Improve handling of consecutive empty ad groups

Issue: #4030
Issue: #4280

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=208055792

RELEASENOTES.md

@@ -1,5 +1,11 @@
 # Release notes #
 
+### 2.8.4 ###
+
+* IMA: Improve handling of consecutive empty ad groups
+  ([#4030](https://github.com/google/ExoPlayer/issues/4030)),
+  ([#4280](https://github.com/google/ExoPlayer/issues/4280)).
+
 ### 2.8.3 ###
 
 * IMA:

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaAdsLoader.java

@@ -619,8 +619,11 @@ public VideoProgressUpdate getContentProgress() {
     } else if (fakeContentProgressElapsedRealtimeMs != C.TIME_UNSET) {
       long elapsedSinceEndMs = SystemClock.elapsedRealtime() - fakeContentProgressElapsedRealtimeMs;
       contentPositionMs = fakeContentProgressOffsetMs + elapsedSinceEndMs;
-      expectedAdGroupIndex =
+      int adGroupIndexForPosition =
           adPlaybackState.getAdGroupIndexForPositionUs(C.msToUs(contentPositionMs));
+      if (adGroupIndexForPosition != C.INDEX_UNSET) {
+        expectedAdGroupIndex = adGroupIndexForPosition;
+      }
     } else if (imaAdState == IMA_AD_STATE_NONE && !playingAd && hasContentDuration) {
       contentPositionMs = player.getCurrentPosition();
       // Update the expected ad group index for the current content position. The update is delayed
@@ -1096,6 +1099,16 @@ private void handleAdGroupLoadError(Exception error) {
     if (pendingAdLoadError == null) {
       pendingAdLoadError = AdLoadException.createForAdGroup(error, adGroupIndex);
     }
+    // Discard the ad break, which makes sure we don't receive duplicate load error events.
+    adsManager.discardAdBreak();
+    // Set the next expected ad group index so we can handle multiple load errors in a row.
+    adGroupIndex++;
+    if (adGroupIndex < adPlaybackState.adGroupCount) {
+      expectedAdGroupIndex = adGroupIndex;
+    } else {
+      expectedAdGroupIndex = C.INDEX_UNSET;
+    }
+    pendingContentPositionMs = C.TIME_UNSET;
   }
 
   private void handleAdPrepareError(int adGroupIndex, int adIndexInAdGroup, Exception exception) {