pixa/internal/imgcache/filestore.go

package imgcache

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
)

// Storage constants.
const (
	// StorageDirPerm is the permission mode for storage directories.
	StorageDirPerm = 0750
	// StorageFilePerm is the permission mode for storage files.
	StorageFilePerm = 0600
	// MinHashLength is the minimum hash length for path splitting.
	MinHashLength = 4
)

// Storage errors.
var (
	ErrNotFound = errors.New("content not found")
)

// FileStore is a directory of files, each named by a hex key. The file for
// a key is at <basedir>/<ab>/<cd>/<key>, where <ab> and <cd> are the key's
// first and second pairs of characters. The cache keeps its source images in
// one FileStore and its variants in another.
type FileStore struct {
	baseDir string
}

// NewFileStore creates the directory baseDir if it is missing and returns
// the FileStore of the files in it.
func NewFileStore(baseDir string) (*FileStore, error) {
	err := os.MkdirAll(baseDir, StorageDirPerm)
	if err != nil {
		return nil, fmt.Errorf("failed to create storage directory: %w", err)
	}

	return &FileStore{baseDir: baseDir}, nil
}

// Write writes data with writeFileSynced to the file for key, replacing any
// file already there.
func (s *FileStore) Write(key string, data []byte) error {
	path := s.keyToPath(key)

	err := os.MkdirAll(filepath.Dir(path), StorageDirPerm)
	if err != nil {
		return fmt.Errorf("failed to create directory: %w", err)
	}

	return writeFileSynced(path, data)
}

// Open opens the file for key and returns it with its size in bytes. A
// missing file is ErrNotFound.
func (s *FileStore) Open(key string) (*os.File, int64, error) {
	f, err := os.Open(s.keyToPath(key))
	if err != nil {
		if os.IsNotExist(err) {
			return nil, 0, ErrNotFound
		}

		return nil, 0, fmt.Errorf("failed to open content: %w", err)
	}

	stat, err := f.Stat()
	if err != nil {
		_ = f.Close()

		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
	}

	return f, stat.Size(), nil
}

// Size returns the size in bytes of the file for key. A missing file is
// ErrNotFound.
func (s *FileStore) Size(key string) (int64, error) {
	stat, err := os.Stat(s.keyToPath(key))
	if err != nil {
		if os.IsNotExist(err) {
			return 0, ErrNotFound
		}

		return 0, fmt.Errorf("failed to stat content: %w", err)
	}

	return stat.Size(), nil
}

// Delete removes the file for key. A missing file is not an error.
func (s *FileStore) Delete(key string) error {
	err := os.Remove(s.keyToPath(key))
	if err != nil && !os.IsNotExist(err) {
		return fmt.Errorf("failed to delete content: %w", err)
	}

	return nil
}

// writeFileSynced writes data to path through a temp file in the same
// directory: the temp file is synced to disk, renamed to path, and the
// directory synced after, so that neither a concurrent reader nor a
// crash finds a partial file under path. When the directory sync fails,
// the file is removed from path, so that a failed write leaves no file
// there.
func writeFileSynced(path string, data []byte) error {
	dir := filepath.Dir(path)

	tmpFile, err := os.CreateTemp(dir, ".tmp-*")
	if err != nil {
		return fmt.Errorf("failed to create temp file: %w", err)
	}

	tmpPath := tmpFile.Name()

	_, err = tmpFile.Write(data)
	if err != nil {
		_ = tmpFile.Close()
		_ = os.Remove(tmpPath)

		return fmt.Errorf("failed to write temp file: %w", err)
	}

	err = tmpFile.Sync()
	if err != nil {
		_ = tmpFile.Close()
		_ = os.Remove(tmpPath)

		return fmt.Errorf("failed to sync temp file: %w", err)
	}

	err = tmpFile.Close()
	if err != nil {
		_ = os.Remove(tmpPath)

		return fmt.Errorf("failed to close temp file: %w", err)
	}

	err = os.Rename(filepath.Clean(tmpPath), filepath.Clean(path))
	if err != nil {
		_ = os.Remove(tmpPath)

		return fmt.Errorf("failed to rename temp file: %w", err)
	}

	err = syncDirectory(dir)
	if err != nil {
		_ = os.Remove(path)

		return err
	}

	return nil
}

// syncDirectory syncs the directory dir to disk, so that a file just
// renamed into it is still there after a crash.
func syncDirectory(dir string) error {
	d, err := os.Open(filepath.Clean(dir))
	if err != nil {
		return fmt.Errorf("failed to open directory: %w", err)
	}

	err = d.Sync()
	_ = d.Close()

	if err != nil {
		return fmt.Errorf("failed to sync directory: %w", err)
	}

	return nil
}

// keyToPath converts a key to a file path: <basedir>/<ab>/<cd>/<key>
func (s *FileStore) keyToPath(key string) string {
	if len(key) < MinHashLength {
		return filepath.Join(s.baseDir, key)
	}

	return filepath.Join(s.baseDir, key[0:2], key[2:4], key)
}