Fixes #1063: PoolCluster#end now accepts a callback function

lib/pool_cluster.js

@@ -118,14 +118,37 @@ class PoolCluster extends EventEmitter {
     namespace.getConnection(cb);
   }
 
-  end() {
+  end(callback) {
+    const cb =
+      callback !== undefined
+        ? callback
+        : err => {
+          if (err) {
+            throw err;
+          }
+        };
     if (this._closed) {
+      process.nextTick(cb);
       return;
     }
     this._closed = true;
+
+    let calledBack = false;
+    let waitingClose = 0;
+    const onEnd = err => {
+      if (!calledBack && (err || --waitingClose <= 0)) {
+        calledBack = true;
+        return cb(err);
+      }
+    };
+
     for (const id in this._nodes) {
+      waitingClose++;
       this._nodes[id].pool.end();
     }
+    if (waitingClose === 0) {
+      process.nextTick(onEnd);
+    }
   }
 
   _findNodeIds(pattern) {
@@ -142,8 +165,8 @@ class PoolCluster extends EventEmitter {
     } else {
       // wild matching
       const keyword = pattern.substring(pattern.length - 1, 0);
-      foundNodeIds = this._serviceableNodeIds.filter(
-        id => id.startsWith(keyword)
+      foundNodeIds = this._serviceableNodeIds.filter(id =>
+        id.startsWith(keyword)
       );
     }
     this._findCaches[pattern] = foundNodeIds;
@@ -183,12 +206,11 @@ class PoolCluster extends EventEmitter {
           // eslint-disable-next-line no-console
           console.warn(`[Error] PoolCluster : ${err}`);
           return cb(null, 'retry');
-        } 
+        }
         return cb(err);
-
-      } 
+      }
       this._decreaseErrorCount(node);
-      
+
       connection._clusterId = node.id;
       return cb(null, connection);
     });