Fix race condition in SoLoader#init. (#99)

Summary:
If two threads call `SoLoader#init` at the same time, there is a chance
they both try to initialize `sSoSources` one after the other, which
eventually leads to an `OverlappingFileLockException`.

To fix this issue, add a double check after obtaining the write lock to
only initialize if `sSoSources` is still null.

Fixes #93.

Pull Request resolved: #99

Differential Revision: D35891724

Pulled By: passy

fbshipit-source-id: fb1bcfaa9df7102c1083ec0c0deb336cef75c0dd

java/com/facebook/soloader/SoLoader.java

@@ -92,7 +92,7 @@ public class SoLoader {
    */
   @GuardedBy("sSoSourcesLock")
   @Nullable
-  private static SoSource[] sSoSources = null;
+  private static volatile SoSource[] sSoSources = null;
 
   @GuardedBy("sSoSourcesLock")
   private static final AtomicInteger sSoSourcesVersion = new AtomicInteger(0);
@@ -278,6 +278,13 @@ private static void initSoSources(Context context, int flags, String[] denyList)
     }
 
     sSoSourcesLock.writeLock().lock();
+
+    // Double check that sSoSources wasn't initialized while waiting for the lock.
+    if (sSoSources != null) {
+      sSoSourcesLock.writeLock().unlock();
+      return;
+    }
+
     try {
       sFlags = flags;