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75 lines
1.7 KiB
75 lines
1.7 KiB
package main
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import (
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"errors"
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"fmt"
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"io"
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"os"
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"path/filepath"
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)
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// checkNew fails if path already exists.
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func checkNew(path string) error {
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if _, err := os.Lstat(path); err == nil {
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return fmt.Errorf("%s already exists; outputs are never overwritten", path)
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} else if !errors.Is(err, os.ErrNotExist) {
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return err
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}
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return nil
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}
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// writeAtomic stages the output in a private temporary file next to path and
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// publishes it only after fn succeeded (spec §56). Publication creates path
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// without replacing an existing file: a hard link where the file system
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// supports it, otherwise an exclusive create and copy. On any failure no
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// partial output remains.
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func writeAtomic(path string, fn func(io.Writer) error) (err error) {
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if err := checkNew(path); err != nil {
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return err
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}
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tmp, err := os.CreateTemp(filepath.Dir(path), ".datekeys-*")
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if err != nil {
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return err
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}
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name := tmp.Name()
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defer func() {
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tmp.Close()
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os.Remove(name)
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}()
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if err := fn(tmp); err != nil {
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return err
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}
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if err := tmp.Sync(); err != nil {
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return err
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}
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if err := tmp.Close(); err != nil {
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return err
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}
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if err := os.Link(name, path); err == nil {
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return nil
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} else if errors.Is(err, os.ErrExist) {
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return fmt.Errorf("%s already exists; outputs are never overwritten", path)
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}
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return copyExclusive(name, path)
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}
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func copyExclusive(from, to string) error {
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src, err := os.Open(from)
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if err != nil {
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return err
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}
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defer src.Close()
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dst, err := os.OpenFile(to, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o600)
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if err != nil {
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return err
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}
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_, copyErr := io.Copy(dst, src)
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syncErr := dst.Sync()
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closeErr := dst.Close()
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if err := errors.Join(copyErr, syncErr, closeErr); err != nil {
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os.Remove(to)
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return err
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}
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return nil
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}
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