Avoid waking all threads when only one is required.

asio/include/asio/detail/impl/win_event.ipp

@@ -29,16 +29,32 @@ namespace asio {
 namespace detail {
 
 win_event::win_event()
-  : event_(::CreateEvent(0, true, false, 0)),
-    state_(0)
+  : state_(0)
 {
-  if (!event_)
+  events_[0] = ::CreateEvent(0, true, false, 0);
+  if (!events_[0])
   {
     DWORD last_error = ::GetLastError();
     asio::error_code ec(last_error,
         asio::error::get_system_category());
     asio::detail::throw_error(ec, "event");
   }
+
+  events_[1] = ::CreateEvent(0, false, false, 0);
+  if (!events_[1])
+  {
+    DWORD last_error = ::GetLastError();
+    ::CloseHandle(events_[0]);
+    asio::error_code ec(last_error,
+        asio::error::get_system_category());
+    asio::detail::throw_error(ec, "event");
+  }
+}
+
+win_event::~win_event()
+{
+  ::CloseHandle(events_[0]);
+  ::CloseHandle(events_[1]);
 }
 
 } // namespace detail

asio/include/asio/detail/win_event.hpp

@@ -36,10 +36,7 @@ public:
   ASIO_DECL win_event();
 
   // Destructor.
-  ~win_event()
-  {
-    ::CloseHandle(event_);
-  }
+  ASIO_DECL ~win_event();
 
   // Signal all waiters.
   template <typename Lock>
@@ -48,7 +45,7 @@ public:
     ASIO_ASSERT(lock.locked());
     (void)lock;
     state_ |= 1;
-    ::SetEvent(event_);
+    ::SetEvent(events_[0]);
   }
 
   // Unlock the mutex and signal one waiter.
@@ -60,7 +57,7 @@ public:
     bool have_waiters = (state_ > 1);
     lock.unlock();
     if (have_waiters)
-      ::SetEvent(event_);
+      ::SetEvent(events_[1]);
   }
 
   // If there's a waiter, unlock the mutex and signal it.
@@ -72,7 +69,7 @@ public:
     if (state_ > 1)
     {
       lock.unlock();
-      ::SetEvent(event_);
+      ::SetEvent(events_[1]);
       return true;
     }
     return false;
@@ -84,7 +81,7 @@ public:
   {
     ASIO_ASSERT(lock.locked());
     (void)lock;
-    ::ResetEvent(event_);
+    ::ResetEvent(events_[0]);
     state_ &= ~std::size_t(1);
   }
 
@@ -97,14 +94,14 @@ public:
     {
       state_ += 2;
       lock.unlock();
-      ::WaitForSingleObject(event_, INFINITE);
+      ::WaitForMultipleObjects(2, events_, false, INFINITE);
       lock.lock();
       state_ -= 2;
     }
   }
 
 private:
-  HANDLE event_;
+  HANDLE events_[2];
   std::size_t state_;
 };