CSHARP-574: special case single matching instance and put lock around call to Random.Next.

Author: rstam

Driver/Internal/ReplicaSetMongoServerProxy.cs

@@ -234,10 +234,17 @@ private MongoServerInstance GetMatchingInstance(List<ConnectedInstanceCollection
                 }
 
                 // stop looking at tagSets if this one yielded any matching instances
-                if (matchingInstances.Count != 0)
+                if (matchingInstances.Count == 1)
                 {
-                    var index = _random.Next(matchingInstances.Count);
-                    return matchingInstances[index]; // randomly selected matching instance
+                    return matchingInstances[0];
+                }
+                else if (matchingInstances.Count != 0)
+                {
+                    lock (_randomLock)
+                    {
+                        var index = _random.Next(matchingInstances.Count);
+                        return matchingInstances[index]; // randomly selected matching instance
+                    }
                 }
             }
 

Driver/Internal/ShardedMongoServerProxy.cs

@@ -64,14 +64,21 @@ protected override MongoServerInstance ChooseServerInstance(ConnectedInstanceCol
             {
                 return null;
             }
+            else if (instancesWithPingTime.Count == 1)
+            {
+                return instancesWithPingTime[0].Instance;
+            }
             else
             {
                 var secondaryAcceptableLatency = Server.Settings.SecondaryAcceptableLatency;
                 var minPingTime = instancesWithPingTime[0].CachedAveragePingTime;
                 var maxPingTime = minPingTime + secondaryAcceptableLatency;
                 var n = instancesWithPingTime.Count(i => i.CachedAveragePingTime <= maxPingTime);
-                var index = _random.Next(n);
-                return instancesWithPingTime[index].Instance; // return random instance
+                lock (_randomLock)
+                {
+                    var index = _random.Next(n);
+                    return instancesWithPingTime[index].Instance; // return random instance
+                }
             }
         }