Two Four One: October 2011

If your team is like my team, you're trying to find ways to automate everything. Perhaps you're also using watir-webdriver to run browser based tests with Cucumber. This works great locally, but when you try to run your tests on a CI environment you start to run into some very strange problems with Firefox testing. You start to get errors around not being able to find the h1 tag on your page. You look at the screen shot you've generated using a method similar to this, and you find that your screenshot shows the text that contains the h1. In fact, your error dumped html file also shows the h1 tag:

	The given selector ".//h1 is either invalid or does not resultin a Webelement. The following error occurred:
	[InvalidSelectorError] Unable to locate elements with the xpath expression .//h1 because of the following error:
	[Exception... "Component returned failure code: 0x80070057 (NS_ERROR_ILLEGAL_VALUE) [nsIDOMXPathEvaluator.createNSResolver]" nsresult: "0x80070057 (NS_ERROR_ILLEGAL_VALUE)" location: "JS frame :: resource://fxdriver/modules/atoms.js :: <TOP_LEVEL> :: line 2360" data: no] (Selenium::WebDriver::Error::InvalidSelectorError)

view raw Selenium Issue 2099 StackTrace.txt hosted with ❤ by GitHub

You've run into selenium issue 2099. It appears what happens with this error is that the test is executing before the page has fully loaded, causing it to fail. Old school race conditions. Nice.

So how do we get around this? There's been a few postings on how to handle this with Capybara, but what about tools that use selenium-webdriver more directly? Here's a monkey patch that hacks over the Ruby bindings to enable tools like watir-webdriver to retry. This patch will retry the find until selenium-webdriver receives an error that is not related to the strange InvalidSelectorError that it gets when the Firefox isn't done loading the page. I just stuff this file into the support directory in a Cucumber suite.

	if WEBDRIVER && BROWSER_TYPE == :firefox

	module Selenium
	module WebDriver
	module SearchContext

	def find_element(*args)
	how, what = extract_args(args)

	unless by = FINDERS[how.to_sym]
	raise ArgumentError, "cannot find element by #{how.inspect}"
	end

	bridge.find_element_by by, what.to_s, ref
	rescue Selenium::WebDriver::Error::InvalidSelectorError => e
	e.message =~ /nsIDOMXPathEvaluator.createNSResolver/ ? retry : raise
	end

	def find_elements(*args)
	how, what = extract_args(args)

	unless by = FINDERS[how.to_sym]
	raise ArgumentError, "cannot find elements by #{how.inspect}"
	end

	bridge.find_elements_by by, what.to_s, ref
	rescue Selenium::WebDriver::Error::InvalidSelectorError => e
	e.message =~ /nsIDOMXPathEvaluator.createNSResolver/ ? retry : raise
	end

	end
	end
	end

	end