proc/pipe: implement args and stat file

This is a generic implementation of helper/pipe.

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

helper/proc/pipe.go

@@ -0,0 +1,100 @@
+package proc
+
+import (
+	"context"
+	"errors"
+	"io"
+	"os"
+)
+
+// NewWriterTo returns a [File] that receives content from wt on fulfillment.
+func NewWriterTo(wt io.WriterTo) File { return &writeToFile{wt: wt} }
+
+// writeToFile exports the read end of a pipe with data written by an [io.WriterTo].
+type writeToFile struct {
+	wt io.WriterTo
+	BaseFile
+}
+
+func (f *writeToFile) ErrCount() int { return 3 }
+func (f *writeToFile) Fulfill(ctx context.Context, dispatchErr func(error)) error {
+	r, w, err := os.Pipe()
+	if err != nil {
+		return err
+	}
+	f.Set(r)
+
+	done := make(chan struct{})
+	go func() { _, err = f.wt.WriteTo(w); dispatchErr(err); dispatchErr(w.Close()); close(done) }()
+	go func() {
+		select {
+		case <-done:
+			dispatchErr(nil)
+		case <-ctx.Done():
+			dispatchErr(w.Close()) // this aborts WriteTo with file already closed
+		}
+	}()
+
+	return nil
+}
+
+// NewStat returns a [File] implementing the behaviour
+// of the receiving end of xdg-dbus-proxy stat fd.
+func NewStat(s *io.Closer) File { return &statFile{s: s} }
+
+var (
+	ErrStatFault = errors.New("generic stat fd fault")
+	ErrStatRead  = errors.New("unexpected stat behaviour")
+)
+
+// statFile implements xdg-dbus-proxy stat fd behaviour.
+type statFile struct {
+	s *io.Closer
+	BaseFile
+}
+
+func (f *statFile) ErrCount() int { return 2 }
+func (f *statFile) Fulfill(ctx context.Context, dispatchErr func(error)) error {
+	r, w, err := os.Pipe()
+	if err != nil {
+		return err
+	}
+	f.Set(w)
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		var n int
+
+		n, err = r.Read(make([]byte, 1))
+		switch n {
+		case -1:
+			if err == nil {
+				err = ErrStatFault
+			}
+			dispatchErr(err)
+		case 0:
+			if err == nil {
+				err = ErrStatRead
+			}
+			dispatchErr(err)
+		case 1:
+			dispatchErr(err)
+		default:
+			panic("unreachable")
+		}
+	}()
+
+	go func() {
+		select {
+		case <-done:
+			dispatchErr(nil)
+		case <-ctx.Done():
+			dispatchErr(r.Close()) // this aborts Read with file already closed
+		}
+	}()
+
+	// this gets closed by the caller
+	*f.s = r
+	return nil
+}