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    <p>This is a bug. I have been able to duplicate it by adding a delay into the non-critical section of <code>task_io_service::do_poll_one</code>. Here is a snippet of the modified <code>task_io_service::do_poll_one()</code> in <a href="http://svn.boost.org/svn/boost/trunk/boost/asio/detail/impl/task_io_service.ipp"><code>booost/asio/detail/impl/task_io_service.ipp</code></a>. The only line added is the sleep.</p> <pre class="lang-cpp prettyprint-override"><code>std::size_t task_io_service::do_poll_one(mutex::scoped_lock&amp; lock, task_io_service::thread_info&amp; this_thread, const boost::system::error_code&amp; ec) { if (stopped_) return 0; operation* o = op_queue_.front(); if (o == &amp;task_operation_) { op_queue_.pop(); lock.unlock(); { task_cleanup c = { this, &amp;lock, &amp;this_thread }; (void)c; // Run the task. May throw an exception. Only block if the operation // queue is empty and we're not polling, otherwise we want to return // as soon as possible. task_-&gt;run(false, this_thread.private_op_queue); boost::this_thread::sleep_for(boost::chrono::seconds(3)); } o = op_queue_.front(); if (o == &amp;task_operation_) return 0; } ... </code></pre> <p>My test driver is fairly basic:</p> <ul> <li>An asynchronous work loop via a timer that will print "." every 3 seconds.</li> <li>Spawn off a single thread that will poll the <code>io_service</code>.</li> <li>Delay to allow the new thread time to poll <code>io_service</code>, and have main call <code>io_service::run()</code> while the poll thread sleeps in <code>task_io_service::do_poll_one()</code>.</li> </ul> <p>Test code:</p> <pre class="lang-cpp prettyprint-override"><code>#include &lt;iostream&gt; #include &lt;boost/asio/io_service.hpp&gt; #include &lt;boost/asio/steady_timer.hpp&gt; #include &lt;boost/chrono.hpp&gt; #include &lt;boost/thread.hpp&gt; boost::asio::io_service io_service; boost::asio::steady_timer timer(io_service); void arm_timer() { std::cout &lt;&lt; "."; std::cout.flush(); timer.expires_from_now(boost::chrono::seconds(3)); timer.async_wait(boost::bind(&amp;arm_timer)); } int main() { // Add asynchronous work loop. arm_timer(); // Spawn poll thread. boost::thread poll_thread( boost::bind(&amp;boost::asio::io_service::poll, boost::ref(io_service))); // Give time for poll thread service reactor. boost::this_thread::sleep_for(boost::chrono::seconds(1)); io_service.run(); } </code></pre> <p>And the debug:</p> <pre>[twsansbury@localhost bug]$ gdb a.out ... (gdb) r Starting program: /home/twsansbury/dev/bug/a.out [Thread debugging using libthread_db enabled] .[New Thread 0xb7feeb90 (LWP 31892)] [Thread 0xb7feeb90 (LWP 31892) exited]</pre> <p>At this point, the <code>arm_timer()</code> has printed "." once (when it was intially armed). The poll thread serviced the reactor in a non-blocking manner, and slept for 3 seconds while <code>op_queue_</code> was empty (<code>task_operation_</code> will be added back to the <code>op_queue_</code> when <code>task_cleanup c</code> exits scope). While the <code>op_queue_</code> was empty, the main thread calls <code>io_service::run()</code>, sees the <code>op_queue_</code> is empty, and makes itself the <code>first_idle_thread_</code>, where it waits on its <code>wakeup_event</code>. The poll thread finishes sleeping, and returns <code>0</code>, leaving the main thread waiting on <code>wakeup_event</code>.</p> <p>After waiting 10~ seconds, plenty of time for the <code>arm_timer()</code> to be ready, I interrupt the debugger:</p> <pre>Program received signal SIGINT, Interrupt. 0x00919402 in __kernel_vsyscall () (gdb) bt #0 0x00919402 in __kernel_vsyscall () #1 0x0081bbc5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/libpthread.so.0 #2 0x00763b3d in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/libc.so.6 #3 0x08059dc2 in void boost::asio::detail::posix_event::wait >(boost::asio::detail::scoped_lock&) () #4 0x0805a009 in boost::asio::detail::task_io_service::do_run_one(boost::asio::detail::scoped_lock&, boost::asio::detail::task_io_service_thread_info&, boost::system::error_code const&) () #5 0x0805a11c in boost::asio::detail::task_io_service::run(boost::system::error_code&) () #6 0x0805a1e2 in boost::asio::io_service::run() () #7 0x0804db78 in main ()</pre> <hr> <p>The side-by-side timeline is as follows:</p> <pre> 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() |</pre> <hr> <p>As best as I could tell, there are no side effects by patching:</p> <pre class="lang-cpp prettyprint-override"><code>if (o == &amp;task_operation_) return 0; </code></pre> <p>to:</p> <pre class="lang-cpp prettyprint-override"><code>if (o == &amp;task_operation_) { if (!one_thread_) wake_one_thread_and_unlock(lock); return 0; } </code></pre> <p>Regardless, I have submitted a <a href="https://svn.boost.org/trac/boost/ticket/8354">bug and fix</a>. Consider keeping an eye on the ticket for an official response.</p>
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