43ea1f5945
Lets operations working on all shards "join" and acquire the same value of something, with that value being based on whenever all shards reach the join. Obvious use case: time stamp after one set of per-shard ops, but before final ones. The generation of the value is guaranteed to happen on the shards that created the join point. Based on the join-ops in CF::snapshot, but abstracted and made caller responsibility. Primary use case is to help deal with the join-problem of truncation. Message-Id: <1456332856-23395-1-git-send-email-calle@scylladb.com>
100 lines
3.1 KiB
C++
100 lines
3.1 KiB
C++
/*
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* Copyright (C) 2016 Cloudius Systems, Ltd.
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*/
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/*
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* This file is part of Scylla.
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*
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* Scylla is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* Scylla is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with Scylla. If not, see <http://www.gnu.org/licenses/>.
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*/
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#pragma once
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#include <memory>
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#include <experimental/optional>
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#include <seastar/core/semaphore.hh>
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#include <seastar/core/shared_ptr.hh>
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/**
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* Joinpoint:
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*
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* Helper type for letting operations working on all shards "join" and acquire
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* the same value of something, with that value being based on whenever that
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* join takes place. (Obvious use case: time stamp after one set of per-shard ops, but
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* before final ones).
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* The generation of the value is guaranteed to happen on the shards that created the
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* join point.
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*/
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namespace utils {
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template<typename T>
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class joinpoint {
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public:
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typedef typename futurize<T>::type type;
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typedef std::function<type()> func_type;
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joinpoint(func_type f)
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: _func(std::move(f))
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, _shard(engine().cpu_id())
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, _enter(0)
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, _wait(0)
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{}
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type value() {
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return smp::submit_to(_shard, [this, id = engine().cpu_id()] {
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_enter.signal();
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if (id == _shard) {
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// We should not generate to common value until all shards
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// have reached this point. Thus the two semaphores.
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return _enter.wait(smp::count).then([this] {
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return _func().then([this](T v) {
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_value = std::move(v);
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_wait.signal(smp::count - 1); // we don't wait
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return make_ready_future<T>(*_value);
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}).handle_exception([this](auto ep) {
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_wait.broken(ep);
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return make_exception_future<T>(ep);
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});
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});
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}
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return _wait.wait().then([this] {
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assert(_value);
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return make_ready_future<T>(*_value);
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});
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});
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}
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private:
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func_type _func;
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shard_id _shard;
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semaphore _enter;
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semaphore _wait;
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std::experimental::optional<T> _value;
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};
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/**
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* Based on the join-code in cf::snapshot.
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* An object that allows us to generate a value for-all-shards
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* at some point down the execution in multiple shards.
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*
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* T type must be copyable, and preferable primitive/trivial
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* or at the very least shard-copy safe.
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*/
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template<typename Func, typename T = std::result_of_t<Func()>>
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joinpoint<T> make_joinpoint(Func && f) {
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return joinpoint<T>([f = std::forward<Func>(f)] {
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return futurize<T>::apply(f);
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});
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}
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}
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