Rails 5.1: "unknown firstpos: NilClass" - Issue reloading application - ruby-on-rails

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

Answer 3

Seems it is Spring hanging or something. Just run spring stop and it should go away. Alternatively you can start the rails console without spring like this:
DISABLE_SPRING=true rails c.

Answer 4

I started having this problem after upgrading Rails from 5.1 to 5.2
It got solved by:
spring stop
spring binstub --all
spring start
rails s

Answer 5

You will not get this bug in production environment and in a test environment (if you don't use Spring).
Because this bug "ArgumentError: unknown firstpos: NilClass" you got in "reload" when it tried to reload some your classes.
In production and test environments all things are in cache, so all your things will be cached and bug will not happen
Unfortunately (for now) for the development environment, I also found only this solution
config.eager_load = false