add autoreset_event for waling up dispatcher context

- dispatcher waits on autoreset_event if ready-queue is empty
- autoreset_event is set if a ready context is pushed to the
  ready-queue

include/boost/fiber/detail/autoreset_event.hpp

@@ -0,0 +1,60 @@
+
+//          Copyright Oliver Kowalke 2013.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_FIBERS_DETAIL_AUTORESET_EVENT_H
+#define BOOST_FIBERS_DETAIL_AUTORESET_EVENT_H
+
+#include <condition_variable>
+#include <chrono>
+#include <mutex>
+
+#include <boost/config.hpp>
+
+#include <boost/fiber/detail/config.hpp>
+
+#ifdef BOOST_HAS_ABI_HEADERS
+#  include BOOST_ABI_PREFIX
+#endif
+
+namespace boost {
+namespace fibers {
+namespace detail {
+
+class autoreset_event {
+private:
+    std::mutex              mtx_;
+    std::condition_variable cnd_;
+    bool                    flag_;
+
+public:
+    autoreset_event() :
+        mtx_(), cnd_(), flag_( false) {
+    }
+
+    autoreset_event( autoreset_event const&) = delete;
+    autoreset_event & operator=( autoreset_event const&) = delete;
+
+    void set() {
+        std::unique_lock< std::mutex > lk( mtx_);
+        flag_ = true;
+        lk.unlock();
+        cnd_.notify_all();
+    }
+
+    void reset( std::chrono::steady_clock::time_point const& time_point) {
+        std::unique_lock< std::mutex > lk( mtx_);
+        cnd_.wait_until( lk, time_point, [=](){ return flag_; });
+        flag_ = false;
+    }
+};
+
+}}}
+
+#ifdef BOOST_HAS_ABI_HEADERS
+#  include BOOST_ABI_SUFFIX
+#endif
+
+#endif // BOOST_FIBERS_DETAIL_AUTORESET_EVENT_H

include/boost/fiber/scheduler.hpp

@@ -11,6 +11,7 @@
 #include <boost/intrusive/list.hpp>
 
 #include <boost/fiber/context.hpp>
+#include <boost/fiber/detail/autoreset_event.hpp>
 #include <boost/fiber/detail/config.hpp>
 
 #ifdef BOOST_HAS_ABI_HEADERS
@@ -38,6 +39,7 @@ private:
     ready_queue_t               ready_queue_;
     terminated_queue_t          terminated_queue_;
     bool                        shutdown_;
+    detail::autoreset_event     ready_queue_ev_;
 
     void resume_( context *, context *);
 

src/scheduler.cpp

@@ -6,6 +6,8 @@
 
 #include "boost/fiber/scheduler.hpp"
 
+#include <chrono>
+
 #include <boost/assert.hpp>
 
 #include "boost/fiber/context.hpp"
@@ -35,9 +37,11 @@ scheduler::resume_( context * actx, context * ctx) {
 
 context *
 scheduler::get_next_() noexcept {
-    BOOST_ASSERT( ! ready_queue_.empty() );
-    context * ctx = & ready_queue_.front();
-    ready_queue_.pop_front();
+    context * ctx( nullptr);
+    if ( ! ready_queue_.empty() ) {
+        ctx = & ready_queue_.front();
+        ready_queue_.pop_front();
+    }
     return ctx;
 }
 
@@ -57,7 +61,8 @@ scheduler::scheduler() noexcept :
     dispatcher_ctx_(),
     ready_queue_(),
     terminated_queue_(),
-    shutdown_( false) {
+    shutdown_( false),
+    ready_queue_ev_() {
 }
 
 scheduler::~scheduler() noexcept {
@@ -99,7 +104,13 @@ void
 scheduler::dispatch() {
     while ( ! shutdown_) {
         release_terminated_();
-        auto ctx = get_next_();
+        context * ctx( nullptr);
+        // loop till we get next ready context
+        while ( nullptr == ( ctx = get_next_() ) ) {
+            // no ready context, wait till signaled
+            ready_queue_ev_.reset(
+                (std::chrono::steady_clock::time_point::max)());
+        }
         // push dispatcher context to ready-queue
         // so that ready-queue never becomes empty
         auto active_ctx = context::active();