Fix Null Pointer Concurrency Issue

• Fixes #513, where in very rare cases a concurrency issue could occur where shared preferences was accessed during a buffer flush before the object was actually initialized
• Fixed by adding synchronization to all accesses/initialization of this object

Author: Nightsd01

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalPrefs.java

@@ -75,6 +75,8 @@ class OneSignalPrefs {
     static HashMap<String, HashMap<String, Object>> prefsToApply;
     public static WritePrefHandlerThread prefsHandler;
 
+    static final Object synchronizer = new Object();
+
     static {
         initializePool();
     }
@@ -113,11 +115,11 @@ public void run() {
         }
 
         private void flushBufferToDisk() {
-            for (String pref : prefsToApply.keySet()) {
-                SharedPreferences prefsToWrite = getSharedPrefsByName(pref);
-                SharedPreferences.Editor editor = prefsToWrite.edit();
-                HashMap<String, Object> prefHash = prefsToApply.get(pref);
-                synchronized (prefHash) {
+            synchronized(synchronizer) {
+                for (String pref : prefsToApply.keySet()) {
+                    SharedPreferences prefsToWrite = getSharedPrefsByName(pref);
+                    SharedPreferences.Editor editor = prefsToWrite.edit();
+                    HashMap<String, Object> prefHash = prefsToApply.get(pref);
                     for (String key : prefHash.keySet()) {
                         Object value = prefHash.get(key);
                         if (value instanceof String)
@@ -130,20 +132,23 @@ else if (value instanceof Long)
                             editor.putLong(key, (Long)value);
                     }
                     prefHash.clear();
+
+                    editor.apply();
                 }
-                editor.apply();
-            }
 
-            lastSyncTime = System.currentTimeMillis();
+                lastSyncTime = System.currentTimeMillis();
+            }
         }
     }
 
     public static void initializePool() {
-        prefsToApply = new HashMap<>();
-        prefsToApply.put(PREFS_ONESIGNAL, new HashMap<String, Object>());
-        prefsToApply.put(PREFS_PLAYER_PURCHASES, new HashMap<String, Object>());
+        synchronized(synchronizer) {
+            prefsToApply = new HashMap<>();
+            prefsToApply.put(PREFS_ONESIGNAL, new HashMap<String, Object>());
+            prefsToApply.put(PREFS_PLAYER_PURCHASES, new HashMap<String, Object>());
 
-        prefsHandler = new WritePrefHandlerThread();
+            prefsHandler = new WritePrefHandlerThread();
+        }
     }
 
     static void saveString(final String prefsName, final String key, final String value) {
@@ -163,11 +168,12 @@ static void saveLong(String prefsName, String key, long value) {
     }
 
     static private void save(String prefsName, String key, Object value) {
-        HashMap<String, Object> pref = prefsToApply.get(prefsName);
-        synchronized (pref) {
+        synchronized(synchronizer) {
+            HashMap<String, Object> pref = prefsToApply.get(prefsName);
             pref.put(key, value);
+
+            prefsHandler.startDelayedWrite();
         }
-        prefsHandler.startDelayedWrite();
     }
 
     static String getString(String prefsName, String key, String defValue) {
@@ -188,34 +194,35 @@ static long getLong(String prefsName, String key, long defValue) {
 
     // If type == Object then this is a contains check
     private static Object get(String prefsName, String key, Class type, Object defValue) {
-        HashMap<String, Object> pref = prefsToApply.get(prefsName);
+        synchronized(synchronizer) {
+            HashMap<String, Object> pref = prefsToApply.get(prefsName);
 
-        synchronized (pref) {
             if (type.equals(Object.class) && pref.containsKey(key))
                 return true;
 
             Object cachedValue = pref.get(key);
             if (cachedValue != null || pref.containsKey(key))
                 return cachedValue;
-        }
 
-        SharedPreferences prefs = getSharedPrefsByName(prefsName);
-        if (prefs != null ) {
-            if (type.equals(String.class))
-               return prefs.getString(key, (String)defValue);
-            else if (type.equals(Boolean.class))
-                return prefs.getBoolean(key, (Boolean)defValue);
-            else if (type.equals(Integer.class))
-                return prefs.getInt(key, (Integer)defValue);
-            else if (type.equals(Long.class))
-                return prefs.getLong(key, (Long)defValue);
-            else if (type.equals(Object.class))
-                return prefs.contains(key);
 
-            return null;
-        }
+            SharedPreferences prefs = getSharedPrefsByName(prefsName);
+            if (prefs != null ) {
+                if (type.equals(String.class))
+                    return prefs.getString(key, (String)defValue);
+                else if (type.equals(Boolean.class))
+                    return prefs.getBoolean(key, (Boolean)defValue);
+                else if (type.equals(Integer.class))
+                    return prefs.getInt(key, (Integer)defValue);
+                else if (type.equals(Long.class))
+                    return prefs.getLong(key, (Long)defValue);
+                else if (type.equals(Object.class))
+                    return prefs.contains(key);
+
+                return null;
+            }
 
-        return defValue;
+            return defValue;
+        }
     }
 
     private static synchronized SharedPreferences getSharedPrefsByName(String prefsName) {