internal/helper: relocate from helper

This package is ugly and is pending removal only kept alive by xdg-dbus-proxy.

Its exported symbols are made available until v0.4.0 where it will be removed for #24.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/helper/args.go

@@ -0,0 +1,59 @@
+package helper
+
+import (
+	"bytes"
+	"io"
+	"syscall"
+)
+
+type argsWt [][]byte
+
+func (a argsWt) WriteTo(w io.Writer) (int64, error) {
+	nt := 0
+	for _, arg := range a {
+		n, err := w.Write(arg)
+		nt += n
+
+		if err != nil {
+			return int64(nt), err
+		}
+	}
+
+	return int64(nt), nil
+}
+
+func (a argsWt) String() string {
+	return string(
+		bytes.TrimSuffix(
+			bytes.ReplaceAll(
+				bytes.Join(a, nil),
+				[]byte{0}, []byte{' '},
+			),
+			[]byte{' '},
+		),
+	)
+}
+
+// NewCheckedArgs returns a checked null-terminated argument writer for a copy of args.
+func NewCheckedArgs(args ...string) (wt io.WriterTo, err error) {
+	a := make(argsWt, len(args))
+	for i, arg := range args {
+		a[i], err = syscall.ByteSliceFromString(arg)
+		if err != nil {
+			return
+		}
+	}
+	wt = a
+	return
+}
+
+// MustNewCheckedArgs returns a checked null-terminated argument writer for a copy of args.
+// If s contains a NUL byte this function panics instead of returning an error.
+func MustNewCheckedArgs(args ...string) io.WriterTo {
+	a, err := NewCheckedArgs(args...)
+	if err != nil {
+		panic(err.Error())
+	}
+
+	return a
+}