1/*
2
3 This Source Code Form is subject to the terms of the Mozilla Public
4 License, v. 2.0. If a copy of the MPL was not distributed with this file,
5 You can obtain one at http://mozilla.org/MPL/2.0/.
6
7 Copyright (c) 2013, Armon Dadgar armon.dadgar@gmail.com
8 Copyright (c) 2013, Mitchell Hashimoto mitchell.hashimoto@gmail.com
9
10 Alternatively, the contents of this file may be used under the terms
11 of the GNU General Public License Version 3 or later, as described below:
12
13 This file is free software: you may copy, redistribute and/or modify
14 it under the terms of the GNU General Public License as published by the
15 Free Software Foundation, either version 3 of the License, or (at your
16 option) any later version.
17
18 This file is distributed in the hope that it will be useful, but
19 WITHOUT ANY WARRANTY; without even the implied warranty of
20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
21 Public License for more details.
22
23 You should have received a copy of the GNU General Public License
24 along with this program. If not, see http://www.gnu.org/licenses/.
25
26*/
27
28// Note: this code originally originate from Hashicorp's Serf but has been changed since to fit git-bug's need.
29
30// Note: this Lamport clock implementation is different than the algorithms you can find, notably Wikipedia or the
31// original Serf implementation. The reason is lie to what constitute an event in this distributed system.
32// Commonly, events happen when messages are sent or received, whereas in git-bug events happen when some data is
33// written, but *not* when read. This is why Witness set the time to the max seen value instead of max seen value +1.
34// See https://cs.stackexchange.com/a/133730/129795
35
36package lamport
37
38import (
39 "sync/atomic"
40)
41
42var _ Clock = &MemClock{}
43
44// MemClock is a thread safe implementation of a lamport clock. It
45// uses efficient atomic operations for all of its functions, falling back
46// to a heavy lock only if there are enough CAS failures.
47type MemClock struct {
48 counter uint64
49}
50
51// NewMemClock create a new clock with the value 1.
52// Value 0 is considered as invalid.
53func NewMemClock() *MemClock {
54 return &MemClock{
55 counter: 1,
56 }
57}
58
59// NewMemClockWithTime create a new clock with a value.
60func NewMemClockWithTime(time uint64) *MemClock {
61 return &MemClock{
62 counter: time,
63 }
64}
65
66// Time is used to return the current value of the lamport clock
67func (mc *MemClock) Time() Time {
68 return Time(atomic.LoadUint64(&mc.counter))
69}
70
71// Increment is used to return the value of the lamport clock and increment it afterwards
72func (mc *MemClock) Increment() (Time, error) {
73 return Time(atomic.AddUint64(&mc.counter, 1)), nil
74}
75
76// Witness is called to update our local clock if necessary after
77// witnessing a clock value received from another process
78func (mc *MemClock) Witness(v Time) error {
79WITNESS:
80 // If the other value is old, we do not need to do anything
81 cur := atomic.LoadUint64(&mc.counter)
82 other := uint64(v)
83 if other <= cur {
84 return nil
85 }
86
87 // Ensure that our local clock is at least one ahead.
88 if !atomic.CompareAndSwapUint64(&mc.counter, cur, other) {
89 // CAS: CompareAndSwap
90 // The CAS failed, so we just retry. Eventually our CAS should
91 // succeed or a future witness will pass us by and our witness
92 // will end.
93 goto WITNESS
94 }
95
96 return nil
97}