From fb386fccddfe381fd6af5656c13fac802bffd316 Mon Sep 17 00:00:00 2001
From: David Vacca <dvacca@fb.com>
Date: Thu, 8 Jul 2021 12:09:34 -0700
Subject: [PATCH] Fix NullPointerException caused by race condition in
 ReactInstanceManager.getViewManagerNames method

Summary:
This diff fixes a NullPointerException that is caused when the method ReactInstanceManager.getViewManagerNames is called at the same time ReactInstanceManager is being destroyed.

Following the stacktrace I noticed that this crash can only happen when RN was destroyed by another thread while this method was being executed

This diff fixes the NullPointerException, but it doesn't fix the root cause race condition that cuases this bug

changelog: [Android][Fixed] Fix NullPointerException caused by race condition in ReactInstanceManager.getViewManagerNames method

Reviewed By: JoshuaGross

Differential Revision: D29616401

fbshipit-source-id: 6ae8ecdd765d2fe3529fef3237f08b182d8ed243
---
 .../facebook/react/ReactInstanceManager.java  | 62 ++++++++++---------
 1 file changed, 32 insertions(+), 30 deletions(-)

diff --git a/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java b/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java
index 84fa847b7a4..e338e5f9c4b 100644
--- a/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java
+++ b/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java
@@ -918,38 +918,40 @@ public class ReactInstanceManager {
 
   public @Nullable List<String> getViewManagerNames() {
     Systrace.beginSection(TRACE_TAG_REACT_JAVA_BRIDGE, "ReactInstanceManager.getViewManagerNames");
-    if (mViewManagerNames == null) {
-      ReactApplicationContext context;
-      synchronized (mReactContextLock) {
-        context = (ReactApplicationContext) getCurrentReactContext();
-        if (context == null || !context.hasActiveReactInstance()) {
-          return null;
-        }
-      }
-
-      synchronized (mPackages) {
-        if (mViewManagerNames == null) {
-          Set<String> uniqueNames = new HashSet<>();
-          for (ReactPackage reactPackage : mPackages) {
-            SystraceMessage.beginSection(
-                    TRACE_TAG_REACT_JAVA_BRIDGE, "ReactInstanceManager.getViewManagerName")
-                .arg("Package", reactPackage.getClass().getSimpleName())
-                .flush();
-            if (reactPackage instanceof ViewManagerOnDemandReactPackage) {
-              List<String> names =
-                  ((ViewManagerOnDemandReactPackage) reactPackage).getViewManagerNames(context);
-              if (names != null) {
-                uniqueNames.addAll(names);
-              }
-            }
-            SystraceMessage.endSection(TRACE_TAG_REACT_JAVA_BRIDGE).flush();
-          }
-          Systrace.endSection(TRACE_TAG_REACT_JAVA_BRIDGE);
-          mViewManagerNames = new ArrayList<>(uniqueNames);
-        }
+    List<String> viewManagerNames = mViewManagerNames;
+    if (viewManagerNames != null) {
+      return viewManagerNames;
+    }
+    ReactApplicationContext context;
+    synchronized (mReactContextLock) {
+      context = (ReactApplicationContext) getCurrentReactContext();
+      if (context == null || !context.hasActiveReactInstance()) {
+        return null;
       }
     }
-    return new ArrayList<>(mViewManagerNames);
+
+    synchronized (mPackages) {
+      if (mViewManagerNames == null) {
+        Set<String> uniqueNames = new HashSet<>();
+        for (ReactPackage reactPackage : mPackages) {
+          SystraceMessage.beginSection(
+                  TRACE_TAG_REACT_JAVA_BRIDGE, "ReactInstanceManager.getViewManagerName")
+              .arg("Package", reactPackage.getClass().getSimpleName())
+              .flush();
+          if (reactPackage instanceof ViewManagerOnDemandReactPackage) {
+            List<String> names =
+                ((ViewManagerOnDemandReactPackage) reactPackage).getViewManagerNames(context);
+            if (names != null) {
+              uniqueNames.addAll(names);
+            }
+          }
+          SystraceMessage.endSection(TRACE_TAG_REACT_JAVA_BRIDGE).flush();
+        }
+        Systrace.endSection(TRACE_TAG_REACT_JAVA_BRIDGE);
+        mViewManagerNames = new ArrayList<>(uniqueNames);
+      }
+      return mViewManagerNames;
+    }
   }
 
   /** Add a listener to be notified of react instance events. */