Allow customisation of transferable headers

Headers specified in the :transfer_headers option are passed (transferred) from responses from the backend through to responses sent to the client (included cached entries).

lib/rack/cache/context.rb

@@ -237,7 +237,7 @@ def validate(entry)
 
  entry = entry.dup
  entry.headers.delete('date')
- %w[Date expires cache-control etag last-modified].each do |name|
+ %w[Date expires cache-control etag last-modified].concat(transfer_headers).each do |name|
  next unless value = response.headers[name]
  entry.headers[name] = value
  end

lib/rack/cache/options.rb

@@ -86,6 +86,13 @@ def option_name(key)
  # Default: ['set-cookie']
  option_accessor :ignore_headers
 
+ # Set of response headers that are transferred from the backend response
+ # onto the cache entry when validating a cached entity (after receiving a
+ # 304 response from the backend) before sending it to the client.
+ #
+ # Default: []
+ option_accessor :transfer_headers
+
  # Set of request headers that trigger "private" cache-control behavior
  # on responses that don't explicitly state whether the response is
  # public or private via a cache-control directive. Applications that use
@@ -149,6 +156,7 @@ def initialize_options(options={})
  'rack-cache.entitystore' => 'heap:/',
  'rack-cache.default_ttl' => 0,
  'rack-cache.ignore_headers' => ['set-cookie'],
+ 'rack-cache.transfer_headers' => [],
  'rack-cache.private_headers' => ['Authorization', 'Cookie'],
  'rack-cache.allow_reload' => false,
  'rack-cache.allow_revalidate' => false,

test/context_test.rb

@@ -776,6 +776,30 @@
  cache.trace.wont_include :miss
  end
 
+ it 'includes specified additional headers from the backend in responses to the client' do
+ count = 0
+ respond_with do |req,res|
+ count += 1
+ res['ETAG'] = '"12345"'
+ res['unique-snowflake'] = '"so-special"'
+ res.status = (count == 1) ? 200 : 304
+ end
+
+ get '/', 'rack-cache.transfer_headers' => ['unique-snowflake']
+ assert app.called?
+ assert response.ok?
+ response.headers.must_include 'unique-snowflake'
+ response['unique-snowflake'].must_equal '"so-special"'
+ cache.trace.must_include :miss
+
+ get '/', 'rack-cache.transfer_headers' => ['unique-snowflake']
+ assert app.called?
+ assert response.ok?
+ response.headers.must_include 'unique-snowflake'
+ response['unique-snowflake'].must_equal '"so-special"'
+ cache.trace.must_include :valid
+ end
+
  it 'replaces cached responses when validation results in non-304 response' do
  timestamp = Time.now.httpdate
  count = 0