Bail on multiple Puma workers

Gemfile

@@ -5,3 +5,4 @@ gemspec
 gem "rails", ">= 7.0.1"
 gem "rake"
 gem "debug"
+gem "puma"

Gemfile.lock

@@ -134,6 +134,8 @@ GEM
       racc (~> 1.4)
     psych (5.1.2)
       stringio
+    puma (6.4.2)
+      nio4r (~> 2.0)
     racc (1.8.1)
     rack (3.1.7)
     rack-session (2.0.0)
@@ -199,6 +201,7 @@ PLATFORMS
 DEPENDENCIES
   debug
   propshaft!
+  puma
   rails (>= 7.0.1)
   rake
 

lib/propshaft/puma_config.rb

@@ -0,0 +1,36 @@
+begin
+  require "puma"
+  require "puma/configuration"
+rescue LoadError
+end
+
+class Propshaft::PumaConfig
+  def initialize
+    if defined?(Puma)
+      @resolved_config = Puma::Configuration.new.load.final_options
+    end
+  end
+
+  def multiple_workers?
+    return false unless @resolved_config
+    @resolved_config[:workers] > 1
+  end
+
+  def self.perform_dev_check!
+    return true unless new.multiple_workers?
+
+    message = "#" * 80 + "\n"
+    message += "# " + " " * 76 + " #\n"
+    message += "# " + "WARNING!!".center(76) + " #\n"
+    message += "# " + " " * 76 + " #\n"
+    message += "# Running multiple Puma workers in development is not supported with Propshaft #\n"
+    message += "# " + " " * 76 + " #\n"
+    message += "# " + "Make sure WEB_CONCURRENCY is not set or ensure".center(76) + " #\n"
+    message += "# " + "workers is not set for development in config/puma.rb.".center(76) + " #\n"
+    message += "# " + " " * 76 + " #\n"
+    message += "#" * 80 + "\n"
+
+    puts message
+    false
+  end
+end

lib/propshaft/railtie.rb

@@ -2,6 +2,7 @@
 require "active_support/ordered_options"
 require "propshaft"
 require "propshaft/quiet_assets"
+require "propshaft/puma_config"
 
 module Propshaft
   class Railtie < ::Rails::Railtie
@@ -43,6 +44,10 @@ class Railtie < ::Rails::Railtie
         app.routes.prepend do
           mount app.assets.server, at: app.assets.config.prefix
         end
+
+        if Rails.env.development?
+          exit 1 unless PumaConfig.perform_dev_check!
+        end
       end
 
       ActiveSupport.on_load(:action_view) do

test/propshaft/puma_config_test.rb

@@ -0,0 +1,29 @@
+require "test_helper"
+require "puma"
+require "propshaft/puma_config"
+require "minitest/mock"
+
+class Propshaft::PumaConfigTest < ActiveSupport::TestCase
+  class PumaConfigurationStub
+    def initialize(workers)
+      @workers = workers
+    end
+
+    def load
+      OpenStruct.new(final_options: { workers: @workers })
+    end
+  end
+
+  {
+    0 => false,
+    1 => false,
+    2 => true
+  }.each_pair do |workers, expected|
+    test "multiple_workers? is #{expected} when config resolves to #{workers}" do
+      mock = PumaConfigurationStub.new(workers)
+      Puma::Configuration.stub :new, mock do
+        assert Propshaft::PumaConfig.new.multiple_workers? == expected
+      end
+    end
+  end
+end