Introspection fails when called multiple times in threads

Start by running the provided multiple_dbus_calls.rb reproducer and inspect the reported paths in lib/dbus/bus.rb, lib/dbus/message_queue.rb, lib/dbus/marshall.rb, and lib/dbus/proxy_object.rb. Trace concurrent introspection through send_sync and message parsing; done means the reproducer no longer raises DBus::InvalidPacketException under multiple threads, with coverage for the failure.

I am trying to build a Rails application that regularly checks network status, Systemd service status etc. showing them on the UI. I need to use Puma which works with threads. When the application makes calls to DBus we start getting DBus::InvalidPacketException over and over during introspection. I am not sure if it is because of using threads, or if threads just makes it easier to reproduce (ex. calling DBus too fast).

I see it happen on OpenSUSE 42.3 and Ubuntu 16.04. Happens with Ruby 2.4.3 and 2.3.6, did not try other Ruby versions. This is reproducable with 2 threads too. Systemd is just an example, issue happens when calling NetworkManager too.

require 'dbus'

PARALLEL = 5
UNIT_NAME = 'ruby_dbus_test.service'

def local_service_unit_is_active(k)
  puts "current thread #{Thread.current[:num]}\n"
  sys_bus = DBus.system_bus
  core_service = sys_bus.service('org.freedesktop.systemd1')
  obj = core_service.object('/org/freedesktop/systemd1')
  obj.default_iface = 'org.freedesktop.systemd1'

  puts "before obj.introspect #{Thread.current[:num]}\n"
  obj.introspect
  puts "after obj.introspect #{Thread.current[:num]}\n"
  obj
end

def build_thread(k)
  Thread.new do
    Thread.current.abort_on_exception = true
    Thread.current[:num] = k

    resp = local_service_unit_is_active(UNIT_NAME)
    puts "\nFINISHED: result for #{k} active: #{resp.bus.inspect}\n"
  end
end

threads = []

PARALLEL.times do |k|
  puts "building thread #{k + 1} out of #{PARALLEL}"
  threads << build_thread(k)
end

threads.each(&:join)

and the errors I get changes time to time but they all look like introspection data is not correct so it is not parsed properly

/home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:183:in `do_parse': DBus::InvalidPacketException (DBus::InvalidPacketException)
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:62:in `block in unmarshall'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:61:in `each'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:61:in `unmarshall'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/message.rb:201:in `unmarshall_buffer'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/message_queue.rb:139:in `message_from_buffer_nonblock'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/message_queue.rb:38:in `pop'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/bus.rb:441:in `wait_for_message'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/bus.rb:458:in `send_sync'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/bus.rb:339:in `send_sync_or_async'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/bus.rb:365:in `introspect_data'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/proxy_object.rb:73:in `introspect'
        from ./multiple_dbus_calls.rb:18:in `local_service_unit_is_active'
        from ./multiple_dbus_calls.rb:28:in `block in build_thread'

/home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:103:in `read_string': String is not nul-terminated (DBus::InvalidPacketException)
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:212:in `do_parse'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:62:in `block in unmarshall'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:61:in `each'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/marshall.rb:61:in `unmarshall'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/message.rb:227:in `unmarshall_buffer'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/message_queue.rb:139:in `message_from_buffer_nonblock'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/message_queue.rb:38:in `pop'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/bus.rb:441:in `wait_for_message'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/bus.rb:453:in `send_sync'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/bus.rb:339:in `send_sync_or_async'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/bus.rb:365:in `introspect_data'
        from /home/zaburt/.rvm/gems/ruby-2.4.3@dbus_test/gems/ruby-dbus-0.14.1/lib/dbus/proxy_object.rb:73:in `introspect'
        from ./multiple_dbus_calls.rb:18:in `local_service_unit_is_active'
        from ./multiple_dbus_calls.rb:28:in `block in build_thread'

You can find the sample script to reproduce the issue (no Rails, just Ruby) and some info here too https://github.com/zaburt/ruby-dbus-test . I can try out any test code or suggestions.