[TimeZone] Fixed race condition with renewing the CurrentTimeZone object.

Fixes bug #1055.

Author: jstedfast

mcs/class/corlib/System/TimeZone.cs

@@ -56,6 +56,8 @@ public abstract class TimeZone
 		// Fields
 		static TimeZone currentTimeZone;
 
+		[NonSerialized]
+		static object tz_lock = new object ();
 		[NonSerialized]
 		static long timezone_check;
 
@@ -68,13 +70,18 @@ protected TimeZone ()
 		public static TimeZone CurrentTimeZone {
 			get {
 				long now = DateTime.GetNow ();
+				TimeZone tz;
 
-				if (currentTimeZone == null || (now - timezone_check) > TimeSpan.TicksPerMinute) {
-					currentTimeZone = new CurrentSystemTimeZone (now);
-					timezone_check = now;
+				lock (tz_lock) {
+					if (currentTimeZone == null || (now - timezone_check) > TimeSpan.TicksPerMinute) {
+						currentTimeZone = new CurrentSystemTimeZone (now);
+						timezone_check = now;
+					}
+					
+					tz = currentTimeZone;
 				}
 
-				return currentTimeZone;
+				return tz;
 			}
 		}