Backport #3639: backport r27401 (preserve errno when releasing GVL)
http://redmine.ruby-lang.org/issues/show/3639

Author: Eric Wong
Status: Open, Priority: Low
Category: core

Not sure if there's going to be another 1.9.1 release, but when using
IO#read_nonblock, I've seen rb_bug() from rb_sys_fail() because errno == 0 under 1.9.1.

So r27401 from 1.9.2/trunk should fix the issue, but I'll probably abandon 1.9.1 for
1.9.2 soon anyways.


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