Merge pull request #2811 from mischief/plan9-lock

pkg/fileutil: add plan9 lockfile support

pkg/fileutil/lock_plan9.go

@@ -0,0 +1,90 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fileutil
+
+import (
+	"errors"
+	"os"
+	"syscall"
+	"time"
+)
+
+var (
+	ErrLocked = errors.New("file already locked")
+)
+
+type Lock interface {
+	Name() string
+	TryLock() error
+	Lock() error
+	Unlock() error
+	Destroy() error
+}
+
+type lock struct {
+	fname string
+	file  *os.File
+}
+
+func (l *lock) Name() string {
+	return l.fname
+}
+
+// TryLock acquires exclusivity on the lock without blocking
+func (l *lock) TryLock() error {
+	err := os.Chmod(l.fname, syscall.DMEXCL|0600)
+	if err != nil {
+		return err
+	}
+
+	f, err := os.Open(l.fname)
+	if err != nil {
+		return ErrLocked
+	}
+
+	l.file = f
+	return nil
+}
+
+// Lock acquires exclusivity on the lock with blocking
+func (l *lock) Lock() error {
+	err := os.Chmod(l.fname, syscall.DMEXCL|0600)
+	if err != nil {
+		return err
+	}
+
+	for {
+		f, err := os.Open(l.fname)
+		if err == nil {
+			l.file = f
+			return nil
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+}
+
+// Unlock unlocks the lock
+func (l *lock) Unlock() error {
+	return l.file.Close()
+}
+
+func (l *lock) Destroy() error {
+	return nil
+}
+
+func NewLock(file string) (Lock, error) {
+	l := &lock{fname: file}
+	return l, nil
+}