Issue #12560 has been reported by Yui NARUSE.

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Bug #12560: backport r55602
https://bugs.ruby-lang.org/issues/12560

* Author: Yui NARUSE
* Status: Closed
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.4.0dev (2016-07-07 trunk 55601) [x86_64-freebsd10.3]
* Backport: 2.1: UNKNOWN, 2.2: REQUIRED, 2.3: REQUIRED
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They may fail parallel test-all.

* ruby_2_3: https://travis-ci.org/ruby/ruby/builds/141708115
* ruby_2_2: https://travis-ci.org/ruby/ruby/builds/142772798



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