Seek nearest SYNC does not adjust stale playlists

`getPlaylistSnapshot()` can return a cached but potentially quite stale playlist
that may be unsuitable for resolving a seek position.

This change fixes a race conditon that can cause a failed attempt to resolve
a seek position with a stale playlist.

Pre-conditions for the race:

* A is the cached snapshot from previous playback of playlist A
* B is the selected playlist at time of the seek
* \* — is current playback position in period
* QQQ — positions in A cannot be seek targets from B, they have rolled out from `Timeline.Window`
* WWW — are positions in A that cannot be resolved in A, but would be in A`

```
+=================*==+
|       Period       |
+=================*==+

     +QQQ-------+
     |     A    |
     +QQQ-------+
         +-------WWW+
         |     B    |
         +-------WWW+

           seek and switch back to A occur here...

          +----------+
          |    A'    |
          +----------+
```

The seek request is issued with `Timeline.Window` from playlist B, yet the call to `getAdjustedSeekPositionUs()`
occurs:

1. After the current track selection swithces back to A
2. But before playlist snapshot A` is loaded, replacing the old A

The check is simple, does basic sanity check on `targetPositionInPlaylistUs` (that it
is < durationUs of the selected playlist).  This covers the positions WWW.

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

@@ -270,28 +270,35 @@ public long getAdjustedSeekPositionUs(long positionUs, SeekParameters seekParame
  return positionUs;
  }
 
+ long startOfPlaylistInPeriodUs =
+ mediaPlaylist.startTimeUs - playlistTracker.getInitialStartTimeUs();
+ long targetPositionInPlaylistUs = positionUs - startOfPlaylistInPeriodUs;
+
+ if (targetPositionInPlaylistUs > mediaPlaylist.durationUs) {
+ return positionUs;
+ }
+
  // Segments start with sync samples (i.e., EXT-X-INDEPENDENT-SEGMENTS is set) and the playlist
- // is non-empty, so we can use segment start times as sync points. Note that in the rare case
- // that (a) an adaptive quality switch occurs between the adjustment and the seek being
+ // is non-empty, and the playlist is fresh enough that the period position falls in side of
+ // the playlist bound, so we can use segment start times as sync points.
+ //
+ // If (a) an adaptive quality switch occurs between the adjustment and the seek being
  // performed, and (b) segment start times are not aligned across variants, it's possible that
  // the adjusted position may not be at a sync point when it was intended to be. However, this is
  // very much an edge case, and getting it wrong is worth it for getting the vast majority of
  // cases right whilst keeping the implementation relatively simple.
- long startOfPlaylistInPeriodUs =
- mediaPlaylist.startTimeUs - playlistTracker.getInitialStartTimeUs();
- long relativePositionUs = positionUs - startOfPlaylistInPeriodUs;
- int segmentIndex =
+ int segIndex =
  Util.binarySearchFloor(
  mediaPlaylist.segments,
- relativePositionUs,
+ targetPositionInPlaylistUs,
  /* inclusive= */ true,
  /* stayInBounds= */ true);
- long firstSyncUs = mediaPlaylist.segments.get(segmentIndex).relativeStartTimeUs;
+ long firstSyncUs = mediaPlaylist.segments.get(segIndex).relativeStartTimeUs;
  long secondSyncUs = firstSyncUs;
- if (segmentIndex != mediaPlaylist.segments.size() - 1) {
- secondSyncUs = mediaPlaylist.segments.get(segmentIndex + 1).relativeStartTimeUs;
+ if (segIndex != mediaPlaylist.segments.size() - 1) {
+ secondSyncUs = mediaPlaylist.segments.get(segIndex + 1).relativeStartTimeUs;
  }
- return seekParameters.resolveSeekPositionUs(relativePositionUs, firstSyncUs, secondSyncUs)
+ return seekParameters.resolveSeekPositionUs(targetPositionInPlaylistUs, firstSyncUs, secondSyncUs)
  + startOfPlaylistInPeriodUs;
  }
 

library/hls/src/test/java/com/google/android/exoplayer2/source/hls/HlsChunkSourceTest.java

@@ -47,8 +47,10 @@
 public class HlsChunkSourceTest {
 
  private static final String PLAYLIST = "media/m3u8/media_playlist";
- private static final String PLAYLIST_INDEPENDENT_SEGMENTS =
- "media/m3u8/media_playlist_independent_segments";
+ private static final String PLAYLIST_LIVE_FIRST =
+ "media/m3u8/media_playlist_live_first";
+ private static final String PLAYLIST_LIVE_SECOND =
+ "media/m3u8/media_playlist_live_second";
  private static final String PLAYLIST_EMPTY = "media/m3u8/media_playlist_empty";
  private static final Uri PLAYLIST_URI = Uri.parse("http://example.com/");
  private static final long PLAYLIST_START_PERIOD_OFFSET_US = 8_000_000L;
@@ -71,7 +73,7 @@ public void setup() throws IOException {
 
  InputStream inputStream =
  TestUtil.getInputStream(
- ApplicationProvider.getApplicationContext(), PLAYLIST_INDEPENDENT_SEGMENTS);
+ ApplicationProvider.getApplicationContext(), PLAYLIST_LIVE_FIRST);
  HlsMediaPlaylist playlist =
  (HlsMediaPlaylist) new HlsPlaylistParser().parse(PLAYLIST_URI, inputStream);
  when(mockPlaylistTracker.getPlaylistSnapshot(eq(PLAYLIST_URI), anyBoolean()))
@@ -161,7 +163,22 @@ public void getAdjustedSeekPositionUs_noIndependentSegments() throws IOException
 
  long adjustedPositionUs =
  testChunkSource.getAdjustedSeekPositionUs(
- playlistTimeToPeriodTimeUs(100_000_000), SeekParameters.EXACT);
+ playlistTimeToPeriodTimeUs(100_000_000), SeekParameters.NEXT_SYNC);
+
+ assertThat(periodTimeToPlaylistTimeUs(adjustedPositionUs)).isEqualTo(100_000_000);
+ }
+
+ @Test
+ public void getAdjustedSeekPositionUs_stalePlaylist() throws IOException {
+ InputStream inputStream =
+ TestUtil.getInputStream(ApplicationProvider.getApplicationContext(), PLAYLIST_LIVE_SECOND);
+ HlsMediaPlaylist playlist =
+ (HlsMediaPlaylist) new HlsPlaylistParser().parse(PLAYLIST_URI, inputStream);
+ when(mockPlaylistTracker.getPlaylistSnapshot(eq(PLAYLIST_URI), anyBoolean()))
+ .thenReturn(playlist);
+ long adjustedPositionUs =
+ testChunkSource.getAdjustedSeekPositionUs(
+ playlistTimeToPeriodTimeUs(100_000_000), SeekParameters.NEXT_SYNC);
 
  assertThat(periodTimeToPlaylistTimeUs(adjustedPositionUs)).isEqualTo(100_000_000);
  }
@@ -177,7 +194,7 @@ public void getAdjustedSeekPositionUs_emptyPlaylist() throws IOException {
 
  long adjustedPositionUs =
  testChunkSource.getAdjustedSeekPositionUs(
- playlistTimeToPeriodTimeUs(100_000_000), SeekParameters.EXACT);
+ playlistTimeToPeriodTimeUs(100_000_000), SeekParameters.NEXT_SYNC);
 
  assertThat(periodTimeToPlaylistTimeUs(adjustedPositionUs)).isEqualTo(100_000_000);
  }

testdata/src/test/assets/media/m3u8/media_playlist_live_first

@@ -0,0 +1,17 @@
+#EXTM3U
+#EXT-X-MEDIA-SEQUENCE:2
+#EXT-X-INDEPENDENT-SEGMENTS
+#EXT-X-PROGRAM-DATE-TIME:2019-02-14T02:00:00.106Z
+#EXT-X-MAP:URI="init.mp4"
+#EXTINF:4,
+2.mp4
+#EXTINF:4,
+3.mp4
+#EXTINF:4,
+4.mp4
+#EXTINF:4,
+5.mp4
+#EXTINF:4,
+6.mp4
+#EXTINF:4,
+7.mp4

testdata/src/test/assets/media/m3u8/media_playlist_live_second

@@ -0,0 +1,15 @@
+#EXTM3U
+#EXT-X-MEDIA-SEQUENCE:4
+#EXT-X-INDEPENDENT-SEGMENTS
+#EXT-X-PROGRAM-DATE-TIME:2019-02-14T02:00:08.106Z
+#EXT-X-MAP:URI="init.mp4"
+#EXTINF:4,
+4.mp4
+#EXTINF:4,
+5.mp4
+#EXTINF:4,
+6.mp4
+#EXTINF:4,
+7.mp4
+#EXTINF:4,
+8.mp4