Handle fatal error that has no backtrace (#2607)

* handle low level error that has no backtrace

* add force_shutdown_after in test case

* skip test on windows for low level error

* remove extra space

* remove two extra lines

* rename test method to lowlevel_error

lib/puma/server.rb

@@ -528,7 +528,8 @@ module Puma
       end
 
       if @leak_stack_on_error
-        [status, {}, ["Puma caught this error: #{e.message} (#{e.class})\n#{e.backtrace.join("\n")}"]]
+        backtrace = e.backtrace.nil? ? '<no backtrace available>' : e.backtrace.join("\n")
+        [status, {}, ["Puma caught this error: #{e.message} (#{e.class})\n#{backtrace}"]]
       else
         [status, {}, ["An unhandled lowlevel error occurred. The application logs may have details.\n"]]
       end

test/test_puma_server.rb

@@ -285,6 +285,25 @@ EOF
     assert_match(/{}\n$/, data)
   end
 
+  def test_lowlevel_error_message
+    skip_if :windows
+    @server = Puma::Server.new @app, @events, {:force_shutdown_after => 2}
+
+    server_run app: ->(env) do
+      require 'json'
+
+      # will raise fatal: machine stack overflow in critical region
+      obj = {}
+      obj['cycle'] = obj
+      ::JSON.dump(obj)
+    end
+
+    data = send_http_and_read "GET / HTTP/1.0\r\n\r\n"
+
+    assert_match(/HTTP\/1.0 500 Internal Server Error/, data)
+    assert (data.size > 0), "Expected response message to be not empty"
+  end
+
   def test_force_shutdown_error_default
     @server = Puma::Server.new @app, @events, {:force_shutdown_after => 2}