util: add a Lamport clock implementation as well as a persistable one

3 files changed, 232 insertions, 0 deletions

diff --git a/util/lamport.go b/util/lamport.go
new file mode 100644
index 00000000..c75e1d15
--- /dev/null
+++ b/util/lamport.go
@@ -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
+	}
+}
diff --git a/util/lamport_test.go b/util/lamport_test.go
new file mode 100644
index 00000000..0bac7d51
--- /dev/null
+++ b/util/lamport_test.go
@@ -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")
+	}
+}
diff --git a/util/persisted_lamport.go b/util/persisted_lamport.go
new file mode 100644
index 00000000..43a2863c
--- /dev/null
+++ b/util/persisted_lamport.go
@@ -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)
+}