jruby: protobuf fields should be defined as methods

[HoneyryderChuck]

What version of protobuf and what language are you using?
Version: 3.19.1
Language: Ruby (JRuby 9.3)

What operating system (Linux, Windows, ...) and version?

Darwin.

What did you do?

First, thanks a lot for reenabling jruby support 🙏

I have code which checks whether a certain field exists.I use the idiomatic .respond_to? in CRuby. Things aren't working the same way in JRuby however.

What did you expect to see

I'd expect the check to work. To compare, here's how it works in CRuby (v3.0):

# given a Google::Protobuf::Duration message
message.respond_to?(:seconds) #=> true
# although the method isn't defined
message.methods - Object.instance_methods #=>  [:to_f, :method_missing, :[], :[]=, :to_h, :to_json, :to_proto]

What did you see instead?

In Jruby, I don't get the same result for the respond_to? call, even if I can call .seconds.

message.respond_to?(:seconds) #=> false
message.seconds #=> 123

I suspect this happens because .seconds gets sent to method_missing, which then infers the value of the field; and the respective .respond_to_missing? method isn't defined, which makes my example break.

[JasonLunn]

Thanks for filing this issue, @HoneyryderChuck. Investigating.

[JasonLunn]

Progress update:

This turns out to be a known issue. There is already a test that covers this issue, and it is disabled on JRuby at the moment (and has been for over 5 years); see https://github.com/protocolbuffers/protobuf/blob/master/ruby/tests/basic.rb#L621

I've attempted to create a fix that would allow me to reenable the test for JRuby, but the issue with JRuby itself seems to still exist. Today I reduced the issue to a reproducible and repeatable failure care and opened #9668 for JRuby.

[JasonLunn]

Marking this issue blocked until #9668 is resolved.

[JasonLunn]

Workaround implemented in #9677

[JasonLunn]

Fix merged.