No PID file created when starting Puma as daemon - ruby-on-rails

Question

I am working on getting my Rails app deployed using Nginx as a reverse proxy. Everything work correctly when starting the app manually using rails s to launch it. All the proper PIDs are created in the tmp/pids/ directory (puma.pid, puma.state, andserver.pid) and the puma.sock is properly created in the tmp/sockets/ directory.
When I attempt to start the same app using rails s -d, to start it as a daemon, everything start normally except the tmp/pids/puma.pid is nowhere to be found which causes my reverse proxy to break. I'll paste a copy of my puma.conf below.
Using:
puma 3.12.6 and rails 5.2.6
threads_count = ENV.fetch("RAILS_MAX_THREADS") { 5 }
threads threads_count, threads_count
port ENV.fetch("PORT") { 3000 }
environment ENV.fetch("RAILS_ENV") { "development" }
workers ENV.fetch("WEB_CONCURRENCY") { 4 }
preload_app!
plugin :tmp_restart
# Prep for Nginx integration
app_dir = File.expand_path("../..", __FILE__)
tmp_dir = "#{app_dir}/tmp"
bind "unix://#{tmp_dir}/sockets/puma.sock"
pidfile "#{tmp_dir}/pids/puma.pid"
state_path "#{tmp_dir}/pids/puma.state"
activate_control_app

Answer 1

It turns out that the problem occurs when launching the Rails server and using the -d switch to daemonize (which I was doing) like this:
rails s -d
However, if I add daemonize true to the puma.conf everything works as expected. So, now I launch the server using rails s with the following puma.conf and the missing puma.pid appears!
# Puma can serve each request in a thread from an internal thread pool.
# The `threads` method setting takes two numbers: a minimum and maximum.
# Any libraries that use thread pools should be configured to match
# the maximum value specified for Puma. Default is set to 5 threads for minimum
# and maximum; this matches the default thread size of Active Record.
#
threads_count = ENV.fetch("RAILS_MAX_THREADS") { 5 }
threads threads_count, threads_count
# Specifies the `port` that Puma will listen on to receive requests; default is 3000.
#
port ENV.fetch("PORT") { 3000 }
# Specifies the `environment` that Puma will run in.
#
environment ENV.fetch("RAILS_ENV") { "development" }
# Specifies the number of `workers` to boot in clustered mode.
# Workers are forked webserver processes. If using threads and workers together
# the concurrency of the application would be max `threads` * `workers`.
# Workers do not work on JRuby or Windows (both of which do not support
# processes).
#
workers ENV.fetch("WEB_CONCURRENCY") { 4 }
# Use the `preload_app!` method when specifying a `workers` number.
# This directive tells Puma to first boot the application and load code
# before forking the application. This takes advantage of Copy On Write
# process behavior so workers use less memory.
#
preload_app!
# Allow puma to be restarted by `rails restart` command.
plugin :tmp_restart
# Prep for Nginx integration
app_dir = File.expand_path("../..", __FILE__)
tmp_dir = "#{app_dir}/tmp"
bind "unix://#{tmp_dir}/sockets/puma.sock"
pidfile "#{tmp_dir}/pids/puma.pid"
state_path "#{tmp_dir}/pids/puma.state"
# Run Puma as a daemon
daemonize true
activate_control_app

Answer 2

to config/puma.rb
add pidfile "tmp/pids/server.pid"