1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
|
// mvar.h
/* Copyright 2009 10gen Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* 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
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* 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 GNU Affero General 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 <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;
};
}
|
