galaxy-game/internal/repo/fs/fs.go

package fs

import (
	"encoding"
	"errors"
	"fmt"
	"math/big"
	"os"
	"path/filepath"
	"time"
)

const (
	defaultPerm   = 0o644
	lockFile      = ".lock"
	oldFileSuffix = ".old"
	newFileSuffix = ".new"
)

type fs struct {
	root string
	lock *os.File
}

func NewFileStorage(path string) (*fs, error) {
	absPath, err := filepath.Abs(path)
	if err != nil {
		return nil, fmt.Errorf("path %s invalid: %s", path, err)
	}
	if ok, err := dirExists(absPath); err != nil {
		return nil, fmt.Errorf("check dir exist: %s", err)
	} else if !ok {
		return nil, errors.New("directory does not exist: " + absPath)
	}

	if ok, err := writable(absPath); err != nil {
		return nil, fmt.Errorf("check dir access: %s", err)
	} else if !ok {
		return nil, errors.New("directory should have read-write access: " + absPath)
	}

	fs := &fs{
		root: path,
	}
	return fs, nil
}

func (f *fs) Lock() (func() error, error) {
	lockPath := f.lockFilePath()
	exists, err := fileExists(lockPath)
	if err != nil {
		return nil, fmt.Errorf("check lock file exists: %s", err)
	}
	if exists {
		return nil, errors.New("lock file already exists")
	}
	fd, err := os.OpenFile(lockPath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o600)
	if err != nil {
		return nil, fmt.Errorf("create lock file: %s", err)
	}
	f.lock = fd
	unlock := func() error {
		if err := f.lock.Close(); err != nil {
			return fmt.Errorf("close lock file: %s", err)
		}
		if err := os.Remove(f.lock.Name()); err != nil {
			return fmt.Errorf("remove lock file: %s", err)
		}
		f.lock = nil
		return nil
	}
	if _, err := f.lock.Write(big.NewInt(time.Now().UnixMilli()).Bytes()); err != nil {
		return nil, errors.Join(fmt.Errorf("write lock file: %s", err), unlock())
	}
	return unlock, nil
}

func (f *fs) Exists(path string) (bool, error) {
	return fileExists(filepath.Join(f.root, path))
}

func (f *fs) WriteSafe(path string, v encoding.BinaryMarshaler) error {
	if v == nil {
		return errors.New("cant't marshal from nil object")
	}

	targetFilePath := filepath.Join(f.root, path)
	if targetFilePath == f.lockFilePath() {
		return errors.New("can't write to the lock file")
	}

	data, err := v.MarshalBinary()
	if err != nil {
		return fmt.Errorf("marshal data: %s", err)
	}

	targetDir := filepath.Dir(targetFilePath)
	if targetDir != f.root {
		ok, err := dirExists(targetDir)
		if err != nil {
			return fmt.Errorf("check target dir exists: %s", err)
		}
		if !ok {
			err := os.MkdirAll(targetDir, os.ModePerm)
			if err != nil {
				return fmt.Errorf("create target dirs: %s", err)
			}
		}
	}
	oldFilePath := targetFilePath + oldFileSuffix

	targetExists, err := fileExists(targetFilePath)
	if err != nil {
		return fmt.Errorf("check target file exists: %s", err)
	}
	if targetExists {
		oldFileExists, err := fileExists(oldFilePath)
		if err != nil {
			return fmt.Errorf("check old file exists: %s", err)
		}
		if oldFileExists {
			return fmt.Errorf("old file exists at: %s", oldFilePath)
		}
	}

	newFilePath := targetFilePath + newFileSuffix
	newFileExists, err := fileExists(newFilePath)
	if err != nil {
		return fmt.Errorf("check new file exists: %s", err)
	}
	if newFileExists {
		return fmt.Errorf("new file exists at: %s", oldFilePath)
	}

	err = os.WriteFile(newFilePath, data, defaultPerm)
	if err != nil {
		return fmt.Errorf("write data to the new file: %s", err)
	}

	if targetExists {
		if err := os.Rename(targetFilePath, oldFilePath); err != nil {
			return fmt.Errorf("rename target file to the old file: %s", err)
		}
	}

	if err := os.Rename(newFilePath, targetFilePath); err != nil {
		return fmt.Errorf("rename new file to the target file: %s", err)
	}

	if targetExists {
		err := os.Remove(oldFilePath)
		if err != nil {
			return fmt.Errorf("remove old file: %s", err)
		}
	}
	return nil
}

func (f *fs) Write(path string, v encoding.BinaryMarshaler) error {
	if f.lock == nil {
		return errors.New("lock must be acquired before write")
	}

	return f.WriteSafe(path, v)
}

func (f *fs) Read(path string, v encoding.BinaryUnmarshaler) error {
	if f.lock == nil {
		return errors.New("lock must be acquired before read")
	}

	return f.readUnsafe(path, v)
}

func (f *fs) ReadSafe(path string, v encoding.BinaryUnmarshaler) error {
	if f.lock != nil {
		timeout := time.NewTimer(time.Millisecond * 100)
		checker := time.NewTicker(time.Millisecond)
	out:
		for {
			select {
			case <-checker.C:
				if f.lock == nil {
					checker.Stop()
					timeout.Stop()
					break out
				}
			case <-timeout.C:
				checker.Stop()
				return errors.New("timeout waiting for lock release")
			}
		}
	}

	return f.readUnsafe(path, v)
}

// readUnsafe reads the file contents without locking mechanism in mind.
// Using readUnsafe directly may cause errors if file being written at the moment.
func (f *fs) readUnsafe(file string, v encoding.BinaryUnmarshaler) error {
	if v == nil {
		return errors.New("can't unmarshal to a nil object")
	}

	targetFilePath := filepath.Join(f.root, file)
	if targetFilePath == f.lockFilePath() {
		return errors.New("can't read from the lock file")
	}

	data, err := os.ReadFile(targetFilePath)
	if err != nil {
		return fmt.Errorf("reading data file: %s", err)
	}

	return v.UnmarshalBinary(data)
}

func (f *fs) lockFilePath() string {
	return filepath.Join(f.root, lockFile)
}