helper: separate helper args fd builder from dbus

This method of passing arguments is used in bubblewrap as well as other tools, this commit separates the argument builder/writer to the helper package and generalise it as an interface.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

helper/args.go

@@ -0,0 +1,97 @@
+package helper
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+	"sync"
+)
+
+var (
+	ErrContainsNull = errors.New("argument contains null character")
+)
+
+// Args is sealed with a slice of arguments for writing to the helper args FD.
+// The sealing args is checked to not contain null characters.
+// Attempting to seal an instance twice will cause a panic.
+type Args interface {
+	Seal(args []string) error
+	io.WriterTo
+	fmt.Stringer
+}
+
+// argsFD implements Args for helpers expecting null terminated arguments to a file descriptor.
+// argsFD must not be copied after first use.
+type argsFD struct {
+	seal []byte
+	sync.RWMutex
+}
+
+func (a *argsFD) Seal(args []string) error {
+	a.Lock()
+	defer a.Unlock()
+
+	if a.seal != nil {
+		panic("args sealed twice")
+	}
+
+	seal := bytes.Buffer{}
+
+	n := 0
+	for _, arg := range args {
+		// reject argument strings containing null
+		if hasNull(arg) {
+			return ErrContainsNull
+		}
+
+		// accumulate buffer size
+		n += len(arg) + 1
+	}
+	seal.Grow(n)
+
+	// write null terminated arguments
+	for _, arg := range args {
+		seal.WriteString(arg)
+		seal.WriteByte('\x00')
+	}
+
+	a.seal = seal.Bytes()
+	return nil
+}
+
+func (a *argsFD) WriteTo(w io.Writer) (int64, error) {
+	if a.seal == nil {
+		panic("attempted to activate unsealed args")
+	}
+
+	n, err := w.Write(a.seal)
+	return int64(n), err
+}
+
+func (a *argsFD) String() string {
+	if a == nil {
+		return "(invalid helper args)"
+	}
+
+	if a.seal == nil {
+		return "(unsealed helper args)"
+	}
+
+	return strings.ReplaceAll(string(a.seal), "\x00", " ")
+}
+
+func hasNull(s string) bool {
+	for _, b := range s {
+		if b == '\x00' {
+			return true
+		}
+	}
+	return false
+}
+
+// NewArgs returns a new instance of Args
+func NewArgs() Args {
+	return new(argsFD)
+}

helper/helper.go

@@ -0,0 +1,4 @@
+/*
+Package helper runs external helpers and manages their status and args FDs.
+*/
+package helper