This example application highlights a race condition between Capybara 1.1.2, Selenium, and JavaScript that removes elements from the DOM.
- Download this repository
- Install gems with
bundle - Run
rspec
Disclaimer: This is my current best guess and understanding of what is happening. I could be very wrong!
The :text matcher in Capybara works by first finding all the nodes
that match the CSS/XPath selector, then selecting nodes that match the
:text condition. This happens in the all method.
When using Selenium, the node returned during the finding phase contains a reference back to the original DOM element. When checking the text of the element, the DOM element is referenced again.
The race condition occurs with the following sequence of events:
- Some JavaScript starts running.
- Capybara / Selenium grabs an element matching the selector from the DOM.
- The JavaScript finally removes the element from the DOM.
- Capybara asks for the text content of the element.
- Selenium can no longer find the element, and throws an exception.
Selenium::WebDriver::Error::StaleElementReferenceError:
Element not found in the cache - perhaps the page has changed since it was looked up
To highlight the issue and make it more reproducible, I've added some
sleeps to the example JavaScript as well as Capybara. These delays
should have no effect on the functionality of either piece of code,
they just emulate a slow machine or unlucky timing.
The example application has a link that removes an element from
the DOM after a delay of one second. All the interesting behavior
occurs in the static file index.html; Rails was simply an easy way
to get all the pieces working together.
The example application uses a custom version of
Capybara based on the 1.1.2 release. This version has a sleep of two
seconds added between the first and second phases of the all method.
Timeouts of 100 and 200ms, respectively, also reproduce the error. Lower values do not reliably reproduce the error.
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin11.4.2]
MacBook Pro, 2.3 GHz Intel Core i7, 16 GB RAM, SSD
OS X 10.8.2