Capistrano task to verify that asset digests are the same on all hosts

assets.rake

# lib/capistrano/tasks/assets.rake

require "concurrent/hash"

# This task tests that hash digests of generated JS and CSS files are the same on every
# asset server. This should always be the case if Sprockets and Uglifier do their job
# well, but I've once had a problem with that. Once bitten, twice shy.
#
# We do not send compiled CSS&JS files to our asset servers, instead we're asking the
# servers to compile and minify them from source. If the process varies between servers
# and the files come out with different hash digests then one web server might send users
# to files that do not exist on the other causing 404 HTTP errors.
#
# See this issue for more details:
#
# https://github.com/rails/sprockets/issues/74

namespace :deploy do
  desc "Verifies that asset digests match on all hosts"
  task :verify_manifests do
    results = Concurrent::Hash.new
    on release_roles(fetch(:assets_roles)) do |host|
      path = detect_manifest_path
      digest = capture :ruby,
        "-r", "json",
        "-r", "digest",
        "-e", "'manifest = JSON.load(File.read(ARGV.first))'",
        "-e", "'items = manifest[\"assets\"].values.join(\"\\n\")'",
        "-e", "'puts Digest::SHA256.hexdigest items'",
        path
      results[host.hostname] = digest
    end

    next if results.size < 2
    next if results.values.uniq.size < 2

    msg = "Generated assets differ between hosts"
    warn msg
    raise msg
  end
end

after "deploy:normalize_assets", "deploy:verify_manifests"