Closes #2950 - Warn when clustered-only hooks are defined in single mode

Clustered-only hooks won't be called in single mode, so this commit adds a log to warn users about that.
Also adds `silence_fork_callback_warning` option to opt-out of the default warning log.

lib/puma/configuration.rb

@@ -157,6 +157,7 @@ class Configuration
       reaping_time: 1,
       remote_address: :socket,
       silence_single_worker_warning: false,
+      silence_fork_callback_warning: false,
       tag: File.basename(Dir.getwd),
       tcp_host: '0.0.0.0'.freeze,
       tcp_port: 9292,

lib/puma/dsl.rb

@@ -585,6 +585,11 @@ def silence_single_worker_warning
       @options[:silence_single_worker_warning] = true
     end
 
+    # Disable warning message when running single mode with callback hook defined.
+    def silence_fork_callback_warning
+      @options[:silence_fork_callback_warning] = true
+    end
+
     # Code to run immediately before master process
     # forks workers (once on boot). These hooks can block if necessary
     # to wait for background operations unknown to Puma to finish before
@@ -600,6 +605,8 @@ def silence_single_worker_warning
     #     puts "Starting workers..."
     #   end
     def before_fork(&block)
+      warn_if_in_single_mode('before_fork')
+
       @options[:before_fork] ||= []
       @options[:before_fork] << block
     end
@@ -615,6 +622,8 @@ def before_fork(&block)
     #     puts 'Before worker boot...'
     #   end
     def on_worker_boot(key = nil, &block)
+      warn_if_in_single_mode('on_worker_boot')
+
       process_hook :before_worker_boot, key, block, 'on_worker_boot'
     end
 
@@ -631,6 +640,8 @@ def on_worker_boot(key = nil, &block)
     #     puts 'On worker shutdown...'
     #   end
     def on_worker_shutdown(key = nil, &block)
+      warn_if_in_single_mode('on_worker_shutdown')
+
       process_hook :before_worker_shutdown, key, block, 'on_worker_shutdown'
     end
 
@@ -645,6 +656,8 @@ def on_worker_shutdown(key = nil, &block)
     #     puts 'Before worker fork...'
     #   end
     def on_worker_fork(&block)
+      warn_if_in_single_mode('on_worker_fork')
+
       process_hook :before_worker_fork, nil, block, 'on_worker_fork'
     end
 
@@ -659,6 +672,8 @@ def on_worker_fork(&block)
     #     puts 'After worker fork...'
     #   end
     def after_worker_fork(&block)
+      warn_if_in_single_mode('after_worker_fork')
+
       process_hook :after_worker_fork, nil, block, 'after_worker_fork'
     end
 
@@ -1073,7 +1088,18 @@ def process_hook(options_key, key, block, meth)
       elsif key.nil?
         @options[options_key] << block
       else
-        raise "'#{method}' key must be String or Symbol"
+        raise "'#{meth}' key must be String or Symbol"
+      end
+    end
+
+    def warn_if_in_single_mode(hook_name)
+      return if @options[:silence_fork_callback_warning]
+
+      if (@options[:workers] || 0) == 0
+        LogWriter::DEFAULT.log(
+          "Warning: You specified code to run in a `#{hook_name}` block, " \
+          "but Puma is configured to run without forking."
+        )
       end
     end
   end

test/test_config.rb

@@ -514,6 +514,22 @@ def test_silence_single_worker_warning_overwrite
     assert_equal true, conf.options[:silence_single_worker_warning]
   end
 
+  def test_silence_fork_callback_warning_default
+    conf = Puma::Configuration.new
+    conf.load
+
+    assert_equal false, conf.options[:silence_fork_callback_warning]
+  end
+
+  def test_silence_fork_callback_warning_overwrite
+    conf = Puma::Configuration.new do |c|
+      c.silence_fork_callback_warning
+    end
+    conf.load
+
+    assert_equal true, conf.options[:silence_fork_callback_warning]
+  end
+
   def test_http_content_length_limit
     assert_nil Puma::Configuration.new.options.default_options[:http_content_length_limit]
 
@@ -529,7 +545,9 @@ def assert_run_hooks(hook_name, options = {})
 
     # test single, not an array
     messages = []
-    conf = Puma::Configuration.new
+    conf = Puma::Configuration.new do |c|
+      c.silence_fork_callback_warning
+    end
     conf.options[hook_name] = -> (a) {
       messages << "#{hook_name} is called with #{a}"
     }
@@ -540,6 +558,8 @@ def assert_run_hooks(hook_name, options = {})
     # test multiple
     messages = []
     conf = Puma::Configuration.new do |c|
+      c.silence_fork_callback_warning
+
       c.send(configured_with) do |a|
         messages << "#{hook_name} is called with #{a} one time"
       end