Add Spring.allow_reloading_disabled opt-in

CHANGELOG.md

@@ -1,3 +1,10 @@
+## Unreleased
+
+* Add `Spring.dangerously_allow_disabling_reloading` opt-in to skip the `:ensure_reloading_is_enabled` initializer check, so projects that want to run with `config.cache_classes = true` / `config.enable_reloading = false` can.
+The default behavior (refuse to boot) is unchanged, as using this option requires a Rails application that uses
+lazy-loader for everything (most importantly, routes & i18n translations).
+
+
 ## 4.5.0
 
 * Skip spring without error if spring is not in installed bundler groups.

README.md

@@ -79,6 +79,31 @@ Spring manages. That setting is typically configured in
 Note: in versions of Rails before 7.1, the setting is called `cache_classes`,
 and it needs to be `false` for Spring to work.
 
+#### Running Spring with reloading disabled (advanced, risky)
+
+If you know what you are doing, and if every reloadable resource is configured
+to materialize *lazily in the forked child*, not in the Spring server process,
+then you can decide to run Spring without Rail's in-process reloaders.
+
+To opt into this, set in `config/spring.rb`:
+
+```ruby
+Spring.dangerously_allow_disabling_reloading = true
+```
+
+If you set this, you should make sure that you are NOT pre-loading reloadable
+resource in the Spring server. One approach is to add boot-time assertions at
+the end of `Spring.after_environment_load`, e.g.:
+
+```ruby
+Spring.after_environment_load do
+  raise "routes were drawn at boot" if Rails.application.routes_reloader.loaded
+  raise "i18n locales where loaded at boot" unless I18n.backend.instance_variable_get(:@translations).nil?
+end
+```
+
+When unsure, leave this disabled.
+
 ### Usage
 
 For this walkthrough I've generated a new Rails application, and run

lib/spring/application.rb

@@ -101,6 +101,8 @@ def preload
       end
 
       Rails::Application.initializer :ensure_reloading_is_enabled, group: :all do
+        next if Spring.dangerously_allow_disabling_reloading
+
         if Rails.application.config.cache_classes
           config_name, set_to = if Rails.application.config.respond_to?(:enable_reloading=)
             ["enable_reloading", "true"]
@@ -110,6 +112,8 @@ def preload
           raise <<-MSG.strip_heredoc
             Spring reloads, and therefore needs the application to have reloading enabled.
             Please, set config.#{config_name} to #{set_to} in config/environments/#{Rails.env}.rb.
+            (If you understand the trade-offs and want to disable Rails' reloader anyway,
+            set `Spring.dangerously_allow_disabling_reloading = true` in config/spring.rb.)
           MSG
         end
       end

lib/spring/configuration.rb

@@ -3,11 +3,17 @@
 module Spring
   @connect_timeout = 5
   @boot_timeout = 20
+  @dangerously_allow_disabling_reloading = false
 
   class << self
     attr_accessor :application_root, :connect_timeout, :boot_timeout
     attr_writer :quiet
 
+    # Opt-in: skip the `:ensure_reloading_is_enabled` initializer so Spring
+    # boots with `config.enable_reloading = false`. Default `false`. See
+    # README "Running Spring with reloading disabled" for what this gives up.
+    attr_accessor :dangerously_allow_disabling_reloading
+
     def gemfile
       require "bundler"
 

test/support/acceptance_test.rb

@@ -158,6 +158,20 @@ def without_gem(name)
         assert_failure "bin/rails runner 1", stderr: expected_error
       end
 
+      test "Spring.dangerously_allow_disabling_reloading bypasses the reloading-enabled check" do
+        config_path = app.path("config/environments/development.rb")
+        config = File.read(config_path)
+        if config.include?("config.cache_classes")
+          config.sub!(/config\.cache_classes\s*=\s*false/, "config.cache_classes = true")
+        else
+          config.sub!(/config.enable_reloading = true/, "config.enable_reloading = true\nconfig.cache_classes = true")
+        end
+        File.write(config_path, config)
+        File.write(app.path("config/spring.rb"), "Spring.dangerously_allow_disabling_reloading = true\n")
+
+        assert_success "bin/rails runner 1"
+      end
+
       test "test changes are picked up" do
         assert_speedup do
           assert_success app.spring_test_command, stdout: "0 failures"