Posted about 4 years ago. Visible to the public.

How to make Webpacker compile once for parallel tests, and only if necessary

Webpack is the future. We're using it in our latest Rails applications.

For tests, we want to compile assets like for production.
For parallel tests, we want to avoid 8 workers compiling the same files at the same time.
When assets did not change, we do not want to spend time compiling them.

Here is our solution for all that.

Its concept should work for all test suites.

Copy the following to config/initializers/webpacker_compile_once.rb. It will patch Webpacker, but only for the test environment:

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# Avoid hardcoded asset hosts in webpack, otherwise all chunks would be loaded through the # Capybara server of the first test process. If that test process hasn't launched a Capybara # server yet, or if it finishes before other processes, chunk loading in other processes will # fail with a message like "ChunkLoadError: Loading chunk 0 failed". ENV['WEBPACKER_ASSET_HOST'] = '' module WebpackerCompileOnce COMPILE_WAIT_TIMEOUT = 180 COMPILE_WAIT_POLL_INTERVAL = 0.3 def compile # Calling #compile will at least once compute SHAs over the content # of all files in app/. Files cannot change in tests, so we immediately # return if we have compiled before. if test? if @compiled_before return true else @compiled_before = true end end if parallel_tests? if parallel_tests_number == 0 # Before compiling Webpacker::Compiler will check whether files in app/ and # other places changed. super else # Wait until the first parallel_tests process has finished compilation. wait_until_fresh true end else # Development, production, non-parallel test run. super end end def fresh? # Stock Webpacker checks if JavaScript changed in app, yarn.lock or config/webpack. # We want to be a bit stricter about freshness than stock Webpacker: # We *do* want to compile if someone manually emptied the packs-test folder in an attempt # to fix a broken build. super && Dir.exist?(public_output_path) && !Dir.empty?(public_output_path) end delegate :public_output_path, to: :config private def test? Rails.env.test? end def parallel_tests? test? && !!parallel_tests_number end def parallel_tests_number if (raw = ENV['TEST_ENV_NUMBER']) raw.to_i end end def caching_watched_files_digest(&block) @cache_watched_files_digest = true block.call ensure @cache_watched_files_digest = false end def watched_files_digest if @cache_watched_files_digest @previous_watched_files_digest ||= super else super end end # Waits until the first parallel_tests process has finished compiling. def wait_until_fresh # We're going to call #fresh? a lot. This will call #watched_files_digest, which # computes SHAs over the contents of all files in app/ and other places. # Because this is expensive we do it only once for the duration of this method. caching_watched_files_digest do elapsed = 0 loop do break if fresh? sleep(COMPILE_WAIT_POLL_INTERVAL) elapsed += COMPILE_WAIT_POLL_INTERVAL if elapsed > COMPILE_WAIT_TIMEOUT raise "First parallel_tests process did not compile within #{COMPILE_WAIT_TIMEOUT} seconds. It may have crashed?" end end end end end Webpacker::Compiler.prepend(WebpackerCompileOnce)

It's a good idea to compile your Webpacker assets once before your end-to-end integration test suite. Otherwise the first rendered javascript_pack_tag would start compilation, causing the helper to freeze for a minute and current Capybara visit to timeeout.

To compile assets before all Cucumber scenarios, copy the following to features/support/webpacker.rb:

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# Compile before the test suite starts. # Otherwise the first rendered javascript_pack_tag would start compilation, # causing the current Capybara visit to time out. Webpacker.compile

That's it. Enjoy! 🎉


Similar scenario: running an application with Webpacker on multiple servers. How to serve identical assets from all servers.

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Arne Hartherz
Last edit:
5 months ago
by Henning Koch
About this deck:
We are makandra and do test-driven, agile Ruby on Rails software development.
License for source code
Posted by Arne Hartherz to makandra dev
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