helper: combine helper ipc setup

The two-step args call is no longer necessary since stat is passed on initialisation.

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

helper/cmd.go

@@ -0,0 +1,87 @@
+package helper
+
+import (
+	"context"
+	"errors"
+	"io"
+	"os"
+	"os/exec"
+	"slices"
+	"sync"
+	"syscall"
+
+	"git.gensokyo.uk/security/fortify/helper/proc"
+)
+
+// NewDirect initialises a new direct Helper instance with wt as the null-terminated argument writer.
+// Function argF returns an array of arguments passed directly to the child process.
+func NewDirect(
+	ctx context.Context,
+	name string,
+	wt io.WriterTo,
+	stat bool,
+	argF func(argsFd, statFd int) []string,
+	cmdF func(cmd *exec.Cmd),
+) Helper {
+	d, args := newHelperCmd(ctx, name, wt, stat, argF, nil)
+	d.Args = append(d.Args, args...)
+	if cmdF != nil {
+		cmdF(d.Cmd)
+	}
+	return d
+}
+
+func newHelperCmd(
+	ctx context.Context,
+	name string,
+	wt io.WriterTo,
+	stat bool,
+	argF func(argsFd, statFd int) []string,
+	extraFiles []*os.File,
+) (cmd *helperCmd, args []string) {
+	cmd = new(helperCmd)
+	cmd.helperFiles, args = newHelperFiles(ctx, wt, stat, argF, extraFiles)
+	cmd.Cmd = commandContext(ctx, name)
+	cmd.Cmd.Cancel = func() error { return cmd.Process.Signal(syscall.SIGTERM) }
+	cmd.WaitDelay = WaitDelay
+	return
+}
+
+// helperCmd provides a [exec.Cmd] wrapper around helper ipc.
+type helperCmd struct {
+	mu sync.RWMutex
+	*helperFiles
+	*exec.Cmd
+}
+
+// finalise sets up the underlying [exec.Cmd] object.
+func (h *helperCmd) finalise() {
+	h.Env = slices.Grow(h.Env, 2)
+	if h.useArgsFd {
+		h.Cmd.Env = append(h.Env, FortifyHelper+"=1")
+	} else {
+		h.Cmd.Env = append(h.Env, FortifyHelper+"=0")
+	}
+	if h.useStatFd {
+		h.Cmd.Env = append(h.Cmd.Env, FortifyStatus+"=1")
+
+		// stat is populated on fulfill
+		h.Cmd.Cancel = func() error { return h.stat.Close() }
+	} else {
+		h.Cmd.Env = append(h.Cmd.Env, FortifyStatus+"=0")
+	}
+}
+
+func (h *helperCmd) Start() error {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	// Check for doubled Start calls before we defer failure cleanup. If the prior
+	// call to Start succeeded, we don't want to spuriously close its pipes.
+	if h.Cmd != nil && h.Cmd.Process != nil {
+		return errors.New("exec: already started")
+	}
+
+	h.finalise()
+	return proc.Fulfill(h.helperFiles.ctx, &h.ExtraFiles, h.Cmd.Start, h.files, h.extraFiles)
+}