Fix handling of ad tags with only preroll and postroll ads

Content progress is only polled if there are midroll ad groups. If the ad tag
had only preroll/postroll ads, the pending content position was not provided to
IMA, which meant that the postroll ad could never play.

Only set the pending content position if there are midroll ads.

Issue: #3715

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185680803

RELEASENOTES.md

@@ -97,6 +97,8 @@
       ([#3676](https://github.com/google/ExoPlayer/issues/3676)).
     * Fix handling of ad tags where ad groups are out of order
       ([#3716](https://github.com/google/ExoPlayer/issues/3716)).
+    * Fix handling of ad tags with only preroll/postroll ad groups
+      ([#3715](https://github.com/google/ExoPlayer/issues/3715)).
     * Propagate ad media preparation errors to IMA so that the ads can be
       skipped.
 * `EventLogger` moved from the demo app into the core library.

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

@@ -851,15 +851,14 @@ private void startAdPlayback() {
     adsRenderingSettings.setMimeTypes(supportedMimeTypes);
 
     // Set up the ad playback state, skipping ads based on the start position as required.
-    pendingContentPositionMs = player.getCurrentPosition();
     long[] adGroupTimesUs = getAdGroupTimesUs(adsManager.getAdCuePoints());
     adPlaybackState = new AdPlaybackState(adGroupTimesUs);
+    long contentPositionMs = player.getCurrentPosition();
     int adGroupIndexForPosition =
-        adPlaybackState.getAdGroupIndexForPositionUs(C.msToUs(pendingContentPositionMs));
+        adPlaybackState.getAdGroupIndexForPositionUs(C.msToUs(contentPositionMs));
     if (adGroupIndexForPosition == 0) {
       podIndexOffset = 0;
     } else if (adGroupIndexForPosition == C.INDEX_UNSET) {
-      pendingContentPositionMs = C.TIME_UNSET;
       // There is no preroll and midroll pod indices start at 1.
       podIndexOffset = -1;
     } else /* adGroupIndexForPosition > 0 */ {
@@ -879,6 +878,11 @@ private void startAdPlayback() {
       podIndexOffset = adGroupIndexForPosition - 1;
     }
 
+    if (hasMidrollAdGroups(adGroupTimesUs)) {
+      // IMA will poll the content position, so provide the player's initial position like a seek.
+      pendingContentPositionMs = contentPositionMs;
+    }
+
     // Start ad playback.
     adsManager.init(adsRenderingSettings);
     updateAdPlaybackState();
@@ -1051,4 +1055,16 @@ private static boolean isAdGroupLoadError(AdError adError) {
     // a single ad, ad group or the whole timeline.
     return adError.getErrorCode() == AdErrorCode.VAST_LINEAR_ASSET_MISMATCH;
   }
+
+  private static boolean hasMidrollAdGroups(long[] adGroupTimesUs) {
+    int count = adGroupTimesUs.length;
+    if (count == 1) {
+      return adGroupTimesUs[0] != 0 && adGroupTimesUs[0] != C.TIME_END_OF_SOURCE;
+    } else if (count == 2) {
+      return adGroupTimesUs[0] != 0 || adGroupTimesUs[1] != C.TIME_END_OF_SOURCE;
+    } else {
+      // There's at least one midroll ad group, as adGroupTimesUs is never empty.
+      return true;
+    }
+  }
 }