[ruby-core:46927] [ruby-trunk - Bug #6822][Assigned] Race Condition with Fiber and Process

Issue #6822 has been updated by shyouhei (Shyouhei Urabe).

Category changed from core to YARV
Status changed from Open to Assigned
Assignee set to ko1 (Koichi Sasada)


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Bug #6822: Race Condition with Fiber and Process
https://bugs.ruby-lang.org/issues/6822#change-28591

Author: MartinBosslet (Martin Bosslet)
Status: Assigned
Priority: Normal
Assignee: ko1 (Koichi Sasada)
Category: YARV
Target version: 2.0.0
ruby -v: ruby 2.0.0dev (2012-05-07 trunk 35550) [x86_64-linux]


If I run the following code

    $stdout.sync = true
    objects = [1, 2, 3]

    fiber = Fiber.new do
      loop do
        objects.each { |obj| Fiber.yield(obj) }
      end
    end

    def run(obj)
      fork do
        puts obj
      end
    end

    def on_child_exit(obj)
      begin
        while Process.wait(-1, Process::WNOHANG)
          run(obj)
        end
      rescue Errno::ECHILD
      end
    end

    trap(:CHLD) { on_child_exit(fiber.resume) }
    4.times { run(fiber.resume) }
    sleep

I get
 
    fiber_process.rb:26:in `resume': double resume (FiberError)

or

    fiber_process.rb:26:in `resume': fiber called across stack rewinding barrier (FiberError)

There is a race condition when two or more children exit. Now I know I can implement
this differently, but this still made me curious. Is this a bug? Let's say I would 
need to use a Fiber, then there is no way how I can do the synchronization manually,
or is there? Using a Mutex to synchronize the Fiber#resume will fail due to the 
non-reentrant behaviour of Mutex#lock (I'll get "in `lock': deadlock; recursive 
locking (ThreadError)"). Is there a way to do this or should Fibers not be used in
this context? 





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