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// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build plan9
package lockedfile
import (
"math/rand"
"os"
"strings"
"time"
)
// Opening an exclusive-use file returns an error.
// The expected error strings are:
//
// - "open/create -- file is locked" (cwfs, kfs)
// - "exclusive lock" (fossil)
// - "exclusive use file already open" (ramfs)
var lockedErrStrings = [...]string{
"file is locked",
"exclusive lock",
"exclusive use file already open",
}
// Even though plan9 doesn't support the Lock/RLock/Unlock functions to
// manipulate already-open files, IsLocked is still meaningful: os.OpenFile
// itself may return errors that indicate that a file with the ModeExclusive bit
// set is already open.
func isLocked(err error) bool {
s := err.Error()
for _, frag := range lockedErrStrings {
if strings.Contains(s, frag) {
return true
}
}
return false
}
func openFile(name string, flag int, perm os.FileMode) (*os.File, error) {
// Plan 9 uses a mode bit instead of explicit lock/unlock syscalls.
//
// Per http://man.cat-v.org/plan_9/5/stat: “Exclusive use files may be open
// for I/O by only one fid at a time across all clients of the server. If a
// second open is attempted, it draws an error.”
//
// So we can try to open a locked file, but if it fails we're on our own to
// figure out when it becomes available. We'll use exponential backoff with
// some jitter and an arbitrary limit of 500ms.
// If the file was unpacked or created by some other program, it might not
// have the ModeExclusive bit set. Set it before we call OpenFile, so that we
// can be confident that a successful OpenFile implies exclusive use.
if fi, err := os.Stat(name); err == nil {
if fi.Mode()&os.ModeExclusive == 0 {
if err := os.Chmod(name, fi.Mode()|os.ModeExclusive); err != nil {
return nil, err
}
}
} else if !os.IsNotExist(err) {
return nil, err
}
nextSleep := 1 * time.Millisecond
const maxSleep = 500 * time.Millisecond
for {
f, err := os.OpenFile(name, flag, perm|os.ModeExclusive)
if err == nil {
return f, nil
}
if !isLocked(err) {
return nil, err
}
time.Sleep(nextSleep)
nextSleep += nextSleep
if nextSleep > maxSleep {
nextSleep = maxSleep
}
// Apply 10% jitter to avoid synchronizing collisions.
nextSleep += time.Duration((0.1*rand.Float64() - 0.05) * float64(nextSleep))
}
}
func closeFile(f *os.File) error {
return f.Close()
}
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