Avoid some more potential data race conditions.

Use `dispatch_once` instead of a `static BOOL` to guard the work.

While at it, remove one use of `@synchronized` to instead standardize on just
`dispatch_once`.

Author: thomasvl

Sources/Core/GTLRBase64.m

@@ -119,24 +119,20 @@ static void CreateDecodingTable(const char *encodingTable,
 
 NSData *GTLRDecodeBase64(NSString *base64Str) {
   static char decodingTable[128];
-  static BOOL hasInited = NO;
-
-  if (!hasInited) {
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
     CreateDecodingTable(gStandardEncodingTable, sizeof(gStandardEncodingTable),
                         decodingTable);
-    hasInited = YES;
-  }
+  });
   return DecodeBase64StringCommon(base64Str, decodingTable, YES /* requirePadding */ );
 }
 
 NSData *GTLRDecodeWebSafeBase64(NSString *base64Str) {
   static char decodingTable[128];
-  static BOOL hasInited = NO;
-
-  if (!hasInited) {
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
     CreateDecodingTable(gWebSafeEncodingTable, sizeof(gWebSafeEncodingTable),
                         decodingTable);
-    hasInited = YES;
-  }
+  });
   return DecodeBase64StringCommon(base64Str, decodingTable, NO /* requirePadding */);
 }

Sources/Core/GTLRRuntimeCommon.m

@@ -484,19 +484,18 @@ typedef NS_ENUM(NSUInteger, GTLRPropertyType) {
     },
   };
 
-  static BOOL hasLookedUpClasses = NO;
-  if (!hasLookedUpClasses) {
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
     // Unfortunately, you can't put [NSString class] into the static structure,
     // so this lookup has to be done at runtime.
-    hasLookedUpClasses = YES;
     for (uint32_t idx = 0; idx < sizeof(kImplInfo)/sizeof(kImplInfo[0]); ++idx) {
       if (kImplInfo[idx].returnClassName) {
         kImplInfo[idx].returnClass = objc_getClass(kImplInfo[idx].returnClassName);
         NSCAssert1(kImplInfo[idx].returnClass != nil,
                    @"GTLRRuntimeCommon: class lookup failed: %s", kImplInfo[idx].returnClassName);
       }
     }
-  }
+  });
 
   const char *attr = property_getAttributes(prop);
 

Sources/Core/GTLRUtilities.m

@@ -85,14 +85,13 @@ BOOL GTLR_AreBoolsEqual(BOOL b1, BOOL b2) {
     if ([str rangeOfString:@"."].location != NSNotFound) {
       // This is a floating-point number.
       // Force the parser to use '.' as the decimal separator.
-      static NSLocale *usLocale = nil;
-      @synchronized([GTLRUtilities class]) {
-        if (usLocale == nil) {
-          usLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
-        }
-        newNum = [NSDecimalNumber decimalNumberWithString:(NSString*)num
-                                                   locale:(id)usLocale];
-      }
+      static NSLocale *usLocale;
+      static dispatch_once_t onceToken;
+      dispatch_once(&onceToken, ^{
+        usLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
+      });
+      newNum = [NSDecimalNumber decimalNumberWithString:(NSString*)num
+                                                 locale:(id)usLocale];
     } else {
       // NSDecimalNumber +decimalNumberWithString:locale:
       // does not correctly create an NSNumber for large values like