Shard SimpleCache files into 10 sub-directories

Issue: #4253
PiperOrigin-RevId: 232659869

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

@@ -26,6 +26,7 @@ import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.NavigableSet;
+import java.util.Random;
 import java.util.Set;
 import java.util.TreeSet;
 
@@ -36,6 +37,14 @@ import java.util.TreeSet;
 public final class SimpleCache implements Cache {
 
   private static final String TAG = "SimpleCache";
+  /**
+   * Cache files are distributed between a number of subdirectories. This helps to avoid poor
+   * performance in cases where the performance of the underlying file system (e.g. FAT32) scales
+   * badly with the number of files per directory. See
+   * https://github.com/google/ExoPlayer/issues/4253.
+   */
+  private static final int SUBDIRECTORY_COUNT = 10;
+
   private static final HashSet<File> lockedCacheDirs = new HashSet<>();
 
   private static boolean cacheFolderLockingDisabled;
@@ -44,6 +53,7 @@ public final class SimpleCache implements Cache {
   private final CacheEvictor evictor;
   private final CachedContentIndex index;
   private final HashMap<String, ArrayList<Listener>> listeners;
+  private final Random random;
 
   private long totalSpace;
   private boolean released;
@@ -128,7 +138,8 @@ public final class SimpleCache implements Cache {
     this.cacheDir = cacheDir;
     this.evictor = evictor;
     this.index = index;
-    this.listeners = new HashMap<>();
+    listeners = new HashMap<>();
+    random = new Random();
 
     // Start cache initialization.
     final ConditionVariable conditionVariable = new ConditionVariable();
@@ -271,8 +282,13 @@ public final class SimpleCache implements Cache {
       removeStaleSpans();
     }
     evictor.onStartFile(this, key, position, length);
-    return SimpleCacheSpan.getCacheFile(
-        cacheDir, cachedContent.id, position, System.currentTimeMillis());
+    // Randomly distribute files into subdirectories with a uniform distribution.
+    File fileDir = new File(cacheDir, Integer.toString(random.nextInt(SUBDIRECTORY_COUNT)));
+    if (!fileDir.exists()) {
+      fileDir.mkdir();
+    }
+    long lastAccessTimestamp = System.currentTimeMillis();
+    return SimpleCacheSpan.getCacheFile(fileDir, cachedContent.id, position, lastAccessTimestamp);
   }
 
   @Override

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

@@ -26,7 +26,9 @@ import java.util.regex.Pattern;
 /** This class stores span metadata in filename. */
 /* package */ final class SimpleCacheSpan extends CacheSpan {
 
-  private static final String SUFFIX = ".v3.exo";
+  /* package */ static final String COMMON_SUFFIX = ".exo";
+
+  private static final String SUFFIX = ".v3" + COMMON_SUFFIX;
   private static final Pattern CACHE_FILE_PATTERN_V1 = Pattern.compile(
       "^(.+)\\.(\\d+)\\.(\\d+)\\.v1\\.exo$", Pattern.DOTALL);
   private static final Pattern CACHE_FILE_PATTERN_V2 = Pattern.compile(

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

@@ -75,7 +75,7 @@ public class SimpleCacheTest {
     NavigableSet<CacheSpan> cachedSpans = simpleCache.getCachedSpans(KEY_1);
     assertThat(cachedSpans.isEmpty()).isTrue();
     assertThat(simpleCache.getCacheSpace()).isEqualTo(0);
-    assertThat(cacheDir.listFiles()).hasLength(0);
+    assertNoCacheFiles(cacheDir);
 
     addCache(simpleCache, KEY_1, 0, 15);
 
@@ -233,7 +233,7 @@ public class SimpleCacheTest {
 
     // Cache should be cleared
     assertThat(simpleCache.getKeys()).isEmpty();
-    assertThat(cacheDir.listFiles()).hasLength(0);
+    assertNoCacheFiles(cacheDir);
   }
 
   @Test
@@ -252,7 +252,7 @@ public class SimpleCacheTest {
 
     // Cache should be cleared
     assertThat(simpleCache.getKeys()).isEmpty();
-    assertThat(cacheDir.listFiles()).hasLength(0);
+    assertNoCacheFiles(cacheDir);
   }
 
   @Test
@@ -391,6 +391,20 @@ public class SimpleCacheTest {
     }
   }
 
+  private static void assertNoCacheFiles(File dir) {
+    File[] files = dir.listFiles();
+    if (files == null) {
+      return;
+    }
+    for (File file : files) {
+      if (file.isDirectory()) {
+        assertNoCacheFiles(file);
+      } else {
+        assertThat(file.getName().endsWith(SimpleCacheSpan.COMMON_SUFFIX)).isFalse();
+      }
+    }
+  }
+
   private static byte[] generateData(String key, int position, int length) {
     byte[] bytes = new byte[length];
     new Random((long) (key.hashCode() ^ position)).nextBytes(bytes);