Fiber#transfer gets confused about thread owners

[chuckremes]

1. What command did you run?

I ran https://gist.github.com/chuckremes/14c9ef06efc0bc62b9dc5b588c06719f

2. What behavior did you expect?

I expected all 8 steps to execute TWICE.

3. What behavior did you get instead?

I observed https://gist.github.com/chuckremes/14c9ef06efc0bc62b9dc5b588c06719f

Main thread executed the 8 steps correctly. Second thread died halfway through.

4. What version of Rubinius?

3.86c109

5. What version of operating system?

OSX 10.12.6

7. How did you build your version of Rubinius?

• I used RVM.
• I used ruby-build.
• I used ruby-install.
• I built manually from a tarball.
• I built manually from a git clone.
• I installed a binary.

8. Does this issue involve proprietary code?

• Yes, this issue involves proprietary code that I cannot share.
• Yes, this issue involves proprietary code, but I am able to share it under certain conditions.
• No, this issue doesn't involve proprietary code.

9. Are you able to help us debug the issue?

• Yes, I'm able to help debug, including running commands under lldb.
• No, I'm not able help debug or I don't have time to help.

[chuckremes]

Spec added to ruby/spec here: ruby/spec#579