common: add optional_yield wrapper

adds a wrapper type that may or may not contain a yield_context
that represents a boost::asio stackful coroutine, along with a
'null_yield' token to designate an empty one

Signed-off-by: Casey Bodley <cbodley@redhat.com>

src/common/async/yield_context.h

@@ -0,0 +1,64 @@
+// -*- 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, Inc
+ *
+ * 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.
+ *
+ */
+
+#pragma once
+
+#include <boost/asio/io_context.hpp>
+
+#include "acconfig.h"
+
+#ifndef HAVE_BOOST_CONTEXT
+
+// hide the dependencies on boost::context and boost::coroutines
+namespace boost::asio {
+struct yield_context;
+}
+
+#else // HAVE_BOOST_CONTEXT
+
+#define BOOST_COROUTINES_NO_DEPRECATION_WARNING
+#include <boost/asio/spawn.hpp>
+
+#endif // HAVE_BOOST_CONTEXT
+
+
+/// optional-like wrapper for a boost::asio::yield_context and its associated
+/// boost::asio::io_context. operations that take an optional_yield argument
+/// will, when passed a non-empty yield context, suspend this coroutine instead
+/// of the blocking the thread of execution
+class optional_yield {
+  boost::asio::io_context *c = nullptr;
+  boost::asio::yield_context *y = nullptr;
+ public:
+  /// construct with a valid io and yield_context
+  explicit optional_yield(boost::asio::io_context& c,
+                          boost::asio::yield_context& y) noexcept
+    : c(&c), y(&y) {}
+
+  /// type tag to construct an empty object
+  struct empty_t {};
+  optional_yield(empty_t) noexcept {}
+
+  /// implicit conversion to bool, returns true if non-empty
+  operator bool() const noexcept { return y; }
+
+  /// return a reference to the associated io_context. only valid if non-empty
+  boost::asio::io_context& get_io_context() const noexcept { return *c; }
+
+  /// return a reference to the yield_context. only valid if non-empty
+  boost::asio::yield_context& get_yield_context() const noexcept { return *y; }
+};
+
+// type tag object to construct an empty optional_yield
+static constexpr optional_yield::empty_t null_yield{};