template.rb

require 'active_support/core_ext/array/wrap'
require 'active_support/core_ext/object/blank'
require 'active_support/core_ext/kernel/singleton_class'

module ActionView
  class Template
    extend ActiveSupport::Autoload

    # === Encodings in ActionView::Template
    #
    # ActionView::Template is one of a few sources of potential
    # encoding issues in Rails. This is because the source for
    # templates are usually read from disk, and Ruby (like most
    # encoding-aware programming languages) assumes that the
    # String retrieved through File IO is encoded in the
    # <tt>default_external</tt> encoding. In Rails, the default
    # <tt>default_external</tt> encoding is UTF-8.
    #
    # As a result, if a user saves their template as ISO-8859-1
    # (for instance, using a non-Unicode-aware text editor),
    # and uses characters outside of the ASCII range, their
    # users will see diamonds with question marks in them in
    # the browser.
    #
    # To mitigate this problem, we use a few strategies:
    # 1. If the source is not valid UTF-8, we raise an exception
    #    when the template is compiled to alert the user
    #    to the problem.
    # 2. The user can specify the encoding using Ruby-style
    #    encoding comments in any template engine. If such
    #    a comment is supplied, Rails will apply that encoding
    #    to the resulting compiled source returned by the
    #    template handler.
    # 3. In all cases, we transcode the resulting String to
    #    the <tt>default_internal</tt> encoding (which defaults
    #    to UTF-8).
    #
    # This means that other parts of Rails can always assume
    # that templates are encoded in UTF-8, even if the original
    # source of the template was not UTF-8.
    #
    # From a user's perspective, the easiest thing to do is
    # to save your templates as UTF-8. If you do this, you
    # do not need to do anything else for things to "just work".
    #
    # === Instructions for template handlers
    #
    # The easiest thing for you to do is to simply ignore
    # encodings. Rails will hand you the template source
    # as the default_internal (generally UTF-8), raising
    # an exception for the user before sending the template
    # to you if it could not determine the original encoding.
    #
    # For the greatest simplicity, you can support only
    # UTF-8 as the <tt>default_internal</tt>. This means
    # that from the perspective of your handler, the
    # entire pipeline is just UTF-8.
    #
    # === Advanced: Handlers with alternate metadata sources
    #
    # If you want to provide an alternate mechanism for
    # specifying encodings (like ERB does via <%# encoding: ... %>),
    # you may indicate that you are willing to accept
    # BINARY data by implementing <tt>self.accepts_binary?</tt>
    # on your handler.
    #
    # If you do, Rails will not raise an exception if
    # the template's encoding could not be determined,
    # assuming that you have another mechanism for
    # making the determination.
    #
    # In this case, make sure you return a String from
    # your handler encoded in the default_internal. Since
    # you are handling out-of-band metadata, you are
    # also responsible for alerting the user to any
    # problems with converting the user's data to
    # the default_internal.
    #
    # To do so, simply raise the raise WrongEncodingError
    # as follows:
    #
    #     raise WrongEncodingError.new(
    #       problematic_string,
    #       expected_encoding
    #     )

    eager_autoload do
      autoload :Error
      autoload :Handler
      autoload :Handlers
      autoload :Text
    end

    extend Template::Handlers

    attr_reader :source, :identifier, :handler, :virtual_path, :formats,
                :original_encoding

    Finalizer = proc do |method_name, mod|
      proc do
        mod.module_eval do
          remove_possible_method method_name
        end
      end
    end

    def initialize(source, identifier, handler, details)
      @source             = source
      @identifier         = identifier
      @handler            = handler
      @original_encoding  = nil

      @virtual_path = details[:virtual_path]
      @method_names = {}

      format   = details[:format] || :html
      @formats = Array.wrap(format).map(&:to_sym)
    end

    def render(view, locals, &block)
      # Notice that we use a bang in this instrumentation because you don't want to
      # consume this in production. This is only slow if it's being listened to.
      ActiveSupport::Notifications.instrument("!render_template.action_view", :virtual_path => @virtual_path) do
        if view.is_a?(ActionView::CompiledTemplates)
          mod = ActionView::CompiledTemplates
        else
          mod = view.singleton_class
        end

        method_name = compile(locals, view, mod)
        view.send(method_name, locals, &block)
      end
    rescue Exception => e
      if e.is_a?(Template::Error)
        e.sub_template_of(self)
        raise e
      else
        raise Template::Error.new(self, view.respond_to?(:assigns) ? view.assigns : {}, e)
      end
    end

    def mime_type
      @mime_type ||= Mime::Type.lookup_by_extension(@formats.first.to_s) if @formats.first
    end

    def variable_name
      @variable_name ||= @virtual_path[%r'_?(\w+)(\.\w+)*$', 1].to_sym
    end

    def counter_name
      @counter_name ||= "#{variable_name}_counter".to_sym
    end

    def inspect
      if defined?(Rails.root)
        identifier.sub("#{Rails.root}/", '')
      else
        identifier
      end
    end

    private
      # Among other things, this method is responsible for properly setting
      # the encoding of the source. Until this point, we assume that the
      # source is BINARY data. If no additional information is supplied,
      # we assume the encoding is the same as Encoding.default_external.
      #
      # The user can also specify the encoding via a comment on the first
      # line of the template (# encoding: NAME-OF-ENCODING). This will work
      # with any template engine, as we process out the encoding comment
      # before passing the source on to the template engine, leaving a
      # blank line in its stead.
      #
      # Note that after we figure out the correct encoding, we then
      # encode the source into Encoding.default_internal. In general,
      # this means that templates will be UTF-8 inside of Rails,
      # regardless of the original source encoding.
      def compile(locals, view, mod)
        method_name = build_method_name(locals)
        return method_name if view.respond_to?(method_name)

        locals_code = locals.keys.map! { |key| "#{key} = local_assigns[:#{key}];" }.join

        if source.encoding_aware?
          if source.sub!(/\A#{ENCODING_FLAG}/, '')
            encoding = $1
          else
            encoding = Encoding.default_external
          end

          # Tag the source with the default external encoding
          # or the encoding specified in the file
          source.force_encoding(encoding)

          # If the original encoding is BINARY, the actual
          # encoding is either stored out-of-band (such as
          # in ERB <%# %> style magic comments) or missing.
          # This is also true if the original encoding is
          # something other than BINARY, but it's invalid.
          if source.encoding != Encoding::BINARY && source.valid_encoding?
            source.encode!
          # If the assumed encoding is incorrect, check to
          # see whether the handler accepts BINARY. If it
          # does, it has another mechanism for determining
          # the true encoding of the String.
          elsif @handler.respond_to?(:accepts_binary?) && @handler.accepts_binary?
            source.force_encoding(Encoding::BINARY)
          # If the handler does not accept BINARY, the
          # assumed encoding (either the default_external,
          # or the explicit encoding specified by the user)
          # is incorrect. We raise an exception here.
          else
            raise WrongEncodingError.new(source, encoding)
          end

          # Don't validate the encoding yet -- the handler
          # may treat the String as raw bytes and extract
          # the encoding some other way
        end

        code = @handler.call(self)

        source = <<-end_src
          def #{method_name}(local_assigns)
            _old_virtual_path, @_virtual_path = @_virtual_path, #{@virtual_path.inspect};_old_output_buffer = @output_buffer;#{locals_code};#{code}
          ensure
            @_virtual_path, @output_buffer = _old_virtual_path, _old_output_buffer
          end
        end_src

        if source.encoding_aware?
          # Handlers should return their source Strings in either the
          # default_internal or BINARY. If the handler returns a BINARY
          # String, we assume its encoding is the one we determined
          # earlier, and encode the resulting source in the default_internal.
          if source.encoding == Encoding::BINARY
            source.force_encoding(Encoding.default_internal)
          end

          # In case we get back a String from a handler that is not in
          # BINARY or the default_internal, encode it to the default_internal
          source.encode!

          # Now, validate that the source we got back from the template
          # handler is valid in the default_internal
          unless source.valid_encoding?
            raise WrongEncodingError.new(@source, Encoding.default_internal)
          end
        end

        begin
          mod.module_eval(source, identifier, 0)
          ObjectSpace.define_finalizer(self, Finalizer[method_name, mod])

          method_name
        rescue Exception => e # errors from template code
          if logger = (view && view.logger)
            logger.debug "ERROR: compiling #{method_name} RAISED #{e}"
            logger.debug "Function body: #{source}"
            logger.debug "Backtrace: #{e.backtrace.join("\n")}"
          end

          raise ActionView::Template::Error.new(self, {}, e)
        end
      end

      def build_method_name(locals)
        # TODO: is locals.keys.hash reliably the same?
        @method_names[locals.keys.hash] ||=
          "_render_template_#{@identifier.hash}_#{__id__}_#{locals.keys.hash}".gsub('-', "_")
      end
  end
end