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// Copyright 2019 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 windows darwin
package robustio
import (
"errors"
"io/ioutil"
"math/rand"
"os"
"syscall"
"time"
)
const arbitraryTimeout = 500 * time.Millisecond
// retry retries ephemeral errors from f up to an arbitrary timeout
// to work around filesystem flakiness on Windows and Darwin.
func retry(f func() (err error, mayRetry bool)) error {
var (
bestErr error
lowestErrno syscall.Errno
start time.Time
nextSleep time.Duration = 1 * time.Millisecond
)
for {
err, mayRetry := f()
if err == nil || !mayRetry {
return err
}
var errno syscall.Errno
if errors.As(err, &errno) && (lowestErrno == 0 || errno < lowestErrno) {
bestErr = err
lowestErrno = errno
} else if bestErr == nil {
bestErr = err
}
if start.IsZero() {
start = time.Now()
} else if d := time.Since(start) + nextSleep; d >= arbitraryTimeout {
break
}
time.Sleep(nextSleep)
nextSleep += time.Duration(rand.Int63n(int64(nextSleep)))
}
return bestErr
}
// rename is like os.Rename, but retries ephemeral errors.
//
// On Windows it wraps os.Rename, which (as of 2019-06-04) uses MoveFileEx with
// MOVEFILE_REPLACE_EXISTING.
//
// Windows also provides a different system call, ReplaceFile,
// that provides similar semantics, but perhaps preserves more metadata. (The
// documentation on the differences between the two is very sparse.)
//
// Empirical error rates with MoveFileEx are lower under modest concurrency, so
// for now we're sticking with what the os package already provides.
func rename(oldpath, newpath string) (err error) {
return retry(func() (err error, mayRetry bool) {
err = os.Rename(oldpath, newpath)
return err, isEphemeralError(err)
})
}
// readFile is like ioutil.ReadFile, but retries ephemeral errors.
func readFile(filename string) ([]byte, error) {
var b []byte
err := retry(func() (err error, mayRetry bool) {
b, err = ioutil.ReadFile(filename)
// Unlike in rename, we do not retry errFileNotFound here: it can occur
// as a spurious error, but the file may also genuinely not exist, so the
// increase in robustness is probably not worth the extra latency.
return err, isEphemeralError(err) && !errors.Is(err, errFileNotFound)
})
return b, err
}
func removeAll(path string) error {
return retry(func() (err error, mayRetry bool) {
err = os.RemoveAll(path)
return err, isEphemeralError(err)
})
}
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