ConnectionManager.close not closing database connection completely (#7756)

* drain doesnt take a callback, it returns a promise instead

* update changelog

* add test

changelog.md

 # Next
 # Next
 - [ADDED] `Model.increment`, to increment multiple rows at a time [#7394](https://github.com/sequelize/sequelize/pull/7394)
 - [ADDED] `Model.increment`, to increment multiple rows at a time [#7394](https://github.com/sequelize/sequelize/pull/7394)
+- [FIXED] calling `.close` on Sequelize db instance does not properly close the connection socket [#7751](https://github.com/sequelize/sequelize/issues/7751)
 
 
 # 4.0.0 (final)
 # 4.0.0 (final)
 - [ADDED] Add `isSoftDeleted` helper method to model instance [#7408](https://github.com/sequelize/sequelize/issues/7408)
 - [ADDED] Add `isSoftDeleted` helper method to model instance [#7408](https://github.com/sequelize/sequelize/issues/7408)

lib/dialects/abstract/connection-manager.js

@@ -61,7 +61,7 @@ class ConnectionManager {
       return Promise.resolve();
       return Promise.resolve();
     }
     }
 
 
-    return this.pool.drain(() => {
+    return this.pool.drain().then(() => {
       debug('connection drain due to process exit');
       debug('connection drain due to process exit');
       return this.pool.clear();
       return this.pool.clear();
     });
     });

test/integration/dialects/abstract/connection-manager.test.js

@@ -146,4 +146,23 @@ describe('Connection Manager', () => {
       });
       });
   });
   });
 
 
+  it('should clear the pool after draining it', () => {
+    const options = {
+      replication: null
+    };
+    const sequelize = Support.createSequelizeInstance(options);
+    const connectionManager = new ConnectionManager(Support.getTestDialect(), sequelize);
+
+    connectionManager.initPools();
+
+    const poolDrainSpy = sandbox.spy(connectionManager.pool, 'drain');
+    const poolClearSpy = sandbox.spy(connectionManager.pool, 'clear');
+
+    return connectionManager.close().then(() => {
+      expect(poolDrainSpy.calledOnce).to.be.true;
+      expect(poolClearSpy.calledOnce).to.be.true;
+    });
+
+  });
+
 });
 });