Rails 5.1: "unknown firstpos: NilClass" - Issue reloading application

Question

Following an upgrade from Rails 5.0 to 5.1 I'm getting this error anytime the app reloads, either from code changes during rails server or if I call reload! from the console.

🌶 13:53$ rc Loading development environment (Rails 5.1.1) 2.3.1 :001 > reload! Reloading... ArgumentError: unknown firstpos: NilClass     from (irb):1 2.3.1 :002 >  

https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/journey/gtg/builder.rb

Application Trace is empty, here's the Framework Trace:

actionpack (5.1.1) lib/action_dispatch/journey/gtg/builder.rb:99:in `firstpos' actionpack (5.1.1) lib/action_dispatch/journey/gtg/builder.rb:22:in `transition_table' actionpack (5.1.1) lib/action_dispatch/journey/routes.rb:58:in `simulator' actionpack (5.1.1) lib/action_dispatch/journey/router.rb:92:in `simulator' actionpack (5.1.1) lib/action_dispatch/journey/router.rb:28:in `eager_load!' actionpack (5.1.1) lib/action_dispatch/routing/route_set.rb:382:in `eager_load!' railties (5.1.1) lib/rails/application/routes_reloader.rb:26:in `each' railties (5.1.1) lib/rails/application/routes_reloader.rb:26:in `execute' railties (5.1.1) lib/rails/application/finisher.rb:141:in `block (2 levels) in <module:Finisher>' activesupport (5.1.1) lib/active_support/callbacks.rb:413:in `instance_exec' activesupport (5.1.1) lib/active_support/callbacks.rb:413:in `block in make_lambda' activesupport (5.1.1) lib/active_support/callbacks.rb:197:in `block (2 levels) in halting' activesupport (5.1.1) lib/active_support/callbacks.rb:601:in `block (2 levels) in default_terminator' activesupport (5.1.1) lib/active_support/callbacks.rb:600:in `catch' activesupport (5.1.1) lib/active_support/callbacks.rb:600:in `block in default_terminator' activesupport (5.1.1) lib/active_support/callbacks.rb:198:in `block in halting' activesupport (5.1.1) lib/active_support/callbacks.rb:507:in `block in invoke_before' activesupport (5.1.1) lib/active_support/callbacks.rb:507:in `each' activesupport (5.1.1) lib/active_support/callbacks.rb:507:in `invoke_before' activesupport (5.1.1) lib/active_support/callbacks.rb:130:in `run_callbacks' activesupport (5.1.1) lib/active_support/execution_wrapper.rb:108:in `run!' activesupport (5.1.1) lib/active_support/reloader.rb:113:in `run!' activesupport (5.1.1) lib/active_support/execution_wrapper.rb:70:in `block in run!' activesupport (5.1.1) lib/active_support/execution_wrapper.rb:67:in `tap' activesupport (5.1.1) lib/active_support/execution_wrapper.rb:67:in `run!' activesupport (5.1.1) lib/active_support/reloader.rb:59:in `run!' actionpack (5.1.1) lib/action_dispatch/middleware/executor.rb:10:in `call' actionpack (5.1.1) lib/action_dispatch/middleware/debug_exceptions.rb:59:in `call' actionpack (5.1.1) lib/action_dispatch/middleware/show_exceptions.rb:31:in `call' railties (5.1.1) lib/rails/rack/logger.rb:36:in `call_app' railties (5.1.1) lib/rails/rack/logger.rb:24:in `block in call' activesupport (5.1.1) lib/active_support/tagged_logging.rb:69:in `block in tagged' activesupport (5.1.1) lib/active_support/tagged_logging.rb:26:in `tagged' activesupport (5.1.1) lib/active_support/tagged_logging.rb:69:in `tagged' railties (5.1.1) lib/rails/rack/logger.rb:24:in `call' sprockets-rails (3.2.0) lib/sprockets/rails/quiet_assets.rb:13:in `call' actionpack (5.1.1) lib/action_dispatch/middleware/remote_ip.rb:79:in `call' request_store (1.3.2) lib/request_store/middleware.rb:9:in `call' actionpack (5.1.1) lib/action_dispatch/middleware/request_id.rb:25:in `call' rack (2.0.3) lib/rack/method_override.rb:22:in `call' rack (2.0.3) lib/rack/runtime.rb:22:in `call' activesupport (5.1.1) lib/active_support/cache/strategy/local_cache_middleware.rb:27:in `call' actionpack (5.1.1) lib/action_dispatch/middleware/executor.rb:12:in `call' actionpack (5.1.1) lib/action_dispatch/middleware/static.rb:125:in `call' rack (2.0.3) lib/rack/sendfile.rb:111:in `call' railties (5.1.1) lib/rails/engine.rb:522:in `call' puma (3.9.1) lib/puma/configuration.rb:224:in `call' puma (3.9.1) lib/puma/server.rb:602:in `handle_request' puma (3.9.1) lib/puma/server.rb:435:in `process_client' puma (3.9.1) lib/puma/server.rb:299:in `block in run' puma (3.9.1) lib/puma/thread_pool.rb:120:in `block in spawn_thread' 

Answer 1

I just faced exactly the same problem. I sovled it by setting:

config/environments/development.rb

from:

# Do not eager load code on boot. config.eager_load = true 

to:

**# Do not eager load code on boot. config.eager_load = false 

Hope this helps! Cheers, Nic.

Answer 2

Workaround found! https://github.com/rails/rails/pull/32296

The pull request is not merged, and will probably only be in 5.2+ anyway. Adding a monkey patch with the one-line change fixed the problem entirely for me.

config/initializers/routes.rb

# MONKEY PATCH!!! # https://github.com/rails/rails/pull/32296 # # Fixes: # * Development mode deadlocks # * ArgumentError: unknown firstpos: NilClass # # Allows use of "config.eager_load = true"   module ActionDispatch   module Journey     class Routes       def simulator         @simulator ||= begin           gtg = GTG::Builder.new(ast).transition_table unless ast.blank?           GTG::Simulator.new(gtg)         end       end     end   end end