Rack::JSONBodyParser: rescue all parser exceptions

Rescue all exceptions raised by the parser as long as it is StandardError
subclass (which should be all the exceptions raised by different libraries).

This is specially needed when using a parser other than JSON. Otherwise we need
to rescue the corresponding exception and raise JSON:ParseError inside the
block.

lib/rack/contrib/json_body_parser.rb

@@ -61,7 +61,7 @@ def call(env)
 
           update_form_hash_with_json_body(env)
         end
-      rescue JSON::ParserError
+      rescue ParserError
         body = { error: 'Failed to parse body as JSON' }.to_json
         header = { 'Content-Type' => 'application/json' }
         return Rack::Response.new(body, 400, header).finish
@@ -71,13 +71,22 @@ def call(env)
 
     private
 
+    class ParserError < StandardError; end
+
     def update_form_hash_with_json_body(env)
       body = env[Rack::RACK_INPUT]
       return unless (body_content = body.read) && !body_content.empty?
 
       body.rewind # somebody might try to read this stream
+
+      begin
+        parsed_body = @parser.call(body_content)
+      rescue StandardError
+        raise ParserError
+      end
+
       env.update(
-        Rack::RACK_REQUEST_FORM_HASH => @parser.call(body_content),
+        Rack::RACK_REQUEST_FORM_HASH => parsed_body,
         Rack::RACK_REQUEST_FORM_INPUT => body
       )
     end

test/spec_rack_json_body_parser_spec.rb

@@ -72,6 +72,17 @@ def create_parser(app, **args, &block)
     _( -> { create_parser(app).call(env) }).must_raise JSON::ParserError
   end
 
+  specify "should rescue StandardError subclasses raised by the parser" do
+    class CustomParserError < StandardError; end
+
+    parser = create_parser(app) do |_body|
+      raise CustomParserError.new
+    end
+
+    res = parser.call(mock_env)
+    _(res[0]).must_equal 400
+  end
+
   describe "contradiction between body and type" do
     specify "should return bad request with a JSON-encoded error message" do
       env = mock_env(input: 'This is not JSON')