De-duplicate DRM acquire & release events by EventDispatcher

When we add DRM pre-acquire support to SampleQueue, we'll dispatch
twice the number of acquire and release events. This is slightly
confusing, since there's the same number of
DrmSessionManager#acquireSession() calls.

We can mitigate this by only dispatching each acquire and release
event to at most one EventDispatcher.

This also changes the events fired when playing a stream with both audio
and video encrypted with the same keys (even without pre-acquisition).

Before: The EventDispatcher would see 2 aquires, 1 key load and 2
release events.

After: The EventDispatcher will see 1 acquire, 1 key load and 1 release.
PiperOrigin-RevId: 358804502

RELEASENOTES.md

@@ -32,6 +32,11 @@
     *   Fix a bug with playback of ads in playlists, where the incorrect period
         index was used when deciding whether to trigger playback of an ad after
         a seek.
+*   DRM:
+    *   Only dispatch DRM session acquire and release events once per period
+        when playing content that uses the same encryption keys for both audio &
+        video tracks (previously separate acquire and release events were
+        dispatched for each track in each period).
 *   VP9 extension: Update to use NDK r22
     ([#8581](https://github.com/google/ExoPlayer/issues/8581)).
 *   FLAC extension: Update to use NDK r22

library/common/src/main/java/com/google/android/exoplayer2/util/CopyOnWriteMultiset.java

@@ -138,4 +138,11 @@ public final class CopyOnWriteMultiset<E extends Object> implements Iterable<E>
       return elements.iterator();
     }
   }
+
+  /** Returns the number of occurrences of an element in this multiset. */
+  public int count(E element) {
+    synchronized (lock) {
+      return elementCounts.containsKey(element) ? elementCounts.get(element) : 0;
+    }
+  }
 }

library/common/src/test/java/com/google/android/exoplayer2/util/CopyOnWriteMultisetTest.java

@@ -107,4 +107,14 @@ public final class CopyOnWriteMultisetTest {
 
     assertThrows(UnsupportedOperationException.class, () -> elementSet.remove("a string"));
   }
+
+  @Test
+  public void count() {
+    CopyOnWriteMultiset<String> multiset = new CopyOnWriteMultiset<>();
+    multiset.add("a string");
+    multiset.add("a string");
+
+    assertThat(multiset.count("a string")).isEqualTo(2);
+    assertThat(multiset.count("another string")).isEqualTo(0);
+  }
 }

library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java

@@ -293,8 +293,11 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
       if (openInternal(true)) {
         doLicense(true);
       }
-    } else if (eventDispatcher != null && isOpen()) {
+    } else if (eventDispatcher != null
-      // If the session is already open then send the acquire event only to the provided dispatcher.
+        && isOpen()
+        && eventDispatchers.count(eventDispatcher) == 1) {
+      // If the session is already open and this is the first instance of eventDispatcher we've
+      // seen, then send the acquire event only to the provided dispatcher.
       // TODO: Add a parameter to onDrmSessionAcquired to indicate whether the session is being
       // re-used or not.
       eventDispatcher.drmSessionAcquired();
@@ -323,9 +326,11 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
       }
     }
     if (eventDispatcher != null) {
-      // Acquire and release events are only sent to the provided dispatcher.
-      eventDispatcher.drmSessionReleased();
       eventDispatchers.remove(eventDispatcher);
+      if (eventDispatchers.count(eventDispatcher) == 0) {
+        // Release events are only sent to the last-attached instance of each EventDispatcher.
+        eventDispatcher.drmSessionReleased();
+      }
     }
     referenceCountListener.onReferenceCountDecremented(this, referenceCount);
   }