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| 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 [email protected] |
| 8 | + Copyright (c) 2013, Mitchell Hashimoto [email protected] |
| 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 | + |
| 36 | +package lamport |
| 37 | + |
| 38 | +import ( |
| 39 | + "sync/atomic" |
| 40 | +) |
| 41 | + |
| 42 | +var _ 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. |
| 47 | +type MemClock struct { |
| 48 | + counter uint64 |
| 49 | +} |
| 50 | + |
| 51 | +// NewMemClock create a new clock with the value 1. |
| 52 | +// Value 0 is considered as invalid. |
| 53 | +func NewMemClock() *MemClock { |
| 54 | + return &MemClock{ |
| 55 | + counter: 1, |
| 56 | + } |
| 57 | +} |
| 58 | + |
| 59 | +// NewMemClockWithTime create a new clock with a value. |
| 60 | +func 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 |
| 67 | +func (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 |
| 72 | +func (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 |
| 78 | +func (mc *MemClock) Witness(v Time) error { |
| 79 | +WITNESS: |
| 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 | +} |
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