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

@@ -109,9 +109,11 @@ public abstract class MongoDbUtils {
 			String username = credentials.getUsername();
 			String password = credentials.hasPassword() ? credentials.getPassword() : null;
 
-			if (!db.authenticate(username, password == null ? null : password.toCharArray())) {
-				throw new CannotGetMongoDbConnectionException("Failed to authenticate to database [" + databaseName
-						+ "], username = [" + username + "], password = [" + password + "]", databaseName, credentials);
+			synchronized (db) {
+				if (!db.authenticate(username, password == null ? null : password.toCharArray())) {
+					throw new CannotGetMongoDbConnectionException("Failed to authenticate to database [" + databaseName
+							+ "], username = [" + username + "], password = [" + password + "]", databaseName, credentials);
+				}
 			}
 		}
 
@@ -159,7 +161,7 @@ public abstract class MongoDbUtils {
 			return false;
 		}
 		DbHolder dbHolder = (DbHolder) TransactionSynchronizationManager.getResource(mongo);
-		return (dbHolder != null && dbHolder.containsDB(db));
+		return dbHolder != null && dbHolder.containsDB(db);
 	}
 
 	/**