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_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")
+}