core: implement future<> sleep()

Seastar applications obviously cannot use the Posix sleep() function, or
the entire thread will block. This patch implements future<> sleep(duration)
where as expected, the future becomes ready when the given duration passes.

For example, one can do:

    sleep(1s).then([] { std::cout << "Done.\n"; });

This can be useful as an example of futures and continuations in the
Seastar tutorial, and people might find other uses for it.

This sleep() is implemented in terms of timer<>. sleep() is easier to use
and more aligned with the rest of Seastar (it uses then() for the
continuation instead of a set_callback() method). The downside of sleep()
compared to a timer is that it cannot be canceled once started.

In this version, sleep() is implemented without shared_ptr, making the
implementation a tiny bit more efficient. There is still a heap allocation
(this is unavoidable because std::function requires a copyable type)
but no reference counting. Unfortunately, this requires us to use bare
"new" and "delete".

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>

core/sleep.hh

@@ -0,0 +1,44 @@
+/*
+ * This file is open source software, licensed to you under the terms
+ * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
+ * distributed with this work for additional information regarding copyright
+ * ownership.  You may not use this file except in compliance with the License.
+ *
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/*
+ * Copyright (C) 2015 Cloudius Systems, Ltd.
+ */
+
+#pragma once
+
+#include <chrono>
+#include <functional>
+
+#include "core/shared_ptr.hh"
+#include "core/reactor.hh"
+#include "core/future.hh"
+
+template <typename Clock = std::chrono::high_resolution_clock, typename Rep, typename Period>
+future<> sleep(std::chrono::duration<Rep, Period> dur) {
+    struct sleeper {
+        promise<> done;
+        timer<Clock> tmr;
+        sleeper(std::chrono::duration<Rep, Period> dur)
+            : tmr([this] { done.set_value(); delete this; })
+        {
+            tmr.arm(dur);
+        }
+    };
+    return (new sleeper(dur))->done.get_future();
+}