🐛 Bug Report

Setting ignoreTrailingSlash: true on main Fastify instance and registering a plugin with tralingSlash prefixed path (prefix: '/attendances/'), prevents avvio from crashing on startup with AssertionError [ERR_ASSERTION]: Method 'GET' already declared for route '/v1/attendances/' error.

As you can see on repro repo, in this case fastify registers two routes on the inner plugin and starts the server:

• GET /v1/attendances/
• GET /v1/attendances//

After the server is ready, calling /v1/attendances// route will result in TypeError: Cannot read property 'length' of undefined in /lib/hooks.js:65:32.

This happens because /lib/route.js#L246 is never reached for /v1/attendances// and therefore some pre* hooks (preParsing, preValidation preSerialization) are never set to null. There is only a default for preHandler hook lib/context.js#L16. The others are supposed to be set in afterRouteAdded function, but for /v1/attendances// the execution goes in lib/route.js#L228 (as already pointed out with anAssertionError [ERR_ASSERTION]: Method 'GET' already declared for route '/v1/attendances/').

The done callback is located in avvio plugin.js#L98 and when called for /v1/attendances// it finds the completed===true because of /v1/attendances/. The bug is generated on lib/route.js#L144 since it calls afterRouteAdded.call two times in a row. In fact, using a non default value for prefixTrailingSlash (so slash or no-slash) it works as expected.

To Reproduce

Clone this repo:
npm i
npm run dev
call the /v1/attendances// rout using postman or similar.

Expected behavior

Avvio must stop loading the plugin and throwing AssertionError [ERR_ASSERTION]: Method 'GET' already declared for route '/v1/attendances/' error.

Your Environment

• node version: 10
• fastify version: >=2.4.1
• os: Mac