[ericproulx]

Summary

• When config.eager_load is on, Grape's Railtie now compiles the APIs mounted in the application's routes as soon as the routes are loaded (config.after_routes_loaded), rather than on the first request each process answers. This mirrors Rails, whose RoutesReloader eager loads its own route sets under the same condition.
• It walks the route set the way bin/rails routes does (route.app.rack_app, recursing into mounted engines). Only the APIs Rails actually serves get compiled, not the ones mounted inside another API or never mounted.
• Without eager loading, as in development, nothing changes. The hook also runs after each reload, and the APIs stay lazy.
• README: drops the advice to call compile! from application.rb, which raises NameError on Rails 7+ (the main autoloader is set up after the app's initializers). It now shows config.after_initialize for an API Rails doesn't see in its routes, such as one wrapped in a Rack::Builder.

What you get: no first-request compile or LOCK wait per worker, and under a preloading server (preload_app!) the compiled router is shared copy-on-write across workers.

Behaviour change

Documented in UPGRADING: compiling builds each endpoint's middleware stack, so a middleware's initialize now runs at boot. Under a preloading server, that is before the workers fork. Also, an API that fails to compile now fails the boot. A compile! call the application already makes is harmless, because compile! is memoized.

Test plan

• spec/integration/rails/railtie_spec.rb: compiled at boot with eager loading, through an engine, and not without eager loading. Each case was mutation-checked (no hook / no eager_load gate / no engine recursion each fail a spec).
• Rails integration specs green on 7.2, 8.0 and 8.1 across 10 seeds. The spec preloads ActionView::Base and points Rails.application at its own app, to keep a multi-app test process from leaking into the boot. It also swaps in a fresh I18n backend, so the I18n.eager_load! an eager boot performs doesn't leak into later specs (explained in the spec's comments).
• Booted a single real app (no stubs) on 7.2 and 8.1: one router built at boot and reused by the first request with eager loading, none until the first request without.
• bundle exec rake and the CI spec set under the Rails gemfiles pass locally.
• CI green.

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[dblock]

should this live in route_set, route_set.compile!?