DateKeys/cmd/datekeys/extract.go

package main

import (
	"crypto/rand"
	"encoding/hex"
	"errors"
	"io"
	"os"
	"path"
	"path/filepath"
	"strings"
	"time"

	"g.activething.com/go/DateKeys/capsule"
)

// dirSink writes the files of a format 3 capsule to a new directory (spec
// §56). Begin claims dir with os.Mkdir, which fails if it exists, and stages
// the tree in dir/.datekeys-*, through an os.Root, which follows no link out
// of dir, with O_EXCL and mode 0600; R10 keeps every entry of the head off
// that name. Commit sets the mtimes and moves each entry of the first level
// into dir. Abort removes dir. A capsule without files creates nothing.
type dirSink struct {
	dir     string
	head    *capsule.Head
	root    *os.Root
	stage   string
	created bool
}

func (s *dirSink) Begin(h *capsule.Head) error {
	s.head = h
	if len(h.Files) == 0 {
		return nil
	}
	if err := os.Mkdir(s.dir, 0o700); err != nil {
		if errors.Is(err, os.ErrExist) {
			return errors.New(s.dir + " already exists; outputs are never overwritten")
		}
		return err
	}
	s.created = true
	root, err := os.OpenRoot(s.dir)
	if err != nil {
		return err
	}
	s.root = root
	var id [8]byte
	_, _ = rand.Read(id[:]) // never fails since Go 1.24
	s.stage = ".datekeys-" + hex.EncodeToString(id[:])
	return root.Mkdir(s.stage, 0o700)
}

// staged is the name, within the root, of file i while it is staged.
func (s *dirSink) staged(i int) string {
	return filepath.FromSlash(s.stage + "/" + s.head.Files[i].Path)
}

func (s *dirSink) Create(i int) (io.WriteCloser, error) {
	name := s.staged(i)
	if dir := filepath.Dir(name); dir != s.stage {
		if err := s.root.MkdirAll(dir, 0o700); err != nil {
			return nil, err
		}
	}
	return s.root.OpenFile(name, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o600)
}

// Commit runs at step 18, once every check of step 17 has passed.
func (s *dirSink) Commit() error {
	if s.root == nil {
		return nil
	}
	for i, f := range s.head.Files {
		if f.HasMTime {
			t := time.Unix(int64(f.MTime), 0)
			if err := s.root.Chtimes(s.staged(i), t, t); err != nil {
				return err
			}
		}
	}
	moved := map[string]bool{}
	for _, f := range s.head.Files {
		first, _, _ := strings.Cut(f.Path, "/")
		if moved[first] {
			continue
		}
		if err := s.root.Rename(filepath.FromSlash(path.Join(s.stage, first)), first); err != nil {
			return err
		}
		moved[first] = true
	}
	if err := s.root.Remove(s.stage); err != nil {
		return err
	}
	// The files are in place: closing the root changes nothing of them.
	s.root.Close()
	s.root = nil
	return nil
}

// Abort removes everything the sink created: dir itself.
func (s *dirSink) Abort() {
	if s.root != nil {
		s.root.Close()
	}
	if s.created {
		os.RemoveAll(s.dir)
	}
}