Fix race condition in timestamp adjustment for HLS

If a Webvtt HlsChunkSource got to schedule its chunk load before the
master HlsChunkSource (the one that downloads the TS or the fMP4
chunks), the player would never get past the buffering state.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132985792

library/src/main/java/com/google/android/exoplayer2/extractor/ts/TimestampAdjuster.java

@@ -58,13 +58,6 @@ public final class TimestampAdjuster {
     lastSampleTimestamp = C.TIME_UNSET;
   }
 
-  /**
-   * Whether this adjuster has been initialized with a first MPEG-2 TS presentation timestamp.
-   */
-  public boolean isInitialized() {
-    return lastSampleTimestamp != C.TIME_UNSET;
-  }
-
   /**
    * Scales and offsets an MPEG-2 TS presentation timestamp considering wraparound.
    *
@@ -92,15 +85,34 @@ public final class TimestampAdjuster {
    * @return The adjusted timestamp in microseconds.
    */
   public long adjustSampleTimestamp(long timeUs) {
-    if (firstSampleTimestampUs != DO_NOT_OFFSET && lastSampleTimestamp == C.TIME_UNSET) {
-      // Calculate the timestamp offset.
-      timestampOffsetUs = firstSampleTimestampUs - timeUs;
-    }
     // Record the adjusted PTS to adjust for wraparound next time.
-    lastSampleTimestamp = timeUs;
+    if (lastSampleTimestamp != C.TIME_UNSET) {
+      lastSampleTimestamp = timeUs;
+    } else {
+      if (firstSampleTimestampUs != DO_NOT_OFFSET) {
+        // Calculate the timestamp offset.
+        timestampOffsetUs = firstSampleTimestampUs - timeUs;
+      }
+      synchronized (this) {
+        lastSampleTimestamp = timeUs;
+        // Notify threads waiting for this adjuster to be initialized.
+        notifyAll();
+      }
+    }
     return timeUs + timestampOffsetUs;
   }
 
+  /**
+   * Blocks the calling thread until this adjuster is initialized.
+   *
+   * @throws InterruptedException If the thread was interrupted.
+   */
+  public synchronized void waitUntilInitialized() throws InterruptedException {
+    while (lastSampleTimestamp == C.TIME_UNSET) {
+      wait();
+    }
+  }
+
   /**
    * Converts a value in MPEG-2 timestamp units to the corresponding value in microseconds.
    *

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

@@ -329,6 +329,8 @@ import java.util.Locale;
     // Configure the extractor that will read the chunk.
     Extractor extractor;
     boolean extractorNeedsInit = true;
+    boolean isTimestampMaster = false;
+    TimestampAdjuster timestampAdjuster = null;
     String lastPathSegment = chunkUri.getLastPathSegment();
     if (lastPathSegment.endsWith(AAC_FILE_EXTENSION)) {
       // TODO: Inject a timestamp adjuster and use it along with ID3 PRIV tag values with owner
@@ -342,25 +344,16 @@ import java.util.Locale;
       extractor = new Mp3Extractor(startTimeUs);
     } else if (lastPathSegment.endsWith(WEBVTT_FILE_EXTENSION)
         || lastPathSegment.endsWith(VTT_FILE_EXTENSION)) {
-      TimestampAdjuster timestampAdjuster = timestampAdjusterProvider.getAdjuster(false,
-          segment.discontinuitySequenceNumber, startTimeUs);
-      if (timestampAdjuster == null) {
-        // The master source has yet to instantiate an adjuster for the discontinuity sequence.
-        // TODO: There's probably an edge case if the master starts playback at a chunk belonging to
-        // a discontinuity sequence greater than the one that this source is trying to start at.
-        return;
-      }
+      timestampAdjuster = timestampAdjusterProvider.getAdjuster(segment.discontinuitySequenceNumber,
+          startTimeUs);
       extractor = new WebvttExtractor(format.language, timestampAdjuster);
     } else if (previous == null
         || previous.discontinuitySequenceNumber != segment.discontinuitySequenceNumber
         || format != previous.trackFormat) {
       // MPEG-2 TS segments, but we need a new extractor.
-      TimestampAdjuster timestampAdjuster = timestampAdjusterProvider.getAdjuster(true,
-          segment.discontinuitySequenceNumber, startTimeUs);
-      if (timestampAdjuster == null) {
-        // The master source has yet to instantiate an adjuster for the discontinuity sequence.
-        return;
-      }
+      isTimestampMaster = true;
+      timestampAdjuster = timestampAdjusterProvider.getAdjuster(segment.discontinuitySequenceNumber,
+          startTimeUs);
       // This flag ensures the change of pid between streams does not affect the sample queues.
       int workaroundFlags = TsExtractor.WORKAROUND_MAP_BY_TYPE;
       String codecs = variants[newVariantIndex].format.codecs;
@@ -384,8 +377,9 @@ import java.util.Locale;
 
     out.chunk = new HlsMediaChunk(dataSource, dataSpec, format,
         trackSelection.getSelectionReason(), trackSelection.getSelectionData(),
-        startTimeUs, endTimeUs, chunkMediaSequence, segment.discontinuitySequenceNumber, extractor,
-        extractorNeedsInit, switchingVariant, encryptionKey, encryptionIv);
+        startTimeUs, endTimeUs, chunkMediaSequence, segment.discontinuitySequenceNumber,
+        isTimestampMaster, timestampAdjuster, extractor, extractorNeedsInit, switchingVariant,
+        encryptionKey, encryptionIv);
   }
 
   /**

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

@@ -19,6 +19,7 @@ import com.google.android.exoplayer2.Format;
 import com.google.android.exoplayer2.extractor.DefaultExtractorInput;
 import com.google.android.exoplayer2.extractor.Extractor;
 import com.google.android.exoplayer2.extractor.ExtractorInput;
+import com.google.android.exoplayer2.extractor.ts.TimestampAdjuster;
 import com.google.android.exoplayer2.source.chunk.MediaChunk;
 import com.google.android.exoplayer2.upstream.DataSource;
 import com.google.android.exoplayer2.upstream.DataSpec;
@@ -51,6 +52,8 @@ import java.util.concurrent.atomic.AtomicInteger;
   private final boolean isEncrypted;
   private final boolean extractorNeedsInit;
   private final boolean shouldSpliceIn;
+  private final boolean isMasterTimestampSource;
+  private final TimestampAdjuster timestampAdjuster;
 
   private int bytesLoaded;
   private HlsSampleStreamWrapper extractorOutput;
@@ -68,6 +71,8 @@ import java.util.concurrent.atomic.AtomicInteger;
    * @param endTimeUs The end time of the media contained by the chunk, in microseconds.
    * @param chunkIndex The media sequence number of the chunk.
    * @param discontinuitySequenceNumber The discontinuity sequence number of the chunk.
+   * @param isMasterTimestampSource True if the chunk can initialize the timestamp adjuster.
+   * @param timestampAdjuster Adjuster corresponding to the provided discontinuity sequence number.
    * @param extractor The extractor to decode samples from the data.
    * @param extractorNeedsInit Whether the extractor needs initializing with the target
    *     {@link HlsSampleStreamWrapper}.
@@ -78,12 +83,14 @@ import java.util.concurrent.atomic.AtomicInteger;
    */
   public HlsMediaChunk(DataSource dataSource, DataSpec dataSpec, Format trackFormat,
       int trackSelectionReason, Object trackSelectionData, long startTimeUs, long endTimeUs,
-      int chunkIndex, int discontinuitySequenceNumber, Extractor extractor,
-      boolean extractorNeedsInit, boolean shouldSpliceIn, byte[] encryptionKey,
-      byte[] encryptionIv) {
+      int chunkIndex, int discontinuitySequenceNumber, boolean isMasterTimestampSource,
+      TimestampAdjuster timestampAdjuster, Extractor extractor, boolean extractorNeedsInit,
+      boolean shouldSpliceIn, byte[] encryptionKey, byte[] encryptionIv) {
     super(buildDataSource(dataSource, encryptionKey, encryptionIv), dataSpec, trackFormat,
         trackSelectionReason, trackSelectionData, startTimeUs, endTimeUs, chunkIndex);
     this.discontinuitySequenceNumber = discontinuitySequenceNumber;
+    this.isMasterTimestampSource = isMasterTimestampSource;
+    this.timestampAdjuster = timestampAdjuster;
     this.extractor = extractor;
     this.extractorNeedsInit = extractorNeedsInit;
     this.shouldSpliceIn = shouldSpliceIn;
@@ -166,6 +173,9 @@ import java.util.concurrent.atomic.AtomicInteger;
       }
       try {
         int result = Extractor.RESULT_CONTINUE;
+        if (!isMasterTimestampSource && timestampAdjuster != null) {
+          timestampAdjuster.waitUntilInitialized();
+        }
         while (result == Extractor.RESULT_CONTINUE && !loadCanceled) {
           result = extractor.read(input, null);
         }

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

@@ -34,22 +34,18 @@ public final class TimestampAdjusterProvider {
   /**
    * Returns a {@link TimestampAdjuster} suitable for adjusting the pts timestamps contained in
    * a chunk with a given discontinuity sequence.
-   * <p>
-   * This method may return null if the master source has yet to initialize a suitable adjuster.
    *
-   * @param isMasterSource True if the calling chunk source is the master.
    * @param discontinuitySequence The chunk's discontinuity sequence.
    * @param startTimeUs The chunk's start time.
    * @return A {@link TimestampAdjuster}.
    */
-  public TimestampAdjuster getAdjuster(boolean isMasterSource, int discontinuitySequence,
-      long startTimeUs) {
+  public TimestampAdjuster getAdjuster(int discontinuitySequence, long startTimeUs) {
     TimestampAdjuster adjuster = timestampAdjusters.get(discontinuitySequence);
-    if (isMasterSource && adjuster == null) {
+    if (adjuster == null) {
       adjuster = new TimestampAdjuster(startTimeUs);
       timestampAdjusters.put(discontinuitySequence, adjuster);
     }
-    return isMasterSource || (adjuster != null && adjuster.isInitialized()) ? adjuster : null;
+    return adjuster;
   }
 
   /**