Re enable single SimpleCache instance assertion and add helper methods

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=193637830

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -37,6 +37,8 @@ public final class SimpleCache implements Cache {
   private static final String TAG = "SimpleCache";
   private static final HashSet<File> lockedCacheDirs = new HashSet<>();
 
+  private static boolean cacheFolderLockingDisabled;
+
   private final File cacheDir;
   private final CacheEvictor evictor;
   private final CachedContentIndex index;
@@ -46,6 +48,31 @@ public final class SimpleCache implements Cache {
   private boolean released;
 
   /**
+   * Returns whether {@code cacheFolder} is locked by a {@link SimpleCache} instance. To unlock the
+   * folder the {@link SimpleCache} instance should be released.
+   */
+  public static synchronized boolean isCacheFolderLocked(File cacheFolder) {
+    return lockedCacheDirs.contains(cacheFolder.getAbsoluteFile());
+  }
+
+  /**
+   * Disables locking the cache folders which {@link SimpleCache} instances are using and releases
+   * any previous lock.
+   *
+   * <p>The locking prevents multiple {@link SimpleCache} instances from being created for the same
+   * folder. Disabling it may cause the cache data to be corrupted. Use at your own risk.
+   *
+   * @deprecated Don't create multiple {@link SimpleCache} instances for the same cache folder. If
+   *     you need to create another instance, make sure you call {@link #release()} on the previous
+   *     instance.
+   */
+  @Deprecated
+  public static synchronized void disableCacheFolderLocking() {
+    cacheFolderLockingDisabled = true;
+    lockedCacheDirs.clear();
+  }
+
+  /**
    * Constructs the cache. The cache will delete any unrecognized files from the directory. Hence
    * the directory cannot be used to store other files.
    *
@@ -93,6 +120,10 @@ public final class SimpleCache implements Cache {
    * @param index The CachedContentIndex to be used.
    */
   /*package*/ SimpleCache(File cacheDir, CacheEvictor evictor, CachedContentIndex index) {
+    if (!lockFolder(cacheDir)) {
+      throw new IllegalStateException("Another SimpleCache instance uses the folder: " + cacheDir);
+    }
+
     this.cacheDir = cacheDir;
     this.evictor = evictor;
     this.index = index;
@@ -122,7 +153,7 @@ public final class SimpleCache implements Cache {
     try {
       removeStaleSpansAndCachedContents();
     } finally {
-      releaseFolder(cacheDir);
+      unlockFolder(cacheDir);
       released = true;
     }
   }
@@ -461,10 +492,15 @@ public final class SimpleCache implements Cache {
   }
 
   private static synchronized boolean lockFolder(File cacheDir) {
+    if (cacheFolderLockingDisabled) {
+      return true;
+    }
     return lockedCacheDirs.add(cacheDir.getAbsoluteFile());
   }
 
-  private static synchronized void releaseFolder(File cacheDir) {
+  private static synchronized void unlockFolder(File cacheDir) {
+    if (!cacheFolderLockingDisabled) {
       lockedCacheDirs.remove(cacheDir.getAbsoluteFile());
     }
+  }
 }

library/core/src/test/java/com/google/android/exoplayer2/upstream/cache/SimpleCacheTest.java

@@ -296,17 +296,17 @@ public class SimpleCacheTest {
     }
   }
 
-  //   @Test
-  //   public void testMultipleSimpleCacheWithSameCacheDirThrowsException() throws Exception {
-  //     new SimpleCache(cacheDir, new NoOpCacheEvictor());
-  //
-  //     try {
-  //       new SimpleCache(cacheDir, new NoOpCacheEvictor());
-  //       assertWithMessage("Exception was expected").fail();
-  //     } catch (IllegalStateException e) {
-  //       // Expected. Do nothing.
-  //     }
-  //   }
+  @Test
+  public void testMultipleSimpleCacheWithSameCacheDirThrowsException() throws Exception {
+    new SimpleCache(cacheDir, new NoOpCacheEvictor());
+
+    try {
+      new SimpleCache(cacheDir, new NoOpCacheEvictor());
+      assertWithMessage("Exception was expected").fail();
+    } catch (IllegalStateException e) {
+      // Expected. Do nothing.
+    }
+  }
 
   @Test
   public void testMultipleSimpleCacheWithSameCacheDirDoesNotThrowsExceptionAfterRelease()