Add Rack::Builder#warmup method for app preloading.

This new `warmup` method takes a block which is invoked after the app
is built. The block can be used to make mock requests that ensure
all application dependencies are loaded before the app starts
serving traffic.

With complex frameworks like Rails, many dependencies are auto-loaded
and data like mime-type and i18n is not loaded into memory by default. This
often means the first few requests handled by an application are quite
slow.

With this patch, config.ru can simply make requests via warmup to
exercise the app before it is used:

  $ tail -4 config.ru
  warmup do |app|
    client = Rack::MockRequest.new(app)
    client.get('/')
  end

2 files changed, 28 insertions, 2 deletions

diff --git a/lib/rack/builder.rb b/lib/rack/builder.rb
index 66dc7bd4..fa3a1ea9 100644
--- a/lib/rack/builder.rb
+++ b/lib/rack/builder.rb
@@ -51,7 +51,7 @@ module Rack
     end
 
     def initialize(default_app = nil,&block)
-      @use, @map, @run = [], nil, default_app
+      @use, @map, @run, @warmup = [], nil, default_app, nil
       instance_eval(&block) if block_given?
     end
 
@@ -104,6 +104,19 @@ module Rack
       @run = app
     end
 
+    # Takes a lambda or block that is used to warm-up the application.
+    #
+    #   warmup do |app|
+    #     client = Rack::MockRequest.new(app)
+    #     client.get('/')
+    #   end
+    #
+    #   use SomeMiddleware
+    #   run MyApp
+    def warmup(prc=nil, &block)
+      @warmup = prc || block
+    end
+
     # Creates a route within the application.
     #
     #   Rack::Builder.app do
@@ -131,7 +144,9 @@ module Rack
     def to_app
       app = @map ? generate_map(@run, @map) : @run
       fail "missing run or map statement" unless app
-      @use.reverse.inject(app) { |a,e| e[a] }
+      app = @use.reverse.inject(app) { |a,e| e[a] }
+      @warmup.call(app) if @warmup
+      app
     end
 
     def call(env)
diff --git a/test/spec_builder.rb b/test/spec_builder.rb
index 0774f597..20ea6681 100644
--- a/test/spec_builder.rb
+++ b/test/spec_builder.rb
@@ -130,6 +130,17 @@ describe Rack::Builder do
     Rack::MockRequest.new(app).get("/foo").should.be.server_error
   end
 
+  it "yields the generated app to a block for warmup" do
+    warmed_up_app = nil
+
+    app = Rack::Builder.new do
+      warmup { |a| warmed_up_app = a }
+      run lambda { |env| [200, {}, []] }
+    end.to_app
+
+    warmed_up_app.should.equal app
+  end
+
   should "initialize apps once" do
     app = builder do
       class AppClass