Recently released rack 2.1.0 introduced the following unit test failures in the Ruby library.
rspec ./spec/thin_http_server_spec.rb:99 # Thrift::ThinHTTPServer::RackApplication 404 response receives a non-POST
rspec ./spec/thin_http_server_spec.rb:105 # Thrift::ThinHTTPServer::RackApplication 404 response receives a header other than application/x-thrift
rspec ./spec/thin_http_server_spec.rb:132 # Thrift::ThinHTTPServer::RackApplication 200 response status code 200
It seems rack 2.1.0 returns status code 500 in some cases, while v2.0.8 or lower returns 404 in such cases. I'm not sure if it's a bug of rack yet, but I'd like to pin its version to 2.0.8 as a workaround for the time being.