helper: use generic extra files interface

This replaces the pipes object and integrates context into helper process lifecycle.

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

helper/helper.go

@@ -2,35 +2,133 @@
 package helper
 
 import (
-	"errors"
+	"context"
+	"fmt"
+	"io"
+	"os"
 	"os/exec"
+	"slices"
+	"syscall"
+	"time"
+
+	"git.gensokyo.uk/security/fortify/helper/proc"
 )
 
 var (
-	ErrStatusFault = errors.New("generic status pipe fault")
-	ErrStatusRead  = errors.New("unexpected status response")
+	WaitDelay = 2 * time.Second
 )
 
 const (
-	// FortifyHelper is set for the process launched by Helper.
+	// FortifyHelper is set to 1 when args fd is enabled and 0 otherwise.
 	FortifyHelper = "FORTIFY_HELPER"
-	// FortifyStatus is 1 when sync fd is enabled and 0 otherwise.
+	// FortifyStatus is set to 1 when stat fd is enabled and 0 otherwise.
 	FortifyStatus = "FORTIFY_STATUS"
 )
 
 type Helper interface {
-	// StartNotify starts the helper process.
-	// A status pipe is passed to the helper if ready is not nil.
-	StartNotify(ready chan error) error
+	// Stdin sets the standard input of Helper.
+	Stdin(r io.Reader) Helper
+	// Stdout sets the standard output of Helper.
+	Stdout(w io.Writer) Helper
+	// Stderr sets the standard error of Helper.
+	Stderr(w io.Writer) Helper
+	// SetEnv sets the environment of Helper.
+	SetEnv(env []string) Helper
+
 	// Start starts the helper process.
-	Start() error
-	// Close closes the status pipe.
-	// If helper is started without the status pipe, Close panics.
-	Close() error
-	// Wait calls wait on the child process and cleans up pipes.
+	// A status pipe is passed to the helper if stat is true.
+	Start(ctx context.Context, stat bool) error
+	// Wait blocks until Helper exits and releases all its resources.
 	Wait() error
-	// Unwrap returns the underlying exec.Cmd instance.
-	Unwrap() *exec.Cmd
+
+	fmt.Stringer
 }
 
-var execCommand = exec.Command
+func newHelperCmd(
+	h Helper, name string,
+	wt io.WriterTo, argF func(argsFd, statFd int) []string,
+	extraFiles []*os.File,
+) (cmd *helperCmd) {
+	cmd = new(helperCmd)
+
+	cmd.r = h
+	cmd.name = name
+
+	cmd.extraFiles = new(proc.ExtraFilesPre)
+	for _, f := range extraFiles {
+		_, v := cmd.extraFiles.Append()
+		*v = f
+	}
+
+	argsFd := -1
+	if wt != nil {
+		f := proc.NewWriterTo(wt)
+		argsFd = int(proc.InitFile(f, cmd.extraFiles))
+		cmd.files = append(cmd.files, f)
+		cmd.hasArgsFd = true
+	}
+	cmd.argF = func(statFd int) []string { return argF(argsFd, statFd) }
+
+	return
+}
+
+// helperCmd wraps Cmd and implements methods shared across all Helper implementations.
+type helperCmd struct {
+	// ref to parent
+	r Helper
+
+	// returns an array of arguments passed directly
+	// to the helper process
+	argF func(statFd int) []string
+	// whether argsFd is present
+	hasArgsFd bool
+
+	// closes statFd
+	stat io.Closer
+	// deferred extraFiles fulfillment
+	files []proc.File
+	// passed through to [proc.Fulfill] and [proc.InitFile]
+	extraFiles *proc.ExtraFilesPre
+
+	name           string
+	stdin          io.Reader
+	stdout, stderr io.Writer
+	env            []string
+	*exec.Cmd
+}
+
+func (h *helperCmd) Stdin(r io.Reader) Helper   { h.stdin = r; return h.r }
+func (h *helperCmd) Stdout(w io.Writer) Helper  { h.stdout = w; return h.r }
+func (h *helperCmd) Stderr(w io.Writer) Helper  { h.stderr = w; return h.r }
+func (h *helperCmd) SetEnv(env []string) Helper { h.env = env; return h.r }
+
+// finalise initialises the underlying [exec.Cmd] object.
+func (h *helperCmd) finalise(ctx context.Context, stat bool) (args []string) {
+	h.Cmd = commandContext(ctx, h.name)
+	h.Cmd.Stdin, h.Cmd.Stdout, h.Cmd.Stderr = h.stdin, h.stdout, h.stderr
+	h.Cmd.Env = slices.Grow(h.env, 2)
+	if h.hasArgsFd {
+		h.Cmd.Env = append(h.Cmd.Env, FortifyHelper+"=1")
+	} else {
+		h.Cmd.Env = append(h.Cmd.Env, FortifyHelper+"=0")
+	}
+
+	h.Cmd.Cancel = func() error { return h.Cmd.Process.Signal(syscall.SIGTERM) }
+	h.Cmd.WaitDelay = WaitDelay
+
+	statFd := -1
+	if stat {
+		f := proc.NewStat(&h.stat)
+		statFd = int(proc.InitFile(f, h.extraFiles))
+		h.files = append(h.files, f)
+		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")
+	}
+	return h.argF(statFd)
+}
+
+var commandContext = exec.CommandContext