id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
8354,Only thread blocked in io_service.run() is not processing event loop.,Tanner Sansbury <twsansbury@…>,chris_kohlhoff,"There is a concurrency issue in {{{task_io_service}}} (Boost 1.48+), that can leave a thread blocked in {{{io_service.run()}}}, waiting for its {{{wakeup_event}}} to be signaled.  If thread A invokes {{{io_service::poll()}}}, runs the reactor while thread B invokes {{{io_service.run()}}}, becoming an idle thread ({{{op_queue_}}} is empty), then if no work is ready to run, thread A {{{io_service::poll()}}} returns {{{0}}} without waking up the idle thread B.  This results in thread B remaining blocked on {{{io_service::run()}}}, but never processing the event loop.

A side-by-side visualization illustrating the problem:
{{{
          poll thread                  |          main thread
---------------------------------------+---------------------------------------
  lock()                               | 
  do_poll_one()                        |                          
  |-- pop task_operation_ from         |
  |   queue_op_                        |
  |-- unlock()                         | lock()
  |-- create task_cleanup              | do_run_one()
  |-- service reactor (non-block)      | `-- queue_op_ is empty
  |-- ~task_cleanup()                  |     |-- set thread as idle
  |   |-- lock()                       |     `-- unlock()
  |   `-- queue_op_.push(              |
  |       task_operation_)             |
  `-- task_operation_ is               | 
      queue_op_.front()                |
      `-- return 0                     | // still waiting on wakeup_event
  unlock()                             |
}}}

Attached files are:
* Patch file that modifies {{{task_io_service.ipp}}} to sleep immediately after {{{do_poll_one}}} runs the reactor.  This is only used to increase the changes of reproducing the concurrency problem.
* Test application:
  * An asynchronous work loop via a timer that will print ""."" every 3 seconds.
  * Spawn off a single thread that will poll the {{{io_service}}}.
  * Delay to allow the new thread time to poll {{{io_service}}}, and have main call {{{io_service::run()}} while the poll thread sleeps in {{{task_io_service::do_poll_one()}}}.
* Patch with suggested fix that invokes {{{wake_one_thread_and_unlock()}}} before {{{do_poll_one}}} returning {{{0}}} if no work is to be done after running the reactor.",Bugs,closed,To Be Determined,asio,Boost Development Trunk,Problem,fixed,,samjmill@… davidjoelschwartz@…