Cleanup order in selenium headless test

TL;DR

In this post I describe on real example how to use important question for every software tester: “Did I do it in proper order?”

For one client I created environment for running selenium webdriver test suite in headless mode for firefox browser. I use docker image that contains latest Jenkins, xvfb server and firefox browser. I was not able to start chrome with xvfb because chrome has much higher security. More about that environment in following posts.

When I run test suite, I got following cryptic exception:

Failure/Error: Unable to find matching line from backtrace
     Selenium::WebDriver::Error::UnknownError:
       Failed to decode response from marionette
     # ./spec/spec_helper.rb:93:in `block (2 levels) in <top (required)>'

Line that triggered exception was:

@driver.quit

With help of this excellent blog post,  I find how to ask right question: “Did I use proper order of calling different api methods?”

I first closed headless driver (which is xvfb server), and then I tried to close selenium webdriver. That lost connection to its X server manager (xvfb).

After I first closed selenium webdriver, and after it I closed headless xvfb server, test suite worked as expected.

When you investigate what went wrong, use this simple question:

“Did I use proper order?”