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// Copyright 2016 The etcd Authors
//
// 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.
// +build linux
package fileutil
import (
"fmt"
"io"
"os"
"syscall"
)
// This used to call syscall.Flock() but that call fails with EBADF on NFS.
// An alternative is lockf() which works on NFS but that call lets a process lock
// the same file twice. Instead, use Linux's non-standard open file descriptor
// locks which will block if the process already holds the file lock.
//
// constants from /usr/include/bits/fcntl-linux.h
const (
F_OFD_GETLK = 37
F_OFD_SETLK = 37
F_OFD_SETLKW = 38
)
var (
wrlck = syscall.Flock_t{
Type: syscall.F_WRLCK,
Whence: int16(io.SeekStart),
Start: 0,
Len: 0,
}
linuxTryLockFile = flockTryLockFile
linuxLockFile = flockLockFile
)
func init() {
// use open file descriptor locks if the system supports it
getlk := syscall.Flock_t{Type: syscall.F_RDLCK}
if err := syscall.FcntlFlock(0, F_OFD_GETLK, &getlk); err == nil {
linuxTryLockFile = ofdTryLockFile
linuxLockFile = ofdLockFile
}
}
func TryLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
return linuxTryLockFile(path, flag, perm)
}
func ofdTryLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
f, err := os.OpenFile(path, flag, perm)
if err != nil {
return nil, fmt.Errorf("ofdTryLockFile failed to open %q (%v)", path, err)
}
flock := wrlck
if err = syscall.FcntlFlock(f.Fd(), F_OFD_SETLK, &flock); err != nil {
f.Close()
if err == syscall.EWOULDBLOCK {
err = ErrLocked
}
return nil, err
}
return &LockedFile{f}, nil
}
func LockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
return linuxLockFile(path, flag, perm)
}
func ofdLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
f, err := os.OpenFile(path, flag, perm)
if err != nil {
return nil, fmt.Errorf("ofdLockFile failed to open %q (%v)", path, err)
}
flock := wrlck
err = syscall.FcntlFlock(f.Fd(), F_OFD_SETLKW, &flock)
if err != nil {
f.Close()
return nil, err
}
return &LockedFile{f}, nil
}
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