Somewhere around the time that I got a Mac, upgraded Firefox, upgraded selenium-webdriver, upgraded Capybara and started running lots of acceptance tests in selenium, I started getting errors like this:

Selenium::WebDriver::Error::UnhandledError: Component returned failure code: 0x80070057 (NS_ERROR_ILLEGAL_VALUE) [nsIDOMXPathEvaluator.createNSResolver]

They don’t appear to correlate with anything particular that my tests are doing– I can run the same tests again immediately and have different tests get this error, or have no tests get this error at all. Like all good Heisenbugs, it was especially hard to trigger when I sat down to dig into this!!!

I did the usual googling and the most helpful thing I found was this Capybara mailing list post explaining different ways of debugging Selenium UnhandledErrors.

I didn’t see anything initially useful in the output when running with $DEBUG on.

Dumping the firefox console log to disk (this post was clearer to me than the capybara docs on how to do this) showed this error that looked like what I saw:

nsCommandProcessor.js:314 - Exception caught by driver: findElements([object Object])
[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 2354"  data: no]

So then I searched for “firefox nsCommandProcessor.js:314 - Exception caught by driver” and found this selenium issue. Which says it’s a timing issue in Selenium between get and findElement. I don’t know right now if there’s a way to sleep in my tests to solve it.

And now i’ve hit a timing issue that I know sleep will cure :D Good night– I might look into a workaround more tomorrow.

My system:

• OSX 10.6.8 (no, I haven't upgraded to Lion yet :P)
• Ruby 1.8.7-p302
• Firefox 5.0.1
• selenium-webdriver 2.0.1
• capybara at 4fc07dbdc814

UPDATE Jul 26: There have been a few updates to the bug today, with some speculation about the cause of the error. One commenter says that they’re seeing the problem with post-redirect-gets, but that’s not what I’m experiencing– I’m getting the problem most with regular old links. The same commenter is suggesting a race condition, so I’m trying adding a sleep 1 after every line in my script that triggers the error– after the click_link, before the next statement (that capybara automatically is doing a wait/find on). This seemed to help at first, but just kept happening at different places instead. I also tried adding a sleep 1 in capybara’s click_link and that did not fix the issue.

BUT! If I add a sleep 1 to be the first line in capybara’s selenium find in lib/capybara/selenium/driver.rb, this seems to be enough to get around the race condition (knock on wood). I’ve run tests that have consistently hit the error a few times now without hitting it, so…

To be clear: The workaround I’ve had good results with is changing def find in lib/capybara/selenium/driver.rb to be:

   def find(selector)
    sleep 1
     browser.find_elements(:xpath, selector).map { |node| Capybara::Selenium::Node.new(self, node) }
   end

Let me know if that works for you. Obviously this is going to make your tests slower, but since you’re running Selenium tests already I’m assuming you aren’t Gary Bernhardt and can put up with slow but passing tests for a while.

And this is a workaround, the bug is with Selenium, NOT with capybara. The real solution will be a fix to this bug, so if you are hitting this, go star that issue.