Detailed changes
@@ -0,0 +1,85 @@
+/*
+
+ This Source Code Form is subject to the terms of the Mozilla Public
+ License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ You can obtain one at http://mozilla.org/MPL/2.0/.
+
+ Copyright (c) 2013, Armon Dadgar armon.dadgar@gmail.com
+ Copyright (c) 2013, Mitchell Hashimoto mitchell.hashimoto@gmail.com
+
+ Alternatively, the contents of this file may be used under the terms
+ of the GNU General Public License Version 3 or later, as described below:
+
+ This file is free software: you may copy, redistribute and/or modify
+ it under the terms of the GNU General Public License as published by the
+ Free Software Foundation, either version 3 of the License, or (at your
+ option) any later version.
+
+ This file 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 General
+ Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see http://www.gnu.org/licenses/.
+
+*/
+
+package util
+
+import (
+ "sync/atomic"
+)
+
+// LamportClock is a thread safe implementation of a lamport clock. It
+// uses efficient atomic operations for all of its functions, falling back
+// to a heavy lock only if there are enough CAS failures.
+type LamportClock struct {
+ counter uint64
+}
+
+// LamportTime is the value of a LamportClock.
+type LamportTime uint64
+
+func NewLamportClock() LamportClock {
+ return LamportClock{
+ counter: 1,
+ }
+}
+
+func NewLamportClockWithTime(time uint64) LamportClock {
+ return LamportClock{
+ counter: time,
+ }
+}
+
+// Time is used to return the current value of the lamport clock
+func (l *LamportClock) Time() LamportTime {
+ return LamportTime(atomic.LoadUint64(&l.counter))
+}
+
+// Increment is used to increment and return the value of the lamport clock
+func (l *LamportClock) Increment() LamportTime {
+ return LamportTime(atomic.AddUint64(&l.counter, 1))
+}
+
+// Witness is called to update our local clock if necessary after
+// witnessing a clock value received from another process
+func (l *LamportClock) Witness(v LamportTime) {
+WITNESS:
+ // If the other value is old, we do not need to do anything
+ cur := atomic.LoadUint64(&l.counter)
+ other := uint64(v)
+ if other < cur {
+ return
+ }
+
+ // Ensure that our local clock is at least one ahead.
+ if !atomic.CompareAndSwapUint64(&l.counter, cur, other+1) {
+ // CAS: CompareAndSwap
+ // The CAS failed, so we just retry. Eventually our CAS should
+ // succeed or a future witness will pass us by and our witness
+ // will end.
+ goto WITNESS
+ }
+}
@@ -0,0 +1,66 @@
+/*
+
+ This Source Code Form is subject to the terms of the Mozilla Public
+ License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ You can obtain one at http://mozilla.org/MPL/2.0/.
+
+ Copyright (c) 2013, Armon Dadgar armon.dadgar@gmail.com
+ Copyright (c) 2013, Mitchell Hashimoto mitchell.hashimoto@gmail.com
+
+ Alternatively, the contents of this file may be used under the terms
+ of the GNU General Public License Version 3 or later, as described below:
+
+ This file is free software: you may copy, redistribute and/or modify
+ it under the terms of the GNU General Public License as published by the
+ Free Software Foundation, either version 3 of the License, or (at your
+ option) any later version.
+
+ This file 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 General
+ Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see http://www.gnu.org/licenses/.
+
+*/
+
+package util
+
+import (
+ "testing"
+)
+
+func TestLamportClock(t *testing.T) {
+ l := &LamportClock{}
+
+ if l.Time() != 0 {
+ t.Fatalf("bad time value")
+ }
+
+ if l.Increment() != 1 {
+ t.Fatalf("bad time value")
+ }
+
+ if l.Time() != 1 {
+ t.Fatalf("bad time value")
+ }
+
+ l.Witness(41)
+
+ if l.Time() != 42 {
+ t.Fatalf("bad time value")
+ }
+
+ l.Witness(41)
+
+ if l.Time() != 42 {
+ t.Fatalf("bad time value")
+ }
+
+ l.Witness(30)
+
+ if l.Time() != 42 {
+ t.Fatalf("bad time value")
+ }
+}
@@ -0,0 +1,81 @@
+package util
+
+import (
+ "fmt"
+ "io/ioutil"
+ "os"
+ "path/filepath"
+)
+
+type PersistedLamport struct {
+ LamportClock
+ filePath string
+}
+
+func NewPersistedLamport(filePath string) *PersistedLamport {
+ clock := &PersistedLamport{
+ filePath: filePath,
+ }
+ return clock
+}
+
+func LoadPersistedLamport(filePath string) (*PersistedLamport, error) {
+ clock := &PersistedLamport{
+ filePath: filePath,
+ }
+
+ err := clock.read()
+ if err != nil {
+ return nil, err
+ }
+
+ return clock, nil
+}
+
+func (c *PersistedLamport) Witness(time LamportTime) error {
+ c.LamportClock.Witness(time)
+ return c.Write()
+}
+
+func (c *PersistedLamport) Time() LamportTime {
+ // Equivalent to:
+ //
+ // res = c.LamportClock.Time()
+ // bugClock.Increment()
+ //
+ // ... but thread safe
+ return c.Increment() - 1
+}
+
+func (c *PersistedLamport) read() error {
+ content, err := ioutil.ReadFile(c.filePath)
+ if err != nil {
+ return err
+ }
+
+ var value uint64
+ n, err := fmt.Sscanf(string(content), "%d", &value)
+
+ if err != nil {
+ return err
+ }
+
+ if n != 1 {
+ return fmt.Errorf("could not read the clock")
+ }
+
+ c.LamportClock = NewLamportClockWithTime(value)
+
+ return nil
+}
+
+func (c *PersistedLamport) Write() error {
+ dir := filepath.Dir(c.filePath)
+ err := os.MkdirAll(dir, 0777)
+ if err != nil {
+ return err
+ }
+
+ data := []byte(fmt.Sprintf("%d", c.LamportClock.Time()))
+ return ioutil.WriteFile(c.filePath, data, 0644)
+}