Respond with a nice 400 if we cannot parse the JSON request

lib/api/errors/parse_error.rb

@@ -0,0 +1,46 @@
+#-- encoding: UTF-8
+#-- copyright
+# OpenProject is a project management system.
+# Copyright (C) 2012-2014 the OpenProject Foundation (OPF)
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See doc/COPYRIGHT.rdoc for more details.
+#++
+
+module API
+  module Errors
+    class ParseError < InvalidRequestBody
+      def initialize(message)
+        super 'The request body was neither empty, nor did it contain a single JSON object.'
+
+        @details = { parseError: clean_parse_error(message) }
+      end
+
+      private
+
+      def clean_parse_error(message)
+        message.gsub(/\s?\[parse.c\:\d+\]/, '')
+      end
+    end
+  end
+end

lib/api/root.rb

@@ -41,11 +41,24 @@ module API
       end
     end
 
+    class Parser
+      def call(object, _env)
+        MultiJson.load(object)
+      rescue MultiJson::ParseError => e
+        error = ::API::Errors::ParseError.new(e.message)
+        representer = ::API::V3::Errors::ErrorRepresenter.new(error)
+
+        throw :error, status: 400, message: representer.to_json
+      end
+    end
+
     content_type 'hal+json', 'application/hal+json; charset=utf-8'
     content_type :json,      'application/json; charset=utf-8'
     format 'hal+json'
     formatter 'hal+json', Formatter.new
 
+    parser :json, Parser.new
+
     helpers do
       def current_user
         return User.current if Rails.env.test?