fix the server group disapearing issue #1835 (#2269)

* fix the server group disapearing issue #1835

* address review comments

src/sql/parts/connection/common/connectionProfileGroup.ts

@@ -153,6 +153,19 @@ export class ConnectionProfileGroup implements IConnectionProfileGroup {
 		return this.parent;
 	}
 
+	public isAncestorOf(node: ConnectionProfileGroup | ConnectionProfile): boolean {
+		let isAncestor = false;
+		let currentNode = node;
+		while (currentNode) {
+			if (currentNode.parent && currentNode.parent.id === this.id) {
+				isAncestor = true;
+				break;
+			}
+			currentNode = currentNode.parent;
+		}
+		return isAncestor;
+	}
+
 	public static getGroupFullNameParts(groupFullName: string): string[] {
 		groupFullName = groupFullName ? groupFullName : '';
 		let groupNames: string[] = groupFullName.split(ConnectionProfileGroup.GroupNameSeparator);

src/sql/parts/objectExplorer/viewlet/dragAndDropController.ts

@@ -74,6 +74,10 @@ export class ServerTreeDragAndDrop implements IDragAndDrop {
 				if (!this._connectionManagementService.canChangeConnectionConfig(source, targetConnectionProfileGroup.id)) {
 					canDragOver = false;
 				}
+			} else if (source instanceof ConnectionProfileGroup) {
+				// Dropping a group to itself or its descendants nodes is not allowed
+				// to avoid creating a circular structure.
+				canDragOver = source.id !== targetElement.id && !source.isAncestorOf(targetElement);
 			}
 
 		} else {