How do I debug stuck rails process in development?

Question

I'm in the process of upgrading my old rails 4.2 app to rails 5. I'm currently trying to make it work at 5.1. When I try to access a page of my app in development, it seems to boot okay and it starts to render. By the browser never receives a response from the server and the server just stops outputting to the logs. I cannot figure out what is going on.

How can I further debug what why it hangs and never responds?

Output of rails s :

=> Booting Puma
=> Rails 5.1.7 application starting in development 
=> Run `rails server -h` for more startup options
Puma starting in single mode...
* Version 4.3.3 (ruby 2.6.5-p114), codename: Mysterious Traveller
* Min threads: 1, max threads: 1
* Environment: development
* Listening on tcp://127.0.0.1:3000
* Listening on tcp://[::1]:3000
Use Ctrl-C to stop
I, [2020-04-27T15:18:05.059708 #75800]  INFO -- : [83adaf81-4bf7-4870-b5f8-b6c0c11a4d2b] Started GET "/" for 127.0.0.1 at 2020-04-27 15:18:05 +0200
D, [2020-04-27T15:18:05.608709 #75800] DEBUG -- : [83adaf81-4bf7-4870-b5f8-b6c0c11a4d2b]    (3.6ms)  SELECT "schema_migrations"."version" FROM "schema_migrations" ORDER BY "schema_migrations"."version" ASC /*application:MyApp*/
I, [2020-04-27T15:18:06.590249 #75800]  INFO -- : [83adaf81-4bf7-4870-b5f8-b6c0c11a4d2b] Processing by MyAppController#front_page as HTML
D, [2020-04-27T15:18:06.613950 #75800] DEBUG -- : [83adaf81-4bf7-4870-b5f8-b6c0c11a4d2b]   User Load (0.7ms)  SELECT  "da"."users".* FROM "da"."users" WHERE "da"."users"."auth_token" = $1 LIMIT $2 /*application:MyApp,line:/app/controllers/controller_concerns/session_handling.rb:35:in `current_user'*/  [["auth_token", "uWOt-zCVDbo8a6AT-SkWUQ"], ["LIMIT", 1]]
I, [2020-04-27T15:18:06.766078 #75800]  INFO -- : [83adaf81-4bf7-4870-b5f8-b6c0c11a4d2b]   Rendering myapp/front_page.html.haml within layouts/front_page
D, [2020-04-27T15:18:06.866720 #75800] DEBUG -- : [83adaf81-4bf7-4870-b5f8-b6c0c11a4d2b]   Car Load (3.1ms)  SELECT  "cars".* FROM "cars" WHERE "cars"."user_id" = $1 AND "cars"."sales_state" = $2 ORDER BY "cars"."id" DESC LIMIT $3 /*application:MyApp,controller:myapp,action:front_page,controller_with_namespace:MyAppController,line:/app/controllers/controller_concerns/user_listings.rb:11:in `last_on_hold_listing'*/  [["user_id", 900000000], ["sales_state", "on_hold"], ["LIMIT", 1]]
D, [2020-04-27T15:18:06.929863 #75800] DEBUG -- : [Redis] command=GET args="da/brands_and_model_names/v1.0"
D, [2020-04-27T15:18:06.932895 #75800] DEBUG -- : [Redis] call_time=2.90 ms
I, [2020-04-27T15:18:07.071685 #75800]  INFO -- : [83adaf81-4bf7-4870-b5f8-b6c0c11a4d2b]   Rendered partials/_extended_search_form.html.haml (193.3ms)
I, [2020-04-27T15:18:07.242322 #75800]  INFO -- : [83adaf81-4bf7-4870-b5f8-b6c0c11a4d2b]   Rendered partials/_front_page_featured_items.html.haml (160.4ms)

It just stops at the point where it looks to be done with the processing in rails and should ship the response to the browser. Instead it just hangs as if I had eg a binding.pry in the code - but I don't. And nothing is emitted in the logs. I even can't stop the server, I have to kill it manually. CPU of the ruby process is idle at 0%.

rails console works fine btw.

What else can I do to figure out where it's getting stuck?

UDPATE

I have a suspicion that it could have something to do with asset compilation. In my old rails 4.2 branch, after reaching the particular point in the log, it would start to compile the assets on the fly in development, however that does not happen in the rails 5.2 branch. Now when running: bin/rails assets:precompile I get:

yarn install v1.22.4
warning package.json: No license field
warning package-lock.json found. Your project contains lock files generated by tools other than Yarn. It is advised not to mix package managers in order to avoid resolution inconsistencies caused by unsynchronized lock files. To clear this warning, remove package-lock.json.
warning No license field
[1/4] 🔍  Resolving packages...
success Already up-to-date.
✨  Done in 0.72s.
yarn install v1.22.4
warning package.json: No license field
warning package-lock.json found. Your project contains lock files generated by tools other than Yarn. It is advised not to mix package managers in order to avoid resolution inconsistencies caused by unsynchronized lock files. To clear this warning, remove package-lock.json.
warning No license field
[1/4] 🔍  Resolving packages...
success Already up-to-date.
✨  Done in 0.60s.
rails aborted!
ExecJS::RuntimeError: SyntaxError: [stdin]:1:1: unexpected //=
compile ((execjs):7528:19)
eval (eval at <anonymous> ((execjs):7539:8), <anonymous>:1:10)
(execjs):7539:8
(execjs):7545:14
(execjs):1:40
Object.<anonymous> ((execjs):1:58)
Module._compile (internal/modules/cjs/loader.js:1156:30)
Object.Module._extensions..js (internal/modules/cjs/loader.js:1176:10)
Module.load (internal/modules/cjs/loader.js:1000:32)
Function.Module._load (internal/modules/cjs/loader.js:899:14)
Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:74:12)
internal/main/run_main_module.js:18:47
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:39:in `exec'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:21:in `eval'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:46:in `call'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/coffee-script-2.4.1/lib/coffee_script.rb:78:in `compile'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/coffee_script_processor.rb:24:in `block in call'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/cache.rb:92:in `fetch'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/coffee_script_processor.rb:23:in `call'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:84:in `call_processor'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:66:in `block in call_processors'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:65:in `reverse_each'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:65:in `call_processors'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:22:in `block in <class:CompositeProcessor>'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:33:in `call'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:84:in `call_processor'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:66:in `block in call_processors'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:65:in `reverse_each'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:65:in `call_processors'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:182:in `load_from_unloaded'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:59:in `block in load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:335:in `fetch_asset_from_dependency_cache'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:43:in `load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/cached_environment.rb:44:in `load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/bundle.rb:41:in `block in call'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/utils.rb:167:in `dfs'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/bundle.rb:42:in `call'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:84:in `call_processor'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:66:in `block in call_processors'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:65:in `reverse_each'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:65:in `call_processors'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:182:in `load_from_unloaded'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:59:in `block in load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:335:in `fetch_asset_from_dependency_cache'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:43:in `load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/cached_environment.rb:44:in `load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/bundle.rb:32:in `block in call'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/bundle.rb:31:in `call'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:84:in `call_processor'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:66:in `block in call_processors'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:65:in `reverse_each'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/processor_utils.rb:65:in `call_processors'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:182:in `load_from_unloaded'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:59:in `block in load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:335:in `fetch_asset_from_dependency_cache'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/loader.rb:43:in `load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/cached_environment.rb:44:in `load'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/base.rb:81:in `find_asset'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/base.rb:88:in `find_all_linked_assets'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/sprockets-4.0.0/lib/sprockets/manifest.rb:125:in `block (2 levels) in find'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/executor/safe_task_executor.rb:24:in `block in execute'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/synchronization/mutex_lockable_object.rb:41:in `block in synchronize'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/synchronization/mutex_lockable_object.rb:41:in `synchronize'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/synchronization/mutex_lockable_object.rb:41:in `synchronize'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/executor/safe_task_executor.rb:19:in `execute'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/promise.rb:563:in `block in realize'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb:353:in `run_task'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb:342:in `block (3 levels) in create_worker'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb:325:in `loop'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb:325:in `block (2 levels) in create_worker'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb:324:in `catch'
/Users/nielsk/.rvm/gems/ruby-2.6.5@rails52/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb:324:in `block in create_worker'
Tasks: TOP => assets:precompile
(See full trace by running task with --trace)

Could this maybe be something?

Answer 1

Check whether you are using the //= syntax to require files in your CoffeeScript files. Apparently that is not supported anymore in sprockets 4.

You should replace all //= in.coffee files with #=

Disclaimer: I'm working on the same project as OP:)