Yes, we use fastthread if the Ruby interpreter's version is < 1.9.
Fastthread actually is required for 1.8.6 because we found a few
threading bugs while developing Phusion Passenger, which were fixed by
loading fastthread.
I see that you're using fastthread 1.0.4. 1.0.4 was recently released
(like a month ago or so) and there's a bug in that version related to
rb_thread_alive_p, as shown in your output. The latest version of
fastthread is 1.0.6; you should upgrade.
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I think that at the time you installed 2.1.2, fastthread was still at
version 1.0.0 (it had been in 1.0.0 for a long time). Then 1.0.4 with
the bug was released, and then you installed Phusion Passenger 2.1.3
before fastthread 1.0.6 was released.
After some investigation, it turns out that in 1.8.6-p0 doesn't
publish rb_thread_alive_p to extensions, i.e. it's a static function.
After digging through the code history, it was revealed that
rb_thread_alive_p was made non-static since 1.8.6-p31.
Fastthread's author has just released version 1.0.7 which fixes this problem.
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Phusion | The Computer Science Company
Web: http://www.phusion.nl/
E-mail: in...@phusion.nl