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moby--moby/pkg/ioutils/fswriters.go

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package ioutils
import (
"io"
"io/ioutil"
"os"
"path/filepath"
)
// NewAtomicFileWriter returns WriteCloser so that writing to it writes to a
// temporary file and closing it atomically changes the temporary file to
// destination path. Writing and closing concurrently is not allowed.
func NewAtomicFileWriter(filename string, perm os.FileMode) (io.WriteCloser, error) {
f, err := ioutil.TempFile(filepath.Dir(filename), ".tmp-"+filepath.Base(filename))
if err != nil {
return nil, err
}
abspath, err := filepath.Abs(filename)
if err != nil {
return nil, err
}
return &atomicFileWriter{
f: f,
fn: abspath,
perm: perm,
}, nil
}
// AtomicWriteFile atomically writes data to a file named by filename.
func AtomicWriteFile(filename string, data []byte, perm os.FileMode) error {
f, err := NewAtomicFileWriter(filename, perm)
if err != nil {
return err
}
n, err := f.Write(data)
if err == nil && n < len(data) {
err = io.ErrShortWrite
f.(*atomicFileWriter).writeErr = err
}
if err1 := f.Close(); err == nil {
err = err1
}
return err
}
type atomicFileWriter struct {
f *os.File
fn string
writeErr error
perm os.FileMode
}
func (w *atomicFileWriter) Write(dt []byte) (int, error) {
n, err := w.f.Write(dt)
if err != nil {
w.writeErr = err
}
return n, err
}
func (w *atomicFileWriter) Close() (retErr error) {
defer func() {
if retErr != nil || w.writeErr != nil {
os.Remove(w.f.Name())
}
}()
if err := w.f.Sync(); err != nil {
w.f.Close()
return err
}
if err := w.f.Close(); err != nil {
return err
}
if err := os.Chmod(w.f.Name(), w.perm); err != nil {
return err
}
if w.writeErr == nil {
return os.Rename(w.f.Name(), w.fn)
}
return nil
}