From 17b6163a261dfbe4b47c9a61de295208344b497d Mon Sep 17 00:00:00 2001
From: Al Sutton <al.sutton@alsutton.com>
Date: Fri, 22 Jan 2021 08:25:24 -0800
Subject: [PATCH] Add synchronized block to protect against race condition
 (#69)

Summary:
Fixes: https://github.com/facebook/SoLoader/issues/60

As mentioned in issue https://github.com/facebook/SoLoader/issues/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: https://github.com/facebook/SoLoader/pull/69

Reviewed By: oprisnik

Differential Revision: D25973362

Pulled By: wizh

fbshipit-source-id: d340bae2b8891c6f095a2560b37ded50cf20ab67
---
 java/com/facebook/soloader/SoLoader.java              |  4 +---
 .../facebook/soloader/nativeloader/NativeLoader.java  | 11 +++++++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/java/com/facebook/soloader/SoLoader.java b/java/com/facebook/soloader/SoLoader.java
index 66f5c23..c1c7837 100644
--- a/java/com/facebook/soloader/SoLoader.java
+++ b/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);
     }
diff --git a/java/com/facebook/soloader/nativeloader/NativeLoader.java b/java/com/facebook/soloader/nativeloader/NativeLoader.java
index f1f460d..535d87d 100644
--- a/java/com/facebook/soloader/nativeloader/NativeLoader.java
+++ b/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);
+    }
+  }
 }