Can you send the refactoring into its own PR? After merging that, this would be simpler to review

can you rebase?

Rebased

Also, it might be a breaking change

This fix is not breaking change and it also allow more usage like below one that should not works before.

const fastify = Fastify()
  
fastify.get('/', async () => '')
fastify.after(() => {
  fastify.head('/', async () => '') 
})

Any fix I can think of would provide side-effect in different way.

1. This approach, make the head route creation in sync and remove it when needed.

• It cause the route rebuild when remove the fastify route. (which can be mitigate by create head route before get.)
• Cold start time may be increase due to route rebuild.

2. Slice the current hook array and store it for later use.

• It cause the memory usage in cold start spark.
• We may not cache the correct hook array.

So, to simplify things. I go for 1. if we need to fix this issue.

This fix is not breaking change and it also allow more usage like below one that should not works before.

What about users using this approach:

function main (fastify, opts, next) {
  fastify.addHook('onRoute', () => {})
  fastify.register(allApplicationRoutes)

  next()
}

They will be affected, won't they?

They will be affected, won't they?

I don't think it will be affected. Or you can describe more on what is the expected behavior you would like to see.

import Fastify from "fastify";
const fastify = Fastify();

fastify.addHook("onRoute", function (options) {
  console.log("onRoute", options.method);
});
fastify.register(function (instance, _, done) {
  instance.get("/", () => {});
  done();
});

await fastify.ready();

main and this PR should both print

onRoute GET
onRoute HEAD

The only thing which would be affected is when it call onRoute and it can be called onRoute multiple times for HEAD route.
We may mitigate the problem by exposing kRouteByFastify to the user.
They can be aware this route is generate by fastify.

The following code works at 4.1.0

const fastify = Fastify();

fastify.get('/', async () => '');
fastify.head('/', async () => '');

At version 4.2.0, I got

FastifyError [Error]: Method 'HEAD' already declared for route '/'
    at Object.addNewRoute (...\node_modules\fastify\lib\route.js:269:19)
    at Object.route (...\node_modules\fastify\lib\route.js:196:19)
    at Object.prepareRoute (...\node_modules\fastify\lib\route.js:143:18)
    at Object._head [as head] (...\node_modules\fastify\fastify.js:247:34)
    at file:///.../t.js:6:9
    at ModuleJob.run (node:internal/modules/esm/module_job:198:25)
    at async Promise.all (index 0)
    at async ESMLoader.import (node:internal/modules/esm/loader:385:24)
    at async loadESM (node:internal/process/esm_loader:88:5)
    at async handleMainPromise (node:internal/modules/run_main:61:12) {
  code: 'FST_ERR_DUPLICATED_ROUTE',
  statusCode: 500
}

Is this expected behavior?

Yes, it is.
It was wrong that we were not throwing

exposeHeadRoute: creates a sibling HEAD route for any GET routes. Defaults to the value of exposeHeadRoutes instance option. If you want a custom HEAD handler without disabling this option, make sure to define it before the GET route.

from: https://github.com/fastify/fastify/blob/main/docs/Reference/Routes.md

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