From: Paul Brannan <pbrannan@...>
Date: 2008-11-11T22:59:27+09:00
Subject: [ruby-core:19821] Re: use of require thread safety

On Tue, Nov 11, 2008 at 10:51:45AM +0900, Nobuyoshi Nakada wrote:
> Hi,
> 
> At Sat, 8 Nov 2008 09:55:59 +0900,
> Roger Pack wrote in [ruby-core:19731]:
> > I'm sure this has been discussed before, but...should there be
> > anything to help in the case that two threads require the same file at
> > the same time [i.e. the second thread requires a file while a first is
> > still loading it and thus continues assuming the file was successfully
> > loaded]?
> 
> While a thread is requiring a given file, another thread which
> requires the same file will be blocked until the former thread
> finishes loading, and the result will be false.

As Charlie pointed out, this behavior can cause a deadlock:

  - Thread 1 requires a.rb
  - Thread 2 requires b.rb
  - Thread 2 requires a.rb and blocks
  - Thread 1 requires b.rb and blocks

[pbrannan@zem tmp]$ cat test.rb
t1 = Thread.new { require 'a' }
t2 = Thread.new { require 'b' }
t1.join
t2.join
[pbrannan@zem tmp]$ cat a.rb
puts "#{Thread.current}: a.rb"
sleep 1
puts "#{Thread.current}: requiring b"
require 'b'
[pbrannan@zem tmp]$ cat b.rb
puts "#{Thread.current}: b.rb"
puts "#{Thread.current}: requiring a"
require 'a'
[pbrannan@zem tmp]$ ruby1.9 test.rb
#<Thread:0x81fe458>: a.rb
#<Thread:0x81fe3e0>: b.rb
#<Thread:0x81fe3e0>: requiring b
#<Thread:0x81fe458>: requiring a
test.rb:3:in `join': Interrupt
        from test.rb:3:in `<main>'

Paul