extend FFI::Library doesn't extend some methods into module

[deepj]

There is no issue under MRI. After initializing the module, TestModule': undefined method 'callback' for TestModule:Module (NoMethodError) exception is occurred. The code below tries to call this method https://www.rubydoc.info/github/ffi/ffi/FFI/Library#callback-instance_method

require 'ffi'

module TestModule
  extend FFI::Library

  callback :g_log_func, [:string, :int, :string, :pointer], :void
end

TestModule

This is a small reproducible example of issue comes from https://github.com/libvips/ruby-vips/blob/428d5c106fb4a64521159c6b0c4f9391eea2eba6/lib/vips.rb#L36

Error:

ruby-ffi.rb:6:in `TestModule': undefined method `callback' for TestModule:Module (NoMethodError)
	from ruby-ffi.rb:3:in `<main>'

[deepj]

It seems to be similar to #1524

[eregon]

Our current implementation of FFI (using Truffle NFI) is not complete, so this means we did not implement that method yet.
It's an open question currently whether we can and how to reuse some of the upstream FFI code (we likely wouldn't want a second libffi copy, so using Truffle NFI would be better).

[deepj]

@eregon Are you sure? Because in case of ruby-vips, the ffi is installed via Bundler, and ruby-vips specs are run via bundle exec rake spec. So I guess, the ffi is used.

[deepj]

In fact I've seen more issues like undefined method 'callback' due to some issue in extending classes/modules in TruffleRuby.

[eregon]

@deepj The ffi gem is installed, but it no-ops when require-d: https://github.com/ffi/ffi/blob/0ee4f7931bb56c7d0e11ef66c51711cb59a80bc4/lib/ffi.rb#L12

[deepj]

@eregon I see, got it.

[eregon]

This is also an issue for ethon (#1524) and sassc.
Related issue in upstream FFI: ffi/ffi#660
In short, I plan to do a pure-Ruby implementation of FFI using Truffle NFI as backend.

[deepj]

@eregon @chrisseaton if there would be a chance to move FFI compability forward it could help to run more native gems on TruffleRuby. RC13/14?

[chrisseaton]

I think we're unlikely to fix this one as fast as the other priority issues, I'm afraid. It's going to be a bit more work.

[chrisseaton]

I'm going to remove the priority tag on this, as we aren't going to be able to solve it in the next month or so, which is what we're aiming at with priority issues now.

[eregon]

I fixed this in 59fb511 and added complete support for the FFI, with a pure-Ruby implementation using Truffle NFI.
It will be in the next release (1.0.0-rc16).
Thank you for the report.