lnd.xprv/chanbackup/backupfile.go

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package chanbackup
import (
"fmt"
"io/ioutil"
"os"
"path/filepath"
"github.com/lightningnetwork/lnd/keychain"
)
const (
// DefaultBackupFileName is the default name of the auto updated static
// channel backup fie.
DefaultBackupFileName = "channel.backup"
// DefaultTempBackupFileName is the default name of the temporary SCB
// file that we'll use to atomically update the primary back up file
// when new channel are detected.
DefaultTempBackupFileName = "temp-dont-use.backup"
)
var (
// ErrNoBackupFileExists is returned if caller attempts to call
// UpdateAndSwap with the file name not set.
ErrNoBackupFileExists = fmt.Errorf("back up file name not set")
// ErrNoTempBackupFile is returned if caller attempts to call
// UpdateAndSwap with the temp back up file name not set.
ErrNoTempBackupFile = fmt.Errorf("temp backup file not set")
)
// MultiFile represents a file on disk that a caller can use to read the packed
// multi backup into an unpacked one, and also atomically update the contents
// on disk once new channels have been opened, and old ones closed. This struct
// relies on an atomic file rename property which most widely use file systems
// have.
type MultiFile struct {
// fileName is the file name of the main back up file.
fileName string
// mainFile is an open handle to the main back up file.
mainFile *os.File
// tempFileName is the name of the file that we'll use to stage a new
// packed multi-chan backup, and the rename to the main back up file.
tempFileName string
// tempFile is an open handle to the temp back up file.
tempFile *os.File
}
// NewMultiFile create a new multi-file instance at the target location on the
// file system.
func NewMultiFile(fileName string) *MultiFile {
// We'll our temporary backup file in the very same directory as the
// main backup file.
backupFileDir := filepath.Dir(fileName)
tempFileName := filepath.Join(
backupFileDir, DefaultTempBackupFileName,
)
return &MultiFile{
fileName: fileName,
tempFileName: tempFileName,
}
}
// UpdateAndSwap will attempt write a new temporary backup file to disk with
// the newBackup encoded, then atomically swap (via rename) the old file for
// the new file by updating the name of the new file to the old.
func (b *MultiFile) UpdateAndSwap(newBackup PackedMulti) error {
// If the main backup file isn't set, then we can't proceed.
if b.fileName == "" {
return ErrNoBackupFileExists
}
log.Infof("Updating backup file at %v", b.fileName)
// If the old back up file still exists, then we'll delete it before
// proceeding.
if _, err := os.Stat(b.tempFileName); err == nil {
log.Infof("Found old temp backup @ %v, removing before swap",
b.tempFileName)
err = os.Remove(b.tempFileName)
if err != nil {
return fmt.Errorf("unable to remove temp "+
"backup file: %v", err)
}
}
// Now that we know the staging area is clear, we'll create the new
// temporary back up file.
var err error
b.tempFile, err = os.Create(b.tempFileName)
if err != nil {
return fmt.Errorf("unable to create temp file: %v", err)
}
// With the file created, we'll write the new packed multi backup and
// remove the temporary file all together once this method exits.
_, err = b.tempFile.Write([]byte(newBackup))
if err != nil {
return fmt.Errorf("unable to write backup to temp file: %v", err)
}
if err := b.tempFile.Sync(); err != nil {
return fmt.Errorf("unable to sync temp file: %v", err)
}
defer os.Remove(b.tempFileName)
log.Infof("Swapping old multi backup file from %v to %v",
b.tempFileName, b.fileName)
// Finally, we'll attempt to atomically rename the temporary file to
// the main back up file. If this succeeds, then we'll only have a
// single file on disk once this method exits.
return os.Rename(b.tempFileName, b.fileName)
}
// ExtractMulti attempts to extract the packed multi backup we currently point
// to into an unpacked version. This method will fail if no backup file
// currently exists as the specified location.
func (b *MultiFile) ExtractMulti(keyChain keychain.KeyRing) (*Multi, error) {
var err error
// If the backup file isn't already set, then we'll attempt to open it
// anew.
if b.mainFile == nil {
// We'll return an error if the main file isn't currently set.
if b.fileName == "" {
return nil, ErrNoBackupFileExists
}
// Otherwise, we'll open the file to prep for reading the
// contents.
b.mainFile, err = os.Open(b.fileName)
if err != nil {
return nil, err
}
}
// Before we start to read the file, we'll ensure that the next read
// call will start from the front of the file.
_, err = b.mainFile.Seek(0, 0)
if err != nil {
return nil, err
}
// With our seek successful, we'll now attempt to read the contents of
// the entire file in one swoop.
multiBytes, err := ioutil.ReadAll(b.mainFile)
if err != nil {
return nil, err
}
// Finally, we'll attempt to unpack the file and return the unpack
// version to the caller.
packedMulti := PackedMulti(multiBytes)
return packedMulti.Unpack(keyChain)
}