Puma not logging when running daemon

Question

When running Puma as a daemon (i.e. using the -d flag), Puma does not seem to log to the location specified by stdout_redirect.

Has anyone seen this behavior before and if so, found a workaround to generate the proper log files for Puma (specifically with the stdout_redirect in place, and specifically for a Ruby on Rails application)?

Answer 1

The reason stdout_redirect is not working is because config/puma.rb is never run when you run rails server -d. You can demonstrate this for yourself by changing the port number or even introducing a syntax error in the file.

Puma runs as a Rack server and is thus launched by the Rack middleware. The Rack code just uses the core Ruby Process class to detach the process:

def daemonize_app
  Process.daemon
end

Unfortunately, Process.daemon does two things by default:

1. Changes the current working directory to root ('/').
2. Redirects stdout and stderr to /dev/null.

So, when the Puma server initializes, all process output is binned and since /config/puma.rb doesn't exist, puma falls back to its hardcoded default settings.

You can actually demonstrate the behavior you want if you temporarily hack the rack code to override the chdir behaviour:

def daemonize_app
  Process.daemon(true)
end

If the first parameter to .daemon is true it does not change the current working directory, and thus the config/puma.rb gets run.

Short of some kind of ugly monkey-patch to Rack, a (possibly also ugly) workaround would be to skip the -d option and roll your own in config/puma.rb. For example:

# ... rest of the puma config

return unless ENV['DAEMONIZE_PUMA'].present?

puts 'Running puma as a daemon...'
Process.daemon(true)

Then:

% export DAEMONIZE_PUMA=true
% rails server
=> Booting Puma
=> Rails 6.1.4 application starting in development
=> Run `bin/rails server --help` for more startup options
Running puma as a daemon...
% tail log/puma.out
=== puma startup: 2021-07-17 10:03:37 +1200 ===