FFI::Struct#layout fails when mkmf is required

[eregon]

$ ruby -rmkmf -rffi -e 'module A; class A < FFI::Struct; layout :a, :int; end; end' |& cat
/home/eregon/.rubies/ruby-2.6.6/lib/ruby/2.6.0/mkmf.rb:1255:in `find_type': wrong number of arguments (given 1, expected 2+) (ArgumentError)
	from /home/eregon/.rubies/ruby-2.6.6/lib/ruby/gems/2.6.0/gems/ffi-1.12.2/lib/ffi/struct.rb:271:in `find_type'
	from /home/eregon/.rubies/ruby-2.6.6/lib/ruby/gems/2.6.0/gems/ffi-1.12.2/lib/ffi/struct.rb:265:in `find_field_type'
	from /home/eregon/.rubies/ruby-2.6.6/lib/ruby/gems/2.6.0/gems/ffi-1.12.2/lib/ffi/struct.rb:305:in `array_layout'
	from /home/eregon/.rubies/ruby-2.6.6/lib/ruby/gems/2.6.0/gems/ffi-1.12.2/lib/ffi/struct.rb:217:in `layout'
	from -e:1:in `<class:A>'
	from -e:1:in `<module:A>'
	from -e:1:in `<main>'

This is due to

ffi/lib/ffi/struct.rb

Line 247 in 33fb45a

(mod < FFI::Library || mod < FFI::Struct || mod.respond_to?(:find_type)) ? mod : nil

and mkmf defining a public find_type on every object by include MakeMakefile 😞

On TruffleRuby, the socket stdlib uses FFI, so this bug ends up failing to install some gems that require socket (indirectly) in extconf.rb, see oracle/truffleruby#1896

[eregon]

I filed an issue for mkmf too since it doesn't seem reasonable to have so many public methods:
https://bugs.ruby-lang.org/issues/16896

But we'll need a workaround in FFI, as old releases won't be fixed.

[eregon]

Code there is related to 510c99c/#256 and b49cfae.
It's not clear why FFI ever checked for respond_to?(:find_type) but that seems quite brittle as this bug shows.

[eregon]

I think the actual intention was to restrict to FFI::Library or FFI::Struct, so the condition should be:

if mod.respond_to?(:find_type) && (mod.is_a?(FFI::Library) || mod < FFI::Struct)
  mod
end

This works and passes all specs.

Another possibility is to check arity, but that's less reliable and not pretty: https://gist.github.com/eregon/61ff7e20aaaa5af35a3e6401603d1f7f

[eregon]

PR to fix this: #777

[larskanis]

I think the actual intention was to restrict to FFI::Library or FFI::Struct

The intention behind respond_to?(:find_type) was maybe to also allow alternative implementations of find_type, but since there's no test case for something like this and I don't remember of anything like this in other projects, I think it's best to make it an AND operation.