(fix) wrap lazy loaded optlyfilemanager in a serial queue (#342)

* don't lazy load file manager since it is used in every method call and seems to be a pretty lightweight object.

* don't need getter method for data store

* had to use a queue for lazy loading since there was some problem creating the file manager in the init call.

* fix using queue

* change queue name to something more unique

* test outside of queue before trying to create file manager

* fix weak self and use strong self inside of dispatch_sync block

Author: thomaszurkan-optimizely

OptimizelySDKShared/OptimizelySDKShared/OPTLYDataStore.m

@@ -43,6 +43,7 @@
 @interface OPTLYDataStore()
 @property (nonatomic, strong) OPTLYFileManager *fileManager;
 @property (nonatomic, strong) id<OPTLYEventDataStore> eventDataStore;
+@property (nonatomic, strong) dispatch_queue_t fileManagerCreateQueue;
 @end
 
 @implementation OPTLYDataStore
@@ -71,6 +72,8 @@ - (id)init {
         filePath = NSTemporaryDirectory();
         _baseDirectory = [filePath stringByAppendingPathComponent:kOptimizelyDirectory];
 #endif
+        
+        _fileManagerCreateQueue = dispatch_queue_create("OptlyFileManagerCreateQueue", DISPATCH_QUEUE_SERIAL);
     }
     return self;
 }
@@ -93,13 +96,6 @@ - (id)init {
     return _eventDataStore;
 }
 
-- (OPTLYFileManager *)fileManager {
-    if (!_fileManager) {
-        _fileManager = [[OPTLYFileManager alloc] initWithBaseDir:self.baseDirectory];
-    }
-    return _fileManager;
-}
-
 - (BOOL)removeAll:(NSError * _Nullable __autoreleasing * _Nullable)error {
     BOOL ok = YES;
     [self removeAllUserData];
@@ -152,6 +148,26 @@ - (void)removeAllUserData
 }
 
 # pragma mark - File Manager Methods
+- (OPTLYFileManager *)fileManager {
+    if (_fileManager == nil) {
+        [self loadFileManagerIfNecessary];
+    }
+    return _fileManager;
+
+}
+
+- (void) loadFileManagerIfNecessary {
+    __block __weak OPTLYDataStore *weakSelf = self;
+    dispatch_sync(_fileManagerCreateQueue, ^{
+        if (weakSelf != nil) {
+            OPTLYDataStore *strongSelf = weakSelf;
+            if (strongSelf->_fileManager == nil) {
+                strongSelf->_fileManager = [[OPTLYFileManager alloc] initWithBaseDir:strongSelf.baseDirectory];
+            }
+        }
+    });
+}
+
 - (BOOL)saveFile:(nonnull NSString *)fileName
             data:(nonnull NSData *)data
             type:(OPTLYDataStoreDataType)dataType