Add synchronized block to protect against race condition (#69)

Summary:
Fixes: #60

As mentioned in issue #60 if the init method is called twice from two different threads it can result in a race condition where both threads receive false for `NativeLoader.isInitialized()`, but, when `NativeLoader.init()` is run for a second time it throws an exception.

Adding a synchronized block here ensures that only one thread will be able to query the initialisation state and trigger the initialisation at a time.

Pull Request resolved: #69

Reviewed By: oprisnik

Differential Revision: D25973362

Pulled By: wizh

fbshipit-source-id: d340bae2b8891c6f095a2560b37ded50cf20ab67

java/com/facebook/soloader/SoLoader.java

@@ -205,9 +205,7 @@ public static void init(Context context, int flags, @Nullable SoFileLoader soFil
       isSystemApp = checkIfSystemApp(context, flags);
       initSoLoader(soFileLoader);
       initSoSources(context, flags, soFileLoader);
-      if (!NativeLoader.isInitialized()) {
-        NativeLoader.init(new NativeLoaderToSoLoaderDelegate());
-      }
+      NativeLoader.initIfUninitialized(new NativeLoaderToSoLoaderDelegate());
     } finally {
       StrictMode.setThreadPolicy(oldPolicy);
     }

java/com/facebook/soloader/nativeloader/NativeLoader.java

@@ -112,4 +112,15 @@ public static synchronized void init(NativeLoaderDelegate delegate) {
   public static synchronized boolean isInitialized() {
     return sDelegate != null;
   }
+
+  /**
+   * Perform an initialization only if {@code NativeLoader} has not already been initialized. This
+   * protects against race conditions where isInitialized and init are called by multiple threads
+   * and both threads end up trying to perform an initialization
+   */
+  public static synchronized void initIfUninitialized(NativeLoaderDelegate delegate) {
+    if (!isInitialized()) {
+      init(delegate);
+    }
+  }
 }