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+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2018 Red Hat
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation. See file COPYING.
+ *
+ */
+
+#ifndef CEPH_ASYNC_FORWARD_HANDLER_H
+#define CEPH_ASYNC_FORWARD_HANDLER_H
+
+#include <boost/asio.hpp>
+
+namespace ceph::async {
+
+/**
+ * A forwarding completion handler for use with boost::asio.
+ *
+ * A completion handler wrapper that invokes the handler's operator() as an
+ * rvalue, regardless of whether the wrapper is invoked as an lvalue or rvalue.
+ * This operation is potentially destructive to the wrapped handler, so is only
+ * suitable for single-use handlers.
+ *
+ * This is useful when combined with bind_handler() and move-only arguments,
+ * because executors will always call the lvalue overload of operator().
+ *
+ * The original Handler's associated allocator and executor are maintained.
+ *
+ * @see forward_handler
+ */
+template <typename Handler>
+struct ForwardingHandler {
+  Handler handler;
+
+  ForwardingHandler(Handler&& handler)
+    : handler(std::move(handler))
+  {}
+
+  template <typename ...Args>
+  void operator()(Args&& ...args) {
+    std::move(handler)(std::forward<Args>(args)...);
+  }
+
+  using allocator_type = boost::asio::associated_allocator_t<Handler>;
+  allocator_type get_allocator() const noexcept {
+    return boost::asio::get_associated_allocator(handler);
+  }
+};
+
+} // namespace ceph::async
+
+namespace boost::asio {
+
+// specialize boost::asio::associated_executor<> for ForwardingHandler
+template <typename Handler, typename Executor>
+struct associated_executor<ceph::async::ForwardingHandler<Handler>, Executor> {
+  using type = boost::asio::associated_executor_t<Handler, Executor>;
+
+  static type get(const ceph::async::ForwardingHandler<Handler>& handler,
+                  const Executor& ex = Executor()) noexcept {
+    return boost::asio::get_associated_executor(handler.handler, ex);
+  }
+};
+
+} // namespace boost::asio
+
+namespace ceph::async {
+
+/**
+ * Returns a single-use completion handler that always forwards on operator().
+ *
+ * Wraps a completion handler such that it is always invoked as an rvalue. This
+ * is necessary when combining executors and bind_handler() with move-only
+ * argument types.
+ *
+ * Example use:
+ *
+ *   auto callback = [] (std::unique_ptr<int>&& p) {};
+ *   auto bound_handler = bind_handler(callback, std::make_unique<int>(5));
+ *   auro handler = forward_handler(std::move(bound_handler));
+ *
+ *   // execute the forwarding handler on an io_context:
+ *   boost::asio::io_context context;
+ *   boost::asio::post(context, std::move(handler));
+ *   context.run();
+ *
+ * @see ForwardingHandler
+ */
+template <typename Handler>
+auto forward_handler(Handler&& h)
+{
+  return ForwardingHandler{std::forward<Handler>(h)};
+}
+
+} // namespace ceph::async
+
+#endif // CEPH_ASYNC_FORWARD_HANDLER_H