last_modified response helper / conditional GET support

ResponseHelpers#last_modified sets the Last-Modified response header
and (potentially) halts execution if an If-Modified-Since request
header is present and matches the time given.

lib/sinatra.rb

@@ -440,6 +440,25 @@ module Sinatra
       response.header['Content-Type'] = type
     end
 
+    # Set the last modified time of the resource (HTTP 'Last-Modified' header)
+    # and halt if conditional GET matches. The +time+ argument is a Time,
+    # DateTime, or other object that responds to +to_time+.
+    #
+    # When the current request includes an 'If-Modified-Since' header that
+    # matches the time specified, execution is immediately halted with a
+    # '304 Not Modified' response.
+    #
+    # Calling this method before perfoming heavy processing (e.g., lengthy
+    # database queries, template rendering, complex logic) can dramatically
+    # increase overall throughput with caching clients.
+    def last_modified(time)
+      time = time.to_time if time.respond_to?(:to_time)
+      time = time.httpdate if time.respond_to?(:httpdate)
+      response.header['Last-Modified'] = time
+      throw :halt, 304 if time == request.env['HTTP_IF_MODIFIED_SINCE']
+      time
+    end
+
   end
 
   module RenderingHelpers

test/app_test.rb

@@ -135,6 +135,23 @@ context "Sinatra" do
     body.should.equal '<feed></feed>'
   end
 
+  specify "supports conditional GETs with last_modified" do
+    modified_at = Time.now
+    get '/maybe' do
+      last_modified modified_at
+      'response body, maybe'
+    end
+
+    get_it '/maybe'
+    should.be.ok
+    body.should.equal 'response body, maybe'
+
+    get_it '/maybe', {},
+      'HTTP_IF_MODIFIED_SINCE' => modified_at.httpdate
+    status.should.equal 304
+    body.should.equal ''
+  end
+
   specify "delegates HEAD requests to GET handlers" do
     get '/invisible' do
       "I am invisible to the world"