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