There seems to be an issue with older versions of FFI missing the size_limit? method. Will check if there's an alternative way to determine the size.

Perhaps use respond_to?? You could check size as well.

@jcupitt yes we could check the whether the size of the pointer equals the "magic" value. That's what size_limit? does here: https://github.com/ffi/ffi/blob/a7f4c951f5004c338fbae237bbae69fbb6ca66c6/lib/ffi/abstract_memory.rb#L41

This helper method was introduced with FFI v1.14.0 but the concept of setting size to LONG_MAX if the memory size is unknown goes way back even before v1.0.0 of FFI.

I'd suggest to define a private constant somewhere as FFI does, i.e INVALID_POINTER_SIZE = FFI::Pointer.new(1).size but where should I define this constant, would Vips::Image be a good place?

Yes, I guess Vips::Image would be OK, though I think you're probably a more experienced rubyist than me, so I'm very happy to leave the choice to you.

@jcupitt okay I added a private constant to the Vips::Image class. All CRuby specs are passing now, JRuby has a custom FFI implementation which needs some further investigation.

@jcupitt the problem seems to be in JRuby's FFI implementation when casting a MemoryPointer to a Pointer it looses the size information. I just added a call to .slice in the specs if used with JRuby.

JRuby

irb(main):002:0> memory = FFI::MemoryPointer.new(:uchar, 1024)
=> #<FFI::MemoryPointer address=0x7fb823ced800 size=1024>
irb(main):003:0> memory.size
=> 1024
irb(main):004:0> ptr = FFI::Pointer.new(memory)
=> #<FFI::Pointer address=0x7fb823ced800>
irb(main):005:0> ptr.size
=> 9223372036854775807

CRuby

irb(main):003:0> memory = FFI::MemoryPointer.new(:uchar, 1024)
=> #<FFI::MemoryPointer address=0x00007f860c902600 size=1024>
irb(main):004:0> memory.size
=> 1024
irb(main):005:0> ptr = FFI::Pointer.new(memory)
=> #<FFI::Pointer address=0x00007f860c902600 size=1024>
irb(main):006:0> ptr.size
=> 1024

Sorry @sled, I though this was still a WIP, I hadn't read your last comment carefully enough. I'll push 2.1.2 with this change.

Thank you for contributing this nice feature!