2 files changed, 40 insertions, 1 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/cookies.rb b/actionpack/lib/action_dispatch/middleware/cookies.rb
index 8c4615c0c1..a4ffd40a66 100644
--- a/actionpack/lib/action_dispatch/middleware/cookies.rb
+++ b/actionpack/lib/action_dispatch/middleware/cookies.rb
@@ -174,7 +174,7 @@ module ActionDispatch
           options = { :value => value }
         end
 
-        value = @cookies[key.to_s] = value
+        @cookies[key.to_s] = value
 
         handle_options(options)
 
diff --git a/actionpack/lib/action_dispatch/middleware/request_id.rb b/actionpack/lib/action_dispatch/middleware/request_id.rb
new file mode 100644
index 0000000000..bee446c8a5
--- /dev/null
+++ b/actionpack/lib/action_dispatch/middleware/request_id.rb
@@ -0,0 +1,39 @@
+require 'securerandom'
+require 'active_support/core_ext/string/access'
+require 'active_support/core_ext/object/blank'
+
+module ActionDispatch
+  # Makes a unique request id available to the action_dispatch.request_id env variable (which is then accessible through
+  # ActionDispatch::Request#uuid) and sends the same id to the client via the X-Request-Id header.
+  #
+  # The unique request id is either based off the X-Request-Id header in the request, which would typically be generated
+  # by a firewall, load balancer, or the web server, or, if this header is not available, a random uuid. If the
+  # header is accepted from the outside world, we sanitize it to a max of 255 chars and alphanumeric and dashes only.
+  #
+  # The unique request id can be used to trace a request end-to-end and would typically end up being part of log files
+  # from multiple pieces of the stack.
+  class RequestId
+    def initialize(app)
+      @app = app
+    end
+
+    def call(env)
+      env["action_dispatch.request_id"] = external_request_id(env) || internal_request_id
+      status, headers, body = @app.call(env)
+
+      headers["X-Request-Id"] = env["action_dispatch.request_id"]
+      [ status, headers, body ]
+    end
+
+    private
+      def external_request_id(env)
+        if request_id = env["HTTP_X_REQUEST_ID"].presence
+          request_id.gsub(/[^\w\-]/, "").first(255)
+        end
+      end
+
+      def internal_request_id
+        SecureRandom.hex(16)
+      end
+  end
+end