in_http: Implement support for CORS preflight requests

To receive a 'application/json' request from another origin, we need
to be able to handle CORS preflight requests. Otherwise, browsers will
refuse to interact with us.

https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#Preflighted_requests

This patch adds support for the feature.

Signed-off-by: Fujimoto Seiji <fujimoto@clear-code.com>

lib/fluent/plugin/in_http.rb

@@ -342,6 +342,10 @@ def on_headers_complete(headers)
             # For multiple X-Forwarded-For headers. Use first header value.
             v = v.first if v.is_a?(Array)
             @remote_addr = v.split(",").first
+          when /Access-Control-Request-Method/i
+            @access_control_request_method = v
+          when /Access-Control-Request-Headers/i
+            @access_control_request_headers = v
           end
         }
         if expect
@@ -367,9 +371,43 @@ def on_body(chunk)
         @body << chunk
       end
 
+      # Web browsers can send an OPTIONS request before performing POST
+      # to check if cross-origin requests are supported.
+      def handle_options_request
+        # Is CORS enabled in the first place?
+        if @cors_allow_origins.nil?
+          return send_response_and_close("403 Forbidden", {}, "")
+        end
+
+        # in_http does not support HTTP methods except POST
+        if @access_control_request_method != 'POST'
+          return send_response_and_close("403 Forbidden", {}, "")
+        end
+
+        header = {
+          "Access-Control-Allow-Methods": "POST",
+          "Access-Control-Allow-Headers": @access_control_request_headers || "",
+        }
+
+        # Check the origin and send back a CORS response
+        if @cors_allow_origins.include?('*')
+          header["Access-Control-Allow-Origin"] = "*"
+          send_response_and_close("200 OK", header, "")
+        elsif @cors_allow_origins.include?(@origin)
+          header["Access-Control-Allow-Origin"] = @origin
+          send_response_and_close("200 OK", header, "")
+        else
+          send_response_and_close("403 Forbidden", {}, "")
+        end
+      end
+
       def on_message_complete
         return if closing?
 
+        if @parser.http_method == 'OPTIONS'
+          return handle_options_request()
+        end
+
         # CORS check
         # ==========
         # For every incoming request, we check if we have some CORS

test/plugin/test_in_http.rb

@@ -624,6 +624,23 @@ def test_cors_allowed_wildcard
     end
   end
 
+  def test_cors_preflight
+    d = create_driver(CONFIG + 'cors_allow_origins ["*"]')
+
+    d.run do
+      header = {
+        "Origin" => "http://foo.com",
+        "Access-Control-Request-Method" => "POST",
+        "Access-Control-Request-Headers" => "Content-Type",
+      }
+      res = options("/cors.test", {}, header)
+
+      assert_equal "200", res.code
+      assert_equal "*", res["Access-Control-Allow-Origin"]
+      assert_equal "POST", res["Access-Control-Allow-Methods"]
+    end
+  end
+
   def test_content_encoding_gzip
     d = create_driver
 
@@ -744,6 +761,12 @@ def test_if_content_type_is_initialized_properly
     assert_equal $test_in_http_connection_object_ids[0], $test_in_http_connection_object_ids[1]
   end
 
+  def options(path, params, header = {})
+    http = Net::HTTP.new("127.0.0.1", PORT)
+    req = Net::HTTP::Options.new(path, header)
+    http.request(req)
+  end
+
   def post(path, params, header = {}, &block)
     http = Net::HTTP.new("127.0.0.1", PORT)
     req = Net::HTTP::Post.new(path, header)