I integrated your comments, let me know your thoughts.

P.S. Plugins.md was updated as well in the initial commit, should I expand more / is it not clear enough?

An interesting and potentially unwanted behaviour of the avvio update has arose.

The fastify instance is passed by reference to the options argument of register and is the same reference as the one passed in to the plugin's function.

This means that it is possible to declare routes on the options argument, use decorate and have the changes reflected to the outer scope if the plugin's function is wrapped with fastify-plugin or even use decorate without fastify-plugin to inject variables that will be locally available to the plugin's function in the fastify instance.

This is showcased in the test suite but a quick example would be:

fastify.register((instance, options, next) => {
  console.log(instance.plugin_opts === 'hello') // Evaluates to true 
  instance.decorate('local', () => 'world')
}, parent => {
  parent.decorate('plugin_opts', 'hello')
  parent.get('/', (request, reply) => {
    reply.send({ hello: parent.local() }) // Will send { "hello": "world" }
  })
})

fastify.after(() => {
  console.log(fastify.plugin_opts === undefined) // Evaluates to true
  console.log(fastify.local === undefined) // Evaluates to true
})

Not sure if this is behaviour we want to remove as it is possible to accidentally declare an externally available plugin if the plugin's function is wrapped by fastify-plugin. Example:

fastify.register(fp((instance, options, next) => {
  instance.decorate('my_plugin', () => 'world')
}), parent => {
  parent.decorate('accidental_plugin', () => 'hello')
})

fastify.after(() => {
  console.log(fastify.my_plugin() === 'world') // Evaluates to true, as it should
  console.log(fastify.accidental_plugin() === 'hello') // Evaluates to true, potentially unwanted behaviour
})

The unwanted behaviour I believe could be removed by calling override once again within the avvio plugin and passing in an overridden instance. Looking forward to your thoughts.

I don't see problems with it.

cc @fastify/fastify what do you think?

I don't see problems with it.
cc @fastify/fastify what do you think?

No problem for me either.

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