Use FFI on JRuby to perform fast restart

lib/puma/cli.rb

@@ -81,8 +81,13 @@ module Puma
     end
 
     def restart!
-      Dir.chdir @restart_dir
-      Kernel.exec(*@restart_argv)
+      if IS_JRUBY
+        require 'puma/jruby_restart'
+        JRubyRestart.chdir_exec(@restart_dir, Gem.ruby, *@restart_argv)
+      else
+        Dir.chdir @restart_dir
+        Kernel.exec(*@restart_argv)
+      end
     end
 
     # Write +str+ to +@stdout+

lib/puma/jruby_restart.rb

@@ -0,0 +1,22 @@
+require 'ffi'
+
+module Puma
+  module JRubyRestart
+    extend FFI::Library
+    ffi_lib 'c'
+
+    attach_function :execlp, [:string, :varargs], :int
+    attach_function :chdir, [:string], :int
+
+    def self.chdir_exec(dir, cmd, *argv)
+      chdir(dir)
+      argv.unshift(cmd)
+      argv = ([:string] * argv.size).zip(argv).flatten
+      argv <<:int
+      argv << 0
+      execlp(cmd, *argv)
+      raise SystemCallError.new(FFI.errno)
+    end
+  end
+end
+