helper: clean up and separate helper process management from dbus

The previous code was poorly documented and made little sense in some parts. This is a generalised and cleaned up implementation in the helper package making use of the Args interface.

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

helper/helper.go

@@ -2,3 +2,183 @@
 Package helper runs external helpers and manages their status and args FDs.
 */
 package helper
+
+import (
+	"errors"
+	"io"
+	"os"
+	"os/exec"
+	"sync"
+)
+
+var (
+	ErrStatusFault = errors.New("generic status pipe fault")
+	ErrStatusRead  = errors.New("unexpected status response")
+)
+
+// Helper wraps *exec.Cmd and manages status and args fd.
+// Args is always 3 and status if set is always 4.
+type Helper struct {
+	lock sync.RWMutex
+	args io.WriterTo
+
+	statP [2]*os.File
+	argsP [2]*os.File
+
+	ready chan error
+
+	// ExtraFiles specifies additional open files to be inherited by the
+	// new process. It does not include standard input, standard output, or
+	// standard error. If non-nil, entry i becomes file descriptor 5+i.
+	ExtraFiles []*os.File
+
+	*exec.Cmd
+}
+
+func (h *Helper) StartNotify(ready chan error) error {
+	h.lock.Lock()
+	defer h.lock.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.Process != nil {
+		return errors.New("exec: already started")
+	}
+
+	// create pipes
+	if pr, pw, err := os.Pipe(); err != nil {
+		return err
+	} else {
+		h.argsP[0], h.argsP[1] = pr, pw
+	}
+	// create status pipes if ready signal is requested
+	if ready != nil {
+		if pr, pw, err := os.Pipe(); err != nil {
+			return err
+		} else {
+			h.statP[0], h.statP[1] = pr, pw
+		}
+	}
+
+	// prepare extra files
+	el := len(h.ExtraFiles)
+	if ready != nil {
+		el += 2
+	} else {
+		el++
+	}
+	ef := make([]*os.File, 0, el)
+	ef = append(ef, h.argsP[0])
+	if ready != nil {
+		ef = append(ef, h.statP[1])
+	}
+	ef = append(ef, h.ExtraFiles...)
+
+	// prepare and start process
+	h.Cmd.ExtraFiles = ef
+	if err := h.Cmd.Start(); err != nil {
+		return err
+	}
+
+	statsP, argsP := h.statP[0], h.argsP[1]
+
+	// write arguments and close args pipe
+	if _, err := h.args.WriteTo(argsP); err != nil {
+		if err1 := h.Cmd.Process.Kill(); err1 != nil {
+			panic(err1)
+		}
+		return err
+	} else {
+		if err = argsP.Close(); err != nil {
+			if err1 := h.Cmd.Process.Kill(); err1 != nil {
+				panic(err1)
+			}
+			return err
+		}
+	}
+
+	if ready != nil {
+		h.ready = ready
+
+		// monitor stat pipe
+		go func() {
+			n, err := statsP.Read(make([]byte, 1))
+			switch n {
+			case -1:
+				if err1 := h.Cmd.Process.Kill(); err1 != nil {
+					panic(err1)
+				}
+				// ensure error is not nil
+				if err == nil {
+					err = ErrStatusFault
+				}
+				ready <- err
+			case 0:
+				// ensure error is not nil
+				if err == nil {
+					err = ErrStatusRead
+				}
+				ready <- err
+			case 1:
+				ready <- nil
+			default:
+				panic("unreachable") // unexpected read count
+			}
+		}()
+	}
+
+	return nil
+}
+
+func (h *Helper) Wait() error {
+	h.lock.RLock()
+	defer h.lock.RUnlock()
+
+	if h.Cmd.Process == nil {
+		return errors.New("exec: not started")
+	}
+	if h.Cmd.ProcessState != nil {
+		return errors.New("exec: Wait was already called")
+	}
+
+	// ensure pipe close
+	defer func() {
+		if err := h.argsP[0].Close(); err != nil && !errors.Is(err, os.ErrClosed) {
+			panic(err)
+		}
+		if err := h.argsP[1].Close(); err != nil && !errors.Is(err, os.ErrClosed) {
+			panic(err)
+		}
+
+		if h.ready != nil {
+			if err := h.statP[0].Close(); err != nil && !errors.Is(err, os.ErrClosed) {
+				panic(err)
+			}
+			if err := h.statP[1].Close(); err != nil && !errors.Is(err, os.ErrClosed) {
+				panic(err)
+			}
+		}
+	}()
+
+	return h.Cmd.Wait()
+}
+
+func (h *Helper) Close() error {
+	if h.ready == nil {
+		panic("attempted to close helper with no status pipe")
+	}
+
+	return h.statP[0].Close()
+}
+
+func (h *Helper) Start() error {
+	return h.StartNotify(nil)
+}
+
+func New(wt io.WriterTo, name string, arg ...string) *Helper {
+	if wt == nil {
+		panic("attempted to create helper with nil argument writer")
+	}
+
+	return &Helper{args: wt, Cmd: exec.Command(name, arg...)}
+}