Add on_refork hook configuration and integration test

lib/puma/dsl.rb

@@ -492,6 +492,28 @@ def after_worker_fork(&block)
 
     alias_method :after_worker_boot, :after_worker_fork
 
+    # When `fork_worker` is enabled, code to run in Worker 0
+    # before all other workers are re-forked from this process,
+    # after the server has temporarily stopped serving requests
+    # (once per complete refork cycle).
+    #
+    # This can be used to trigger extra garbage-collection to maximize
+    # copy-on-write efficiency, or close any connections to remote servers
+    # (database, Redis, ...) that were opened while the server was running.
+    #
+    # This can be called multiple times to add several hooks.
+    #
+    # @note Cluster mode with `fork_worker` enabled only.
+    # @example
+    #   on_refork do
+    #     3.times {GC.start}
+    #   end
+
+    def on_refork(&block)
+      @options[:before_refork] ||= []
+      @options[:before_refork] << block
+    end
+
     # Code to run out-of-band when the worker is idle.
     # These hooks run immediately after a request has finished
     # processing and there are no busy threads on the worker.

test/helpers/integration.rb

@@ -44,12 +44,18 @@ def teardown
 
   private
 
-  def cli_server(argv, unix: false)
+  def cli_server(argv, unix: false, config: nil)
+    if config
+      config_file = Tempfile.new(%w(config .rb))
+      config_file.write config
+      config_file.close
+      config = "-C #{config_file.path}"
+    end
     if unix
-      cmd = "#{BASE} bin/puma -b unix://#{@bind_path} #{argv}"
+      cmd = "#{BASE} bin/puma #{config} -b unix://#{@bind_path} #{argv}"
     else
       @tcp_port = UniquePort.call
-      cmd = "#{BASE} bin/puma -b tcp://#{HOST}:#{@tcp_port} #{argv}"
+      cmd = "#{BASE} bin/puma #{config} -b tcp://#{HOST}:#{@tcp_port} #{argv}"
     end
     @server = IO.popen(cmd, "r")
     wait_for_server_to_boot

test/test_integration_cluster.rb

@@ -157,13 +157,21 @@ def test_worker_timeout
 RUBY
   end
 
+  def test_refork
+    refork = Tempfile.new('refork')
+    cli_server "-w #{WORKERS} test/rackup/sleep.ru", config: <<RUBY
+fork_worker 1
+on_refork {File.write('#{refork.path}', 'Reforked')}
+RUBY
+    pids = get_worker_pids
+    read_body(connect('sleep1')) until refork.read == 'Reforked'
+    refute_includes pids, get_worker_pids(1, WORKERS - 1)
+  end
+
   private
 
   def worker_timeout(timeout, iterations, config)
-    config_file = Tempfile.new(%w(worker_timeout .rb))
-    config_file.write config
-    config_file.close
-    cli_server "-w #{WORKERS} -t 1:1 -C #{config_file.path} test/rackup/hello.ru"
+    cli_server "-w #{WORKERS} -t 1:1 test/rackup/hello.ru", config: config
 
     pids = []
     Timeout.timeout(iterations * timeout + 1) do