1// Copyright 2016 The etcd Authors 2// 3// Licensed under the Apache License, Version 2.0 (the "License"); 4// you may not use this file except in compliance with the License. 5// You may obtain a copy of the License at 6// 7// http://www.apache.org/licenses/LICENSE-2.0 8// 9// Unless required by applicable law or agreed to in writing, software 10// distributed under the License is distributed on an "AS IS" BASIS, 11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12// See the License for the specific language governing permissions and 13// limitations under the License. 14 15//go:build linux 16// +build linux 17 18package fileutil 19 20import ( 21 "fmt" 22 "io" 23 "os" 24 "syscall" 25 26 "golang.org/x/sys/unix" 27) 28 29// This used to call syscall.Flock() but that call fails with EBADF on NFS. 30// An alternative is lockf() which works on NFS but that call lets a process lock 31// the same file twice. Instead, use Linux's non-standard open file descriptor 32// locks which will block if the process already holds the file lock. 33 34var ( 35 wrlck = syscall.Flock_t{ 36 Type: syscall.F_WRLCK, 37 Whence: int16(io.SeekStart), 38 Start: 0, 39 Len: 0, 40 } 41 42 linuxTryLockFile = flockTryLockFile 43 linuxLockFile = flockLockFile 44) 45 46func init() { 47 // use open file descriptor locks if the system supports it 48 getlk := syscall.Flock_t{Type: syscall.F_RDLCK} 49 if err := syscall.FcntlFlock(0, unix.F_OFD_GETLK, &getlk); err == nil { 50 linuxTryLockFile = ofdTryLockFile 51 linuxLockFile = ofdLockFile 52 } 53} 54 55func TryLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) { 56 return linuxTryLockFile(path, flag, perm) 57} 58 59func ofdTryLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) { 60 f, err := os.OpenFile(path, flag, perm) 61 if err != nil { 62 return nil, fmt.Errorf("ofdTryLockFile failed to open %q (%v)", path, err) 63 } 64 65 flock := wrlck 66 if err = syscall.FcntlFlock(f.Fd(), unix.F_OFD_SETLK, &flock); err != nil { 67 f.Close() 68 if err == syscall.EWOULDBLOCK { 69 err = ErrLocked 70 } 71 return nil, err 72 } 73 return &LockedFile{f}, nil 74} 75 76func LockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) { 77 return linuxLockFile(path, flag, perm) 78} 79 80func ofdLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) { 81 f, err := os.OpenFile(path, flag, perm) 82 if err != nil { 83 return nil, fmt.Errorf("ofdLockFile failed to open %q (%v)", path, err) 84 } 85 86 flock := wrlck 87 err = syscall.FcntlFlock(f.Fd(), unix.F_OFD_SETLKW, &flock) 88 if err != nil { 89 f.Close() 90 return nil, err 91 } 92 return &LockedFile{f}, nil 93} 94