internal/helper: relocate from helper

This package is ugly and is pending removal only kept alive by xdg-dbus-proxy.

Its exported symbols are made available until v0.4.0 where it will be removed for #24.

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

internal/helper/stub.go

@@ -0,0 +1,102 @@
+package helper
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+	"syscall"
+)
+
+// InternalHelperStub is an internal function but exported because it is cross-package;
+// it is part of the implementation of the helper stub.
+func InternalHelperStub() {
+	// this test mocks the helper process
+	var ap, sp string
+	if v, ok := os.LookupEnv(HakureiHelper); !ok {
+		return
+	} else {
+		ap = v
+	}
+	if v, ok := os.LookupEnv(HakureiStatus); !ok {
+		panic(HakureiStatus)
+	} else {
+		sp = v
+	}
+
+	genericStub(flagRestoreFiles(1, ap, sp))
+
+	os.Exit(0)
+}
+
+func newFile(fd int, name, p string) *os.File {
+	present := false
+	switch p {
+	case "0":
+	case "1":
+		present = true
+	default:
+		panic(fmt.Sprintf("%s fd has unexpected presence value %q", name, p))
+	}
+
+	f := os.NewFile(uintptr(fd), name)
+	if !present && f != nil {
+		panic(fmt.Sprintf("%s fd set but not present", name))
+	}
+	if present && f == nil {
+		panic(fmt.Sprintf("%s fd preset but unset", name))
+	}
+
+	return f
+}
+
+func flagRestoreFiles(offset int, ap, sp string) (argsFile, statFile *os.File) {
+	argsFd := flag.Int("args", -1, "")
+	statFd := flag.Int("fd", -1, "")
+	_ = flag.CommandLine.Parse(os.Args[offset:])
+	argsFile = newFile(*argsFd, "args", ap)
+	statFile = newFile(*statFd, "stat", sp)
+	return
+}
+
+func genericStub(argsFile, statFile *os.File) {
+	if argsFile != nil {
+		// this output is checked by parent
+		if _, err := io.Copy(os.Stdout, argsFile); err != nil {
+			panic("cannot read args: " + err.Error())
+		}
+	}
+
+	if statFile != nil {
+		// simulate status pipe behaviour
+		var epoll int
+		if fd, err := syscall.EpollCreate1(0); err != nil {
+			panic("cannot open epoll fd: " + err.Error())
+		} else {
+			defer func() {
+				if err = syscall.Close(fd); err != nil {
+					panic("cannot close epoll fd: " + err.Error())
+				}
+			}()
+			epoll = fd
+		}
+		if err := syscall.EpollCtl(epoll, syscall.EPOLL_CTL_ADD, int(statFile.Fd()), &syscall.EpollEvent{}); err != nil {
+			panic("cannot add status pipe to epoll: " + err.Error())
+		}
+
+		if _, err := statFile.Write([]byte{'x'}); err != nil {
+			panic("cannot write to status pipe: " + err.Error())
+		}
+
+		// wait for status pipe close
+		events := make([]syscall.EpollEvent, 1)
+		if _, err := syscall.EpollWait(epoll, events, -1); err != nil {
+			panic("cannot poll status pipe: " + err.Error())
+		}
+		if events[0].Events != syscall.EPOLLERR {
+			panic(strconv.Itoa(int(events[0].Events)))
+
+		}
+	}
+}