/usr/include/mongo/util/concurrency/mvar.h is in mongodb-dev 1:2.4.9-1ubuntu2.
This file is owned by root:root, with mode 0o644.
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/* Copyright 2009 10gen Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* 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.
*/
#pragma once
#include <boost/thread/recursive_mutex.hpp>
#include <boost/thread/condition.hpp>
namespace mongo {
/* This is based on haskell's MVar synchronization primitive:
* http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent-MVar.html
*
* It is a thread-safe queue that can hold at most one object.
* You can also think of it as a box that can be either full or empty.
*/
template <typename T>
class MVar {
public:
enum State {EMPTY=0, FULL};
// create an empty MVar
MVar()
: _state(EMPTY)
{}
// creates a full MVar
MVar(const T& val)
: _state(FULL)
, _value(val)
{}
// puts val into the MVar and returns true or returns false if full
// never blocks
bool tryPut(const T& val) {
// intentionally repeat test before and after lock
if (_state == FULL) return false;
Mutex::scoped_lock lock(_mutex);
if (_state == FULL) return false;
_state = FULL;
_value = val;
// unblock threads waiting to 'take'
_condition.notify_all();
return true;
}
// puts val into the MVar
// will block if the MVar is already full
void put(const T& val) {
Mutex::scoped_lock lock(_mutex);
while (!tryPut(val)) {
// unlocks lock while waiting and relocks before returning
_condition.wait(lock);
}
}
// takes val out of the MVar and returns true or returns false if empty
// never blocks
bool tryTake(T& out) {
// intentionally repeat test before and after lock
if (_state == EMPTY) return false;
Mutex::scoped_lock lock(_mutex);
if (_state == EMPTY) return false;
_state = EMPTY;
out = _value;
// unblock threads waiting to 'put'
_condition.notify_all();
return true;
}
// takes val out of the MVar
// will block if the MVar is empty
T take() {
T ret = T();
Mutex::scoped_lock lock(_mutex);
while (!tryTake(ret)) {
// unlocks lock while waiting and relocks before returning
_condition.wait(lock);
}
return ret;
}
// Note: this is fast because there is no locking, but state could
// change before you get a chance to act on it.
// Mainly useful for sanity checks / asserts.
State getState() { return _state; }
private:
State _state;
T _value;
typedef boost::recursive_mutex Mutex;
Mutex _mutex;
boost::condition _condition;
};
}
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