Don't retry when "no space left on device" in local backend

Also adds relevant documentation to the restic.Backend interface.

internal/backend/local/local.go

@@ -13,6 +13,8 @@ import (
 	"github.com/restic/restic/internal/backend"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/fs"
+
+	"github.com/cenkalti/backoff/v4"
 )
 
 // Local is a backend in a local directory.
@@ -80,7 +82,7 @@ func (b *Local) IsNotExist(err error) bool {
 }
 
 // Save stores data in the backend at the handle.
-func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) error {
+func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) (err error) {
 	debug.Log("Save %v", h)
 	if err := h.Valid(); err != nil {
 		return err
@@ -88,8 +90,16 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 
 	filename := b.Filename(h)
 
+	defer func() {
+		// Mark non-retriable errors as such (currently only
+		// "no space left on device").
+		if errors.Is(err, syscall.ENOSPC) {
+			err = backoff.Permanent(err)
+		}
+	}()
+
 	// create new file
-	f, err := fs.OpenFile(filename, os.O_CREATE|os.O_EXCL|os.O_WRONLY, backend.Modes.File)
+	f, err := openFile(filename, os.O_CREATE|os.O_EXCL|os.O_WRONLY, backend.Modes.File)
 
 	if b.IsNotExist(err) {
 		debug.Log("error %v: creating dir", err)
@@ -100,7 +110,7 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 			debug.Log("error creating dir %v: %v", filepath.Dir(filename), mkdirErr)
 		} else {
 			// try again
-			f, err = fs.OpenFile(filename, os.O_CREATE|os.O_EXCL|os.O_WRONLY, backend.Modes.File)
+			f, err = openFile(filename, os.O_CREATE|os.O_EXCL|os.O_WRONLY, backend.Modes.File)
 		}
 	}
 
@@ -141,6 +151,8 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 	return nil
 }
 
+var openFile = fs.OpenFile // Overridden by test.
+
 // Load runs fn with a reader that yields the contents of the file at h at the
 // given offset.
 func (b *Local) Load(ctx context.Context, h restic.Handle, length int, offset int64, fn func(rd io.Reader) error) error {

internal/backend/local/local_internal_test.go

@@ -0,0 +1,42 @@
+package local
+
+import (
+	"context"
+	"errors"
+	"os"
+	"syscall"
+	"testing"
+
+	"github.com/restic/restic/internal/restic"
+	rtest "github.com/restic/restic/internal/test"
+
+	"github.com/cenkalti/backoff/v4"
+)
+
+func TestNoSpacePermanent(t *testing.T) {
+	oldOpenFile := openFile
+	defer func() {
+		openFile = oldOpenFile
+	}()
+
+	openFile = func(name string, flags int, mode os.FileMode) (*os.File, error) {
+		// The actual error from os.OpenFile is *os.PathError.
+		// Other functions called inside Save may return *os.SyscallError.
+		return nil, os.NewSyscallError("open", syscall.ENOSPC)
+	}
+
+	dir, cleanup := rtest.TempDir(t)
+	defer cleanup()
+
+	be, err := Open(context.Background(), Config{Path: dir})
+	rtest.OK(t, err)
+	defer be.Close()
+
+	h := restic.Handle{Type: restic.ConfigFile}
+	err = be.Save(context.Background(), h, nil)
+	_, ok := err.(*backoff.PermanentError)
+	rtest.Assert(t, ok,
+		"error type should be backoff.PermanentError, got %T", err)
+	rtest.Assert(t, errors.Is(err, syscall.ENOSPC),
+		"could not recover original ENOSPC error")
+}