Return proper exit code for TERM signal

Attempt at returning the proper exit code (128+15) when TERM signal
is sent to the server, for both single and clustered mode.

The changes are achieved by restoring signal from within the trap
and accordingly killing the process using TERM event.

Added plus, stopping single mode gracefully now.

lib/puma/cluster.rb

@@ -376,6 +376,9 @@ def setup_signals
           exit! 0
         else
           stop
+
+          Signal.trap("SIGTERM", "DEFAULT")
+          Process.kill("TERM", Process.pid)
         end
       end
     end

lib/puma/launcher.rb

@@ -392,7 +392,10 @@ def setup_signals
 
       begin
         Signal.trap "SIGTERM" do
-          stop
+          graceful_stop
+
+          Signal.trap("SIGTERM", "DEFAULT")
+          Process.kill("TERM", Process.pid)
         end
       rescue Exception
         log "*** SIGTERM not implemented, signal based gracefully stopping unavailable!"

test/test_integration.rb

@@ -218,4 +218,26 @@ def test_restart_restores_environment
     assert_includes new_reply, "Hello RAND"
     refute_equal initial_reply, new_reply
   end
+
+  def test_term_signal_exit_code_in_single_mode
+    server("-q test/rackup/hello.ru")
+
+    Process.kill(:TERM, @server.pid)
+
+    _, status = Process.wait2(@server.pid)
+    assert_equal 15, status
+
+    @server = nil
+  end
+
+  def test_term_signal_exit_code_in_clustered_mode
+    server("-q -w 2 test/rackup/hello.ru")
+
+    Process.kill(:TERM, @server.pid)
+
+    _, status = Process.wait2(@server.pid)
+    assert_equal 15, status
+
+    @server = nil
+  end
 end