Reflect Input Request IDs

reflect-request-ids.diff

diff --git a/lib/api/middleware/request_id.rb b/lib/api/middleware/request_id.rb
index 9367bf7..255f4e4 100644
--- a/lib/api/middleware/request_id.rb
+++ b/lib/api/middleware/request_id.rb
@@ -8,7 +8,8 @@ def initialize(app)
     end
 
     def call(env)
-      request_ids = [SecureRandom.uuid] + extract_request_ids(env)
+      extracted_request_ids = extract_request_ids(env)
+      request_ids = [SecureRandom.uuid] + extracted_request_ids
 
       # make ID of the request accessible to consumers down the stack
       env["REQUEST_ID"] = request_ids[0]
@@ -19,8 +20,15 @@ def call(env)
 
       status, headers, response = @app.call(env)
 
-      # tag all responses with a request ID
-      headers["Request-Id"] = request_ids[0]
+      # Tag all responses with a request ID.
+      #
+      # If valid request IDs were sent in from a client, pass those back.
+      # Otherwise, give them the default generated request ID.
+      headers["Request-Id"] = if extracted_request_ids.size > 0
+        extracted_request_ids.join(",")
+      else
+        request_ids[0]
+      end
 
       [status, headers, response]
     end
diff --git a/spec/middleware/request_id_spec.rb b/spec/middleware/request_id_spec.rb
new file mode 100644
index 0000000..1b78ad7
--- /dev/null
+++ b/spec/middleware/request_id_spec.rb
@@ -0,0 +1,49 @@
+require "spec_helper"
+
+describe API::Middleware::RequestID do
+  include Rack::Test::Methods
+
+  def app
+    Rack::Builder.new do
+      use Rack::Lint
+      use API::Middleware::RequestID
+      run Sinatra.new {
+        get "/" do
+          MultiJson.encode(env["REQUEST_IDS"])
+        end
+      }
+    end
+  end
+
+  it "generates a request ID for requests" do
+    get "/"
+    request_ids = MultiJson.decode(last_response.body)
+    assert_equal 1, request_ids.count
+    assert_match API::Middleware::RequestID::UUID_PATTERN, request_ids[0]
+  end
+
+  it "sets a request ID in the Request-Id response header" do
+    get "/"
+    assert_match API::Middleware::RequestID::UUID_PATTERN,
+      last_response.headers["Request-Id"]
+  end
+
+  it "allows request IDs to be passed into requests" do
+    input_ids = [SecureRandom.uuid, SecureRandom.uuid]
+    input = input_ids.join(",")
+    header "Request-Id", input
+    get "/"
+    request_ids = MultiJson.decode(last_response.body)
+    assert_equal 3, request_ids.count
+    assert_includes request_ids, input_ids[0]
+    assert_includes request_ids, input_ids[1]
+  end
+
+  it "sets input request IDs in the Request-Id response header" do
+    input_ids = [SecureRandom.uuid, SecureRandom.uuid]
+    input = input_ids.join(",")
+    header "Request-Id", input
+    get "/"
+    assert_equal input, last_response.headers["Request-Id"]
+  end
+end