From c03d1c273622c2a6de263f246bd5718eec08fb1f Mon Sep 17 00:00:00 2001
From: akr <akr@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Mon, 2 Jul 2012 13:15:29 +0000
Subject: * thread.c (rb_thread_aref): add explanation for why Thread#[] and  
 Thread#[]= are fiber-local and not thread-local.   reported by Julien A. 
 [ruby-core:41606] [ruby-trunk - Bug #5750]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36269 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 thread.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

(limited to 'thread.c')

diff --git a/thread.c b/thread.c
index 279dc81aea..5c1eabb223 100644
--- a/thread.c
+++ b/thread.c
@@ -2078,6 +2078,39 @@ rb_thread_local_aref(VALUE thread, ID id)
  *     #<Thread:0x00000002a54220 dead>: A
  *     #<Thread:0x00000002a541a8 dead>: B
  *     #<Thread:0x00000002a54130 dead>: C
+ *
+ *  Thread#[] and Thread#[]= are not thread-local but fiber-local.
+ *  This confusion was not exist until Ruby 1.8 because
+ *  fiber is available since Ruby 1.9.
+ *  Ruby 1.9 chooses that the methods behaves fiber-local to save
+ *  following idiom for dynamic scope.
+ *
+ *    def meth(newvalue)
+ *      begin
+ *        oldvalue = Thread.current[:name]
+ *        Thread.current[:name] = newvalue
+ *        yield
+ *      ensure
+ *        Thread.current[:name] = oldvalue
+ *      end
+ *    end
+ *
+ *  The idiom may not work as dynamic scope if the methods are thread-local
+ *  and a given block switches fiber.
+ *
+ *    f = Fiber.new {
+ *      meth(1) {
+ *        Fiber.yield
+ *      }
+ *    }
+ *    meth(2) {
+ *      f.resume
+ *    }
+ *    f.resume
+ *    p Thread.current[:name]
+ *    #=> nil if fiber-local
+ *    #=> 2 if thread-local (The value 2 is leaked to outside of meth method.)
+ *
  */
 
 static VALUE
-- 
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