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# frozen_string_literal: true
module Rack
# Middleware that enables conditional GET using If-None-Match and
# If-Modified-Since. The application should set either or both of the
# Last-Modified or Etag response headers according to RFC 2616. When
# either of the conditions is met, the response body is set to be zero
# length and the response status is set to 304 Not Modified.
#
# Applications that defer response body generation until the body's each
# message is received will avoid response body generation completely when
# a conditional GET matches.
#
# Adapted from Michael Klishin's Merb implementation:
# https://github.com/wycats/merb/blob/master/merb-core/lib/merb-core/rack/middleware/conditional_get.rb
class ConditionalGet
def initialize(app)
@app = app
end
# Return empty 304 response if the response has not been
# modified since the last request.
def call(env)
case env[REQUEST_METHOD]
when "GET", "HEAD"
status, headers, body = @app.call(env)
headers = Utils::HeaderHash[headers]
if status == 200 && fresh?(env, headers)
status = 304
headers.delete(CONTENT_TYPE)
headers.delete(CONTENT_LENGTH)
original_body = body
body = Rack::BodyProxy.new([]) do
original_body.close if original_body.respond_to?(:close)
end
end
[status, headers, body]
else
@app.call(env)
end
end
private
# Return whether the response has not been modified since the
# last request.
def fresh?(env, headers)
# If-None-Match has priority over If-Modified-Since per RFC 7232
if none_match = env['HTTP_IF_NONE_MATCH']
etag_matches?(none_match, headers)
elsif (modified_since = env['HTTP_IF_MODIFIED_SINCE']) && (modified_since = to_rfc2822(modified_since))
modified_since?(modified_since, headers)
end
end
# Whether the ETag response header matches the If-None-Match request header.
# If so, the request has not been modified.
def etag_matches?(none_match, headers)
headers['ETag'] == none_match
end
# Whether the Last-Modified response header matches the If-Modified-Since
# request header. If so, the request has not been modified.
def modified_since?(modified_since, headers)
last_modified = to_rfc2822(headers['Last-Modified']) and
modified_since >= last_modified
end
# Return a Time object for the given string (which should be in RFC2822
# format), or nil if the string cannot be parsed.
def to_rfc2822(since)
# shortest possible valid date is the obsolete: 1 Nov 97 09:55 A
# anything shorter is invalid, this avoids exceptions for common cases
# most common being the empty string
if since && since.length >= 16
# NOTE: there is no trivial way to write this in a non exception way
# _rfc2822 returns a hash but is not that usable
Time.rfc2822(since) rescue nil
end
end
end
end