module Roda::RodaPlugins::Caching::RequestMethods

def etag(value, opts=OPTS)

etag that doesn't match, immediately returns a response with a 412 status.
When the current request includes an If-Match header with a

depending on the request method.
matching etag, immediately returns a response with a 304 or 412 status,
When the current request includes an If-None-Match header with a

:new_resource :: Whether this etag should match an etag of * (true for POST, false otherwise)
:weak :: Use a weak cache validator (a strong cache validator is the default)
Options:
identifies the current version of the resource.
The +value+ argument is an identifier that uniquely

Set the response entity tag using the ETag header. 
def etag(value, opts=OPTS)
  # Before touching this code, please double check RFC 2616 14.24 and 14.26.
  weak = opts[:weak]
  new_resource = opts.fetch(:new_resource){post?}
  res = response
  e = env
  res[RodaResponseHeaders::ETAG] = etag = "#{'W/' if weak}\"#{value}\""
  status = res.status
  if (!status || (status >= 200 && status < 300) || status == 304)
    if etag_matches?(e['HTTP_IF_NONE_MATCH'], etag, new_resource)
      res.status = (request_method =~ /\AGET|HEAD|OPTIONS|TRACE\z/i ? 304 : 412)
      halt
    end
    if ifm = e['HTTP_IF_MATCH']
      unless etag_matches?(ifm, etag, new_resource)
        res.status = 412
        halt
      end
    end
  end
end