Precompile shipit assets before benchmark iterations

The shipit-engine ships CoffeeScript assets that require a Node.js
runtime to compile via Sprockets. Without precompilation, Sprockets
shells out to Node.js on the first request during warmup/benchmark
iterations, adding noise to timing measurements.

Run assets:precompile once before any iterations, and skip it on
subsequent runs by checking for the Sprockets manifest file.

Fixes #493

Author: k0kubun

benchmarks/shipit/benchmark.rb

@@ -14,6 +14,15 @@
 require_relative 'config/environment'
 require_relative "route_generator"
 
+# Precompile assets once so that Sprockets never shells out to Node.js during
+# warmup/benchmark iterations (shipit-engine ships CoffeeScript assets).
+unless Dir.glob(File.join(__dir__, 'public/assets/.sprockets-manifest*.json')).any?
+  puts "Precompiling assets..."
+  Rails.application.load_tasks
+  Rake::Task['assets:precompile'].invoke
+  puts "Assets precompiled."
+end
+
 # For an in-mem DB, we need to load all data on every boot
 mem_db = ActiveRecord::Base.connection.raw_connection
 file_db = SQLite3::Database.new('db/production.committed.sqlite3')