Add ability to specify the level of concurrency desired, which is currently

used by Win32's I/O completion ports.

asio/include/asio/detail/task_io_service.hpp

@@ -47,6 +47,10 @@ public:
   {
   }
 
+  void init(size_t concurrency_hint)
+  {
+  }
+
   // Destroy all user-defined handler objects owned by the service.
   void shutdown_service()
   {

asio/include/asio/detail/win_iocp_io_service.hpp

@@ -47,11 +47,17 @@ public:
   // Constructor.
   win_iocp_io_service(asio::io_service& io_service)
     : asio::io_service::service(io_service),
-      iocp_(::CreateIoCompletionPort(INVALID_HANDLE_VALUE, 0, 0, 0)),
+      iocp_(),
       outstanding_work_(0),
       interrupted_(0),
       shutdown_(0)
   {
+  }
+
+  void init(size_t concurrency_hint)
+  {
+    iocp_.handle = ::CreateIoCompletionPort(INVALID_HANDLE_VALUE, 0, 0,
+        static_cast<DWORD>((std::min<size_t>)(concurrency_hint, DWORD(~0))));
     if (!iocp_.handle)
     {
       DWORD last_error = ::GetLastError();
@@ -377,8 +383,8 @@ private:
   struct iocp_holder
   {
     HANDLE handle;
-    iocp_holder(HANDLE h) : handle(h) {}
-    ~iocp_holder() { ::CloseHandle(handle); }
+    iocp_holder() : handle(0) {}
+    ~iocp_holder() { if (handle) ::CloseHandle(handle); }
   } iocp_;
 
   // The count of unfinished work.

asio/include/asio/impl/io_service.ipp

@@ -17,6 +17,10 @@
 
 #include "asio/detail/push_options.hpp"
 
+#include "asio/detail/push_options.hpp"
+#include <limits>
+#include "asio/detail/pop_options.hpp"
+
 #include "asio/detail/epoll_reactor.hpp"
 #include "asio/detail/kqueue_reactor.hpp"
 #include "asio/detail/select_reactor.hpp"
@@ -29,6 +33,14 @@ inline io_service::io_service()
   : service_registry_(*this),
     impl_(service_registry_.use_service<impl_type>())
 {
+  impl_.init((std::numeric_limits<size_t>::max)());
+}
+
+inline io_service::io_service(size_t concurrency_hint)
+  : service_registry_(*this),
+    impl_(service_registry_.use_service<impl_type>())
+{
+  impl_.init(concurrency_hint);
 }
 
 inline size_t io_service::run()

asio/include/asio/io_service.hpp

@@ -87,6 +87,13 @@ public:
   /// Default constructor.
   io_service();
 
+  /// Construct with a hint about the required level of concurrency.
+  /**
+   * @param concurrency_hint A suggestion to the implementation on how many
+   * threads it should allow to run simultaneously.
+   */
+  explicit io_service(size_t concurrency_hint);
+
   /// Run the io_service's event processing loop.
   /**
    * The run() function blocks until all work has finished and there are no