Fixed concurrency issue on multiple files upload

src/main/java/it/inaf/ia2/transfer/controller/PutFileController.java

@@ -83,11 +83,17 @@ public class PutFileController {
 
         File file = path.toFile();
 
+        /**
+         * This block must be synchronized, to avoid concurrency issues when
+         * multiple files are uploaded to a new folder in parallel.
+         */
+        synchronized (this) {
             if (!file.getParentFile().exists()) {
                 if (!file.getParentFile().mkdirs()) {
                     throw new IllegalStateException("Unable to create parent folder: " + file.getParentFile().getAbsolutePath());
                 }
             }
+        }
 
         Files.copy(is, file.toPath());
     }