DATAMONGO-532 - Synchronize DB authentication.

In multithreaded environments Mongo database authentication can be triggered twice if two or more threads refer to the same db instance. This is now prevented by synchronizing calls to db.authenticate(…).

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoDbUtils.java

@@ -97,10 +97,12 @@ public abstract class MongoDbUtils {
 		boolean credentialsGiven = username != null && password != null;
 		if (credentialsGiven && !db.isAuthenticated()) {
 			// Note, can only authenticate once against the same com.mongodb.DB object.
-			if (!db.authenticate(username, password)) {
-				throw new CannotGetMongoDbConnectionException("Failed to authenticate to database [" + databaseName
-						+ "], username = [" + username + "], password = [" + new String(password) + "]", databaseName, username,
-						password);
+			synchronized (db) {
+				if (!db.authenticate(username, password)) {
+					throw new CannotGetMongoDbConnectionException("Failed to authenticate to database [" + databaseName
+							+ "], username = [" + username + "], password = [" + new String(password) + "]", databaseName, username,
+							password);
+				}
 			}
 		}
 
@@ -144,7 +146,7 @@ public abstract class MongoDbUtils {
 			return false;
 		}
 		DbHolder dbHolder = (DbHolder) TransactionSynchronizationManager.getResource(mongo);
-		return (dbHolder != null && dbHolder.containsDB(db));
+		return dbHolder != null && dbHolder.containsDB(db);
 	}
 
 	/**