rails--rails/actionview/lib/action_view/digestor.rb

require 'thread_safe'
require 'action_view/dependency_tracker'
require 'monitor'

module ActionView
  class Digestor
    cattr_reader(:cache)
    @@cache          = ThreadSafe::Cache.new
    @@digest_monitor = Monitor.new

    class << self
      def digest(name, format, finder, options = {})
        cache_key = ([name, format] + Array.wrap(options[:dependencies])).join('.')
        # this is a correctly done double-checked locking idiom
        # (ThreadSafe::Cache's lookups have volatile semantics)
        @@cache[cache_key] || @@digest_monitor.synchronize do
          @@cache.fetch(cache_key) do # re-check under lock
            compute_and_store_digest(cache_key, name, format, finder, options)
          end
        end
      end

      private
      def compute_and_store_digest(cache_key, name, format, finder, options) # called under @@digest_monitor lock
        klass = if options[:partial] || name.include?("/_")
          # Prevent re-entry or else recursive templates will blow the stack.
          # There is no need to worry about other threads seeing the +false+ value,
          # as they will then have to wait for this thread to let go of the @@digest_monitor lock.
          pre_stored = @@cache.put_if_absent(cache_key, false).nil? # put_if_absent returns nil on insertion
          PartialDigestor
        else
          Digestor
        end

        @@cache[cache_key] = digest = klass.new(name, format, finder, options).digest # Store the actual digest
      ensure
        @@cache.delete_pair(cache_key, false) if pre_stored && !digest # something went wrong, make sure not to corrupt the @@cache
      end
    end

    attr_reader :name, :format, :finder, :options

    def initialize(name, format, finder, options={})
      @name, @format, @finder, @options = name, format, finder, options
    end

    def digest
      Digest::MD5.hexdigest("#{source}-#{dependency_digest}").tap do |digest|
        logger.try :info, "Cache digest for #{name}.#{format}: #{digest}"
      end
    rescue ActionView::MissingTemplate
      logger.try :error, "Couldn't find template for digesting: #{name}.#{format}"
      ''
    end

    def dependencies
      DependencyTracker.find_dependencies(name, template)
    rescue ActionView::MissingTemplate
      [] # File doesn't exist, so no dependencies
    end

    def nested_dependencies
      dependencies.collect do |dependency|
        dependencies = PartialDigestor.new(dependency, format, finder).nested_dependencies
        dependencies.any? ? { dependency => dependencies } : dependency
      end
    end

    private

      def logger
        ActionView::Base.logger
      end

      def logical_name
        name.gsub(%r|/_|, "/")
      end

      def partial?
        false
      end

      def template
        @template ||= finder.find(logical_name, [], partial?, formats: [ format ])
      end

      def source
        template.source
      end

      def dependency_digest
        template_digests = dependencies.collect do |template_name|
          Digestor.digest(template_name, format, finder, partial: true)
        end

        (template_digests + injected_dependencies).join("-")
      end

      def injected_dependencies
        Array.wrap(options[:dependencies])
      end
  end

  class PartialDigestor < Digestor # :nodoc:
    def partial?
      true
    end
  end
end