUri, segment.url);
     Uri chunkUri = UriUtil.resolveToUri(mediaPlaylist.baseUri, segment.url);
     DataSpec dataSpec = new DataSpec(chunkUri, segment.byterangeOffset, segment.byterangeLength,
     DataSpec dataSpec = new DataSpec(chunkUri, segment.byterangeOffset, segment.byterangeLength,
         null);
         null);
-    out.chunk = new HlsMediaChunk(dataSource, dataSpec, initDataSpec, variants[newVariantIndex],
+    out.chunk = new HlsMediaChunk(dataSource, dataSpec, initDataSpec, selectedUrl,
         trackSelection.getSelectionReason(), trackSelection.getSelectionData(),
         trackSelection.getSelectionReason(), trackSelection.getSelectionData(),
         startTimeUs, startTimeUs + segment.durationUs, chunkMediaSequence, discontinuitySequence,
         startTimeUs, startTimeUs + segment.durationUs, chunkMediaSequence, discontinuitySequence,
         isTimestampMaster, timestampAdjuster, previous, encryptionKey, encryptionIv);
         isTimestampMaster, timestampAdjuster, previous, encryptionKey, encryptionIv);

library/src/main/java/com/google/android/exoplayer2/source/hls/playlist/HlsPlaylistTracker.java

@@ -166,8 +166,24 @@ public final class HlsPlaylistTracker implements Loader.Callback<ParsingLoadable
    *     be null if no snapshot has been loaded yet.
    *     be null if no snapshot has been loaded yet.
    */
    */
   public HlsMediaPlaylist getPlaylistSnapshot(HlsUrl url) {
   public HlsMediaPlaylist getPlaylistSnapshot(HlsUrl url) {
+    HlsMediaPlaylist snapshot = playlistBundles.get(url).getPlaylistSnapshot();
+    if (snapshot != null) {
       maybeSetPrimaryUrl(url);
       maybeSetPrimaryUrl(url);
-    return playlistBundles.get(url).getPlaylistSnapshot();
+    }
+    return snapshot;
+  }
+
+  /**
+   * Returns whether the snapshot of the playlist referenced by the provided {@link HlsUrl} is
+   * valid, meaning all the segments referenced by the playlist are expected to be available. If the
+   * playlist is not valid then some of the segments may no longer be available.
+
+   * @param url The {@link HlsUrl}.
+   * @return Whether the snapshot of the playlist referenced by the provided {@link HlsUrl} is
+   *     valid.
+   */
+  public boolean isSnapshotValid(HlsUrl url) {
+    return playlistBundles.get(url).isSnapshotValid();
   }
   }
 
 
   /**
   /**
@@ -412,6 +428,7 @@ public final class HlsPlaylistTracker implements Loader.Callback<ParsingLoadable
     private final ParsingLoadable<HlsPlaylist> mediaPlaylistLoadable;
     private final ParsingLoadable<HlsPlaylist> mediaPlaylistLoadable;
 
 
     private HlsMediaPlaylist playlistSnapshot;
     private HlsMediaPlaylist playlistSnapshot;
+    private long lastSnapshotLoadMs;
     private long lastSnapshotAccessTimeMs;
     private long lastSnapshotAccessTimeMs;
     private long blacklistUntilMs;
     private long blacklistUntilMs;
 
 
@@ -429,6 +446,17 @@ public final class HlsPlaylistTracker implements Loader.Callback<ParsingLoadable
       return playlistSnapshot;
       return playlistSnapshot;
     }
     }
 
 
+    public boolean isSnapshotValid() {
+      if (playlistSnapshot == null) {
+        return false;
+      }
+      // TODO: Return true for event playlists once playlist types are supported.
+      long currentTimeMs = SystemClock.elapsedRealtime();
+      long snapshotValidityDurationMs = Math.max(30000, C.usToMs(playlistSnapshot.durationUs));
+      return playlistSnapshot.hasEndTag
+          || lastSnapshotLoadMs + snapshotValidityDurationMs > currentTimeMs;
+    }
+
     public void release() {
     public void release() {
       mediaPlaylistLoader.release();
       mediaPlaylistLoader.release();
     }
     }
@@ -488,6 +516,7 @@ public final class HlsPlaylistTracker implements Loader.Callback<ParsingLoadable
 
 
     private void processLoadedPlaylist(HlsMediaPlaylist loadedPlaylist) {
     private void processLoadedPlaylist(HlsMediaPlaylist loadedPlaylist) {
       HlsMediaPlaylist oldPlaylist = playlistSnapshot;
       HlsMediaPlaylist oldPlaylist = playlistSnapshot;
+      lastSnapshotLoadMs = SystemClock.elapsedRealtime();
       playlistSnapshot = getLatestPlaylistSnapshot(oldPlaylist, loadedPlaylist);
       playlistSnapshot = getLatestPlaylistSnapshot(oldPlaylist, loadedPlaylist);
       long refreshDelayUs = C.TIME_UNSET;
       long refreshDelayUs = C.TIME_UNSET;
       if (playlistSnapshot != oldPlaylist) {
       if (playlistSnapshot != oldPlaylist) {