🐛 Bug Report

When an instance onSend hook executes asynchronously, responding from a route hook does not always prevent the route handler from being called.

To Reproduce

Setup

1. npm init -y && npm i -S fastify@2.6.0

2. Add the code from this gist to index.js

   Bare-bones snippet

   const fastify = require('fastify')();
   
   // This issue occurs with both callback and promise based `onSend` hooks
   fastify.addHook('onSend', (request, reply, payload, done) => {
       setTimeout(() => {
           done(null, payload);
       }, 50);
   });
   fastify.route({
       method: 'GET',
       path: '/',
       preHandler: async function(request, reply) {
           console.log('GET / -> preHandler()');
           reply.redirect('/login');
       },
       handler: function(request, reply) {
           console.log('GET / -> handler() !! This should never be shown !!');
           reply.send({ hello: 'world' });
       }
   });
   fastify.route({
       method: 'GET',
       path: '/login',
       handler: function(request, reply) {
           console.log('GET /login -> handler()');
           reply.send('Please log in\n');
       }
   });
   fastify.listen(3000);

3. node ./index.js

Steps to reproduce

1. curl -L localhost:3000 (Or via a browser, postman, etc.)
2. Notice that the / route handler was called, even though we responded in the preHandler hook.

   server listening on port 3000
   GET / -> preHandler()
   GET / -> handler() !! This should never be shown !!
   GET /login -> handler()
   

Additional notes (from gist code comments)

• This issue occurs when either the callback or the promise based onSend hook is used.
• This issue occurs when the promise based preHandler hook is used, but not when the callback based version is used.

Expected behavior

1. curl -L localhost:3000
2. The handler for the / route should not be called.

   server listening on port 3000
   GET / -> preHandler()
   GET /login -> handler()
   

Your Environment

• node version: 10.16.0
• fastify version: 2.6.0
• os: Mac