Firefox needs some more RAM to run your Rails system tests

A quick fix for an annoying (and not very descriptive) error Browsing context has been discarded when setting up Ruby on Rails system tests with Firefox headless.

Here is how that might look like:

Error:
PublicPagesTest#test_visiting_the_index:
Selenium::WebDriver::Error::NoSuchWindowError: Browsing context has been discarded
test/system/public_pages_test.rb:7:in `block in '

Error:
PublicPagesTest#test_visiting_the_index:
Selenium::WebDriver::Error::UnknownError: Failed to decode response from marionette

Alternatively:

Error:
PublicPagesTest#test_visiting_the_index:
Selenium::WebDriver::Error::SessionNotCreatedError: Tried to run command without establishing a connection
    test/system/public_pages_test.rb:7:in `block in <class:PublicPagesTest>'

Error:
PublicPagesTest#test_visiting_the_index:
Selenium::WebDriver::Error::SessionNotCreatedError: Tried to run command without establishing a connection

This happens especially if you are running this in a constrained environment such as Linux container (with Docker) or a VM (with Vagrant). Simply put there is not enough memory.

If you are running Vagrant as me the actual fix is to increase the memory limit:

# Vagrantfile
  ...
  config.vm.provider :libvirt do |libvirt|
    libvirt.memory = 4096
  end
  ...

And now the tests work!

$ rails test:system
Run options: --seed 53251

# Running:

Capybara starting Puma...
* Version 3.12.0 , codename: Llamas in Pajamas
* Min threads: 0, max threads: 4
* Listening on tcp://127.0.0.1:36151
.

Finished in 5.667349s, 0.1764 runs/s, 0.1764 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips