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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'
JP Duffy
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    can you try this link https://collectiveidea.com/blog/archives/2016/07/22/solutions-to-potential-upgrade-problems-in-rails-5 – skam Aug 29 '17 at 11:32

5 Answers5

15

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.

Nic
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    I prefer to use eager_load on dev boot as a sort of static code analyzer. If you have a syntax error *anywhere* you know about it immediately. That said, I tried it anyway and unfortunately it didn't have any effect. I also tried building a new rails 5.1.1 app and both ways worked as expected, with and without spring... humm – JP Duffy Jun 14 '17 at 14:24
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    worked for me, but there still must be another way to do it (or why would there be this option anyway from 5.1 on) – Maxim Zubarev Jul 02 '17 at 14:35
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    It seems very undesirable to disable a key feature of Rails just to get around this problem, let alone a feature that may be necessary for some. – Todd Sep 11 '17 at 23:27
10

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
JP Duffy
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    For the record: 32296 was closed in favor of https://github.com/rails/rails/pull/33118 and the bug is confirmed to still exist in 5.2.0 – genkilabs Jun 20 '18 at 16:45
  • This was the only thing that worked for me. I wrapped mine with https://github.com/ingoweiss/gem-patching (old gem, still works) `Gem.patching("rails", "5.2.0") do...end` so I won't forget to remove it – Chris Beck Aug 07 '18 at 20:39
  • Problem still there with `5.2.1` – bragboy Aug 21 '18 at 17:27
6

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.

Maxim Zubarev
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2

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
user9869932
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1

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
Игорь Хлебников
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