Restore automatic detection of processor count in default puma config

It has to be reverted because the previous implementation wasn't
cgroup aware so it would often start way too many processes on various
shared hosting platforms.

Thanks to ruby-concurrency/concurrent-ruby#1038
concurrent-ruby 1.3 now offer a cgroups aware method to detect how
many processors we can actually use.

Gemfile.lock

@@ -89,7 +89,7 @@ PATH
     activesupport (8.0.0.alpha)
       base64
       bigdecimal
-      concurrent-ruby (~> 1.0, >= 1.0.2)
+      concurrent-ruby (~> 1.0, >= 1.3.1)
       connection_pool (>= 2.2.5)
       drb
       i18n (>= 1.6, < 2)
@@ -182,7 +182,7 @@ GEM
     cgi (0.4.1)
     chef-utils (18.3.0)
       concurrent-ruby
-    concurrent-ruby (1.2.2)
+    concurrent-ruby (1.3.1)
     connection_pool (2.4.1)
     crack (0.4.5)
       rexml

activesupport/activesupport.gemspec

@@ -36,7 +36,7 @@ Gem::Specification.new do |s|
 
   s.add_dependency "i18n",            ">= 1.6", "< 2"
   s.add_dependency "tzinfo",          "~> 2.0", ">= 2.0.5"
-  s.add_dependency "concurrent-ruby", "~> 1.0", ">= 1.0.2"
+  s.add_dependency "concurrent-ruby", "~> 1.0", ">= 1.3.1"
   s.add_dependency "connection_pool", ">= 2.2.5"
   s.add_dependency "minitest",        ">= 5.1"
   s.add_dependency "base64"

railties/lib/rails/generators/rails/app/templates/config/puma.rb.tt

@@ -32,10 +32,9 @@ when "production"
   # If you are running more than 1 thread per process, the workers count
   # should be equal to the number of processors (CPU cores) in production.
   #
-  # It defaults to 1 because it's impossible to reliably detect how many
-  # CPU cores are available. Make sure to set the `WEB_CONCURRENCY` environment
-  # variable to match the number of processors.
-  workers_count = Integer(ENV.fetch("WEB_CONCURRENCY", 1))
+  # Automatically detect the number of available processors in production.
+  require "concurrent-ruby"
+  workers_count = Integer(ENV.fetch("WEB_CONCURRENCY") { Concurrent.available_processor_count })
   workers workers_count if workers_count > 1
 
   preload_app!