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sftp: Implement atomic uploads

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Create a temporary file with a sufficiently random name to essentially
avoid any chance of conflicts. Once the upload has finished remove the
temporary suffix. Interrupted upload thus will be ignored by restic.
This commit is contained in:
Michael Eischer
2021-09-22 22:28:57 +02:00
parent cc90f2ba6b
commit 5ec312ca06
2 changed files with 33 additions and 4 deletions
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27
internal/backend/sftp/sftp.go
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@@ -3,6 +3,8 @@ package sftp
import (
"bufio"
"context"
"crypto/rand"
"encoding/hex"
"fmt"
"hash"
"io"
@@ -252,6 +254,17 @@ func Join(parts ...string) string {
return path.Clean(path.Join(parts...))
}
// tempSuffix generates a random string suffix that should be sufficiently long
// to avoid accidential conflicts
func tempSuffix() string {
var nonce [16]byte
_, err := rand.Read(nonce[:])
if err != nil {
panic(err)
}
return hex.EncodeToString(nonce[:])
}
// Save stores data in the backend at the handle.
func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) error {
debug.Log("Save %v", h)
@@ -264,10 +277,11 @@ func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader
}
filename := r.Filename(h)
tmpFilename := filename + "-restic-temp-" + tempSuffix()
dirname := r.Dirname(h)
// create new file
f, err := r.c.OpenFile(filename, os.O_CREATE|os.O_EXCL|os.O_WRONLY)
f, err := r.c.OpenFile(tmpFilename, os.O_CREATE|os.O_EXCL|os.O_WRONLY)
if r.IsNotExist(err) {
// error is caused by a missing directory, try to create it
@@ -276,7 +290,7 @@ func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader
debug.Log("error creating dir %v: %v", r.Dirname(h), mkdirErr)
} else {
// try again
f, err = r.c.OpenFile(filename, os.O_CREATE|os.O_EXCL|os.O_WRONLY)
f, err = r.c.OpenFile(tmpFilename, os.O_CREATE|os.O_EXCL|os.O_WRONLY)
}
}
@@ -298,7 +312,7 @@ func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader
rmErr := r.c.Remove(f.Name())
if rmErr != nil {
debug.Log("sftp: failed to remove broken file %v: %v",
filename, rmErr)
f.Name(), rmErr)
}
err = r.checkNoSpace(dirname, rd.Length(), err)
@@ -318,7 +332,12 @@ func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader
}
err = f.Close()
return errors.Wrap(err, "Close")
if err != nil {
return errors.Wrap(err, "Close")
}
err = r.c.Rename(tmpFilename, filename)
return errors.Wrap(err, "Rename")
}
// checkNoSpace checks if err was likely caused by lack of available space