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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <atomic>
#include <condition_variable>
#include <list>
#include "mongo/platform/atomic_word.h"
#include "mongo/stdx/mutex.h"
#include "mongo/util/concurrency/with_lock.h"
namespace mongo {
namespace stdx {
class condition_variable;
}
/**
* Notifyable is a slim type meant to allow integration of special kinds of waiters for
* stdx::condition_variable. Specifially, the notify() on this type will be called directly from
* stdx::condition_varibale::notify_(one|all).
*
* See Waitable for the stdx::condition_variable integration.
*/
class Notifyable {
friend class stdx::condition_variable;
public:
Notifyable() = default;
// !!! PAY ATTENTION, THERE IS DANGER HERE !!!
//
// Implementers of the notifyable api must be level triggered by notify, rather than edge
// triggered.
//
// I.e. a call to notify() must either unblock the notifyable immediately, if it is currently
// blocked, or unblock it the next time it would wait, if it is not currently blocked.
//
// In addition to unblocking, the notifyable should also atomically consume that notification
// state as a result of waking. I.e. any number of calls to notify before or during a wait must
// unblock exactly one wait.
//
// Notifyable::notify is not like condition_variable::notify_X()
virtual void notify() noexcept = 0;
protected:
~Notifyable() = default;
private:
// Notifyable's own a list node which they splice into a stdx::condition_variable's wait list
// when waiting on one. It's important that we pre-allocate this entry on construction.
//
// Note that the notifyable** in this list is only used and meaningful while the notifyable
// waits on a condition variable (so the ownership of a self pointer doesn't really have
// implications for copy/move, as the objects shouldn't be moved/copied while waiting)
std::list<Notifyable*> _handleContainer{this};
};
class Waitable;
namespace stdx {
using condition_variable_any = ::std::condition_variable_any; // NOLINT
using cv_status = ::std::cv_status; // NOLINT
using ::std::notify_all_at_thread_exit; // NOLINT
/**
* We wrap std::condition_variable to allow us to register Notifyables which can "wait" on the
* condvar without actually waiting on the std::condition_variable. This allows us to possibly do
* productive work in those types, rather than sleeping in the os.
*/
class condition_variable : private std::condition_variable { // NOLINT
public:
using std::condition_variable::condition_variable; // NOLINT
void notify_one() noexcept {
if (_notifyableCount.load()) {
stdx::lock_guard lk(_mutex);
if (_notifyNextNotifyable(lk)) {
return;
}
}
std::condition_variable::notify_one(); // NOLINT
}
void notify_all() noexcept {
if (_notifyableCount.load()) {
stdx::lock_guard lk(_mutex);
while (_notifyNextNotifyable(lk)) {
}
}
std::condition_variable::notify_all(); // NOLINT
}
using std::condition_variable::native_handle; // NOLINT
using std::condition_variable::wait; // NOLINT
using std::condition_variable::wait_for; // NOLINT
using std::condition_variable::wait_until; // NOLINT
private:
friend class ::mongo::Waitable;
/**
* Runs the callback with the Notifyable registered on the condvar. This ensures that for the
* duration of the callback execution, a notification on the condvar will trigger a notify() to
* the Notifyable.
*
* The scheme here is that list entries are spliced back to their Notifyable from the
* notification list when notified (so that they don't eat multiple notify_one's). We detect
* that condition by noting that our list isn't empty (in which case we should avoid a double
* splice back).
*
* The method is private, and accessed via friendship in Waitable.
*/
template <typename Callback>
void _runWithNotifyable(Notifyable& notifyable, Callback&& cb) noexcept {
static_assert(noexcept(std::forward<Callback>(cb)()),
"Only noexcept functions may be invoked with _runWithNotifyable");
auto iter = [&] {
stdx::lock_guard localMutex(_mutex);
_notifyableCount.addAndFetch(1);
_notifyables.splice(_notifyables.end(),
notifyable._handleContainer,
notifyable._handleContainer.begin());
return --_notifyables.end();
}();
// The supplied callback should do the equivalent of waiting on this condition_variable
// (i.e. return on notify), as well as any other work the waiter would like to do while
// waiting.
std::forward<Callback>(cb)();
stdx::lock_guard localMutex(_mutex);
// If our list isn't empty, we were notified, and spliced back in _notifyNextNotifyable.
// If it is empty, we need to stash our wait queue iterator ourselves.
if (notifyable._handleContainer.empty()) {
_notifyableCount.subtractAndFetch(1);
_spliceBack(localMutex, iter);
}
}
/**
* Notifies the next notifyable.
*
* Returns true if there was a notifyable to be notified.
*
* Note that as part of notifying, we splice back iterators to _runWithNotifyable callers. This
* is safe because we hold _mutex while we do so, and our splicing communicates that those
* waiters need not clear themselves from the notification list on wakeup.
*/
bool _notifyNextNotifyable(WithLock wl) noexcept {
auto iter = _notifyables.begin();
if (iter == _notifyables.end()) {
return false;
}
_notifyableCount.subtractAndFetch(1);
(*iter)->notify();
_spliceBack(wl, iter);
return true;
}
/**
* Splice the notifyable iterator back into the notifyable (out from this condvar's wait list)
*/
void _spliceBack(WithLock, std::list<Notifyable*>::iterator iter) {
auto notifyable = *iter;
notifyable->_handleContainer.splice(
notifyable->_handleContainer.begin(), _notifyables, iter);
}
AtomicWord<unsigned long long> _notifyableCount;
stdx::mutex _mutex;
std::list<Notifyable*> _notifyables;
};
} // namespace stdx
} // namespace mongo
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