Always re-send format if sample buffer is null

Also clear the playingPeriodHolder in the case the renderers
are being disabled. This is required to ensure that
setPlayingPeriodHolder isn't turned into a no-op, which will
break the seek.

Issue: #2330

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=144635220

library/src/androidTest/java/com/google/android/exoplayer2/ExoPlayerTest.java

@@ -514,13 +514,14 @@ public final class ExoPlayerTest extends TestCase {
 
     @Override
     public int readData(FormatHolder formatHolder, DecoderInputBuffer buffer) {
-      if (readFormat) {
+      if (buffer == null || !readFormat) {
+        formatHolder.format = format;
+        readFormat = true;
+        return C.RESULT_FORMAT_READ;
+      } else {
         buffer.setFlags(C.BUFFER_FLAG_END_OF_STREAM);
         return C.RESULT_BUFFER_READ;
       }
-      formatHolder.format = format;
-      readFormat = true;
-      return C.RESULT_FORMAT_READ;
     }
 
     @Override

library/src/main/java/com/google/android/exoplayer2/BaseRenderer.java

@@ -251,11 +251,11 @@ public abstract class BaseRenderer implements Renderer, RendererCapabilities {
    * {@link C#RESULT_BUFFER_READ} is only returned if {@link #setCurrentStreamFinal()} has been
    * called. {@link C#RESULT_NOTHING_READ} is returned otherwise.
    *
-   * @see SampleStream#readData(FormatHolder, DecoderInputBuffer)
    * @param formatHolder A {@link FormatHolder} to populate in the case of reading a format.
    * @param buffer A {@link DecoderInputBuffer} to populate in the case of reading a sample or the
    *     end of the stream. If the end of the stream has been reached, the
-   *     {@link C#BUFFER_FLAG_END_OF_STREAM} flag will be set on the buffer.
+   *     {@link C#BUFFER_FLAG_END_OF_STREAM} flag will be set on the buffer. May be null if the
+   *     caller requires that the format of the stream be read even if it's not changing.
    * @return The result, which can be {@link C#RESULT_NOTHING_READ}, {@link C#RESULT_FORMAT_READ} or
    *     {@link C#RESULT_BUFFER_READ}.
    */

library/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -610,6 +610,7 @@ import java.io.IOException;
       enabledRenderers = new Renderer[0];
       rendererMediaClock = null;
       rendererMediaClockSource = null;
+      playingPeriodHolder = null;
     }
 
     // Update the holders.

library/src/main/java/com/google/android/exoplayer2/extractor/DefaultTrackOutput.java

@@ -265,7 +265,8 @@ public final class DefaultTrackOutput implements TrackOutput {
    * @param formatHolder A {@link FormatHolder} to populate in the case of reading a format.
    * @param buffer A {@link DecoderInputBuffer} to populate in the case of reading a sample or the
    *     end of the stream. If the end of the stream has been reached, the
-   *     {@link C#BUFFER_FLAG_END_OF_STREAM} flag will be set on the buffer.
+   *     {@link C#BUFFER_FLAG_END_OF_STREAM} flag will be set on the buffer.  May be null if the
+   *     caller requires that the format of the stream be read even if it's not changing.
    * @param loadingFinished True if an empty queue should be considered the end of the stream.
    * @param decodeOnlyUntilUs If a buffer is read, the {@link C#BUFFER_FLAG_DECODE_ONLY} flag will
    *     be set if the buffer's timestamp is less than this value.
@@ -751,7 +752,8 @@ public final class DefaultTrackOutput implements TrackOutput {
      *     about the sample, but not its data. The size and absolute position of the data in the
      *     rolling buffer is stored in {@code extrasHolder}, along with an encryption id if present
      *     and the absolute position of the first byte that may still be required after the current
-     *     sample has been read.
+     *     sample has been read. May be null if the caller requires that the format of the stream be
+     *     read even if it's not changing.
      * @param downstreamFormat The current downstream {@link Format}. If the format of the next
      *     sample is different to the current downstream format then a format will be read.
      * @param extrasHolder The holder into which extra sample information should be written.
@@ -761,14 +763,14 @@ public final class DefaultTrackOutput implements TrackOutput {
     public synchronized int readData(FormatHolder formatHolder, DecoderInputBuffer buffer,
         Format downstreamFormat, BufferExtrasHolder extrasHolder) {
       if (queueSize == 0) {
-        if (upstreamFormat != null && upstreamFormat != downstreamFormat) {
+        if (upstreamFormat != null && (buffer == null || upstreamFormat != downstreamFormat)) {
           formatHolder.format = upstreamFormat;
           return C.RESULT_FORMAT_READ;
         }
         return C.RESULT_NOTHING_READ;
       }
 
-      if (formats[relativeReadIndex] != downstreamFormat) {
+      if (buffer == null || formats[relativeReadIndex] != downstreamFormat) {
         formatHolder.format = formats[relativeReadIndex];
         return C.RESULT_FORMAT_READ;
       }

library/src/main/java/com/google/android/exoplayer2/source/ClippingMediaPeriod.java

@@ -235,6 +235,9 @@ public final class ClippingMediaPeriod implements MediaPeriod, MediaPeriod.Callb
       if (pendingDiscontinuity) {
         return C.RESULT_NOTHING_READ;
       }
+      if (buffer == null) {
+        return stream.readData(formatHolder, null);
+      }
       if (sentEos) {
         buffer.setFlags(C.BUFFER_FLAG_END_OF_STREAM);
         return C.RESULT_BUFFER_READ;

library/src/main/java/com/google/android/exoplayer2/source/SampleStream.java

@@ -44,11 +44,17 @@ public interface SampleStream {
 
   /**
    * Attempts to read from the stream.
+   * <p>
+   * If no data is available then {@link C#RESULT_NOTHING_READ} is returned. If the format of the
+   * media is changing or if {@code buffer == null} then {@code formatHolder} is populated and
+   * {@link C#RESULT_FORMAT_READ} is returned. Else {@code buffer} is populated and
+   * {@link C#RESULT_BUFFER_READ} is returned.
    *
    * @param formatHolder A {@link FormatHolder} to populate in the case of reading a format.
    * @param buffer A {@link DecoderInputBuffer} to populate in the case of reading a sample or the
    *     end of the stream. If the end of the stream has been reached, the
-   *     {@link C#BUFFER_FLAG_END_OF_STREAM} flag will be set on the buffer.
+   *     {@link C#BUFFER_FLAG_END_OF_STREAM} flag will be set on the buffer. May be null if the
+   *     caller requires that the format of the stream be read even if it's not changing.
    * @return The result, which can be {@link C#RESULT_NOTHING_READ}, {@link C#RESULT_FORMAT_READ} or
    *     {@link C#RESULT_BUFFER_READ}.
    */

library/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaPeriod.java

@@ -206,13 +206,13 @@ import java.util.Arrays;
 
     @Override
     public int readData(FormatHolder formatHolder, DecoderInputBuffer buffer) {
-      if (streamState == STREAM_STATE_END_OF_STREAM) {
+      if (buffer == null || streamState == STREAM_STATE_SEND_FORMAT) {
-        buffer.addFlag(C.BUFFER_FLAG_END_OF_STREAM);
-        return C.RESULT_BUFFER_READ;
-      } else if (streamState == STREAM_STATE_SEND_FORMAT) {
         formatHolder.format = format;
         streamState = STREAM_STATE_SEND_SAMPLE;
         return C.RESULT_FORMAT_READ;
+      } else if (streamState == STREAM_STATE_END_OF_STREAM) {
+        buffer.addFlag(C.BUFFER_FLAG_END_OF_STREAM);
+        return C.RESULT_BUFFER_READ;
       }
 
       Assertions.checkState(streamState == STREAM_STATE_SEND_SAMPLE);

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

@@ -26,7 +26,7 @@ import java.io.IOException;
 /* package */ final class HlsSampleStream implements SampleStream {
 
   public final int group;
-  
+
   private final HlsSampleStreamWrapper sampleStreamWrapper;
 
   public HlsSampleStream(HlsSampleStreamWrapper sampleStreamWrapper, int group) {