First draft

.gitignore

@@ -1,3 +1,5 @@
+reap
+
 # Compiled Object files, Static and Dynamic libs (Shared Objects)
 *.o
 *.a

Dockerfile

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+FROM alpine:latest
+RUN echo "http://dl-4.alpinelinux.org/alpine/edge/testing" >>/etc/apk/repositories && apk add --update runit && rm -rf /var/cache/apk/*
+
+ADD reap /
+
+ADD foo /
+RUN mkdir -p /etc/service/foo
+ADD run-foo /etc/service/foo/run
+
+ADD bar /
+RUN mkdir -p /etc/service/bar
+ADD run-bar /etc/service/bar/run
+
+ENTRYPOINT ["/reap"]

README.md

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+# reap
+
+If you have a Docker container that's a collection of runit-supervised daemons,
+this process is suitable for use as the ENTRYPOINT.
+
+```Docker
+FROM alpine:latest
+RUN echo "http://dl-4.alpinelinux.org/alpine/edge/testing" >>/etc/apk/repositories && apk add --update runit && rm -rf /var/cache/apk/*
+
+ADD foo /
+RUN mkdir -p /etc/service/foo
+ADD run-foo /etc/service/foo/run
+
+ADD bar /
+RUN mkdir -p /etc/service/bar
+ADD run-bar /etc/service/bar/run
+
+ADD reap /
+ENTRYPOINT ["/reap"]
+```
+
+**Why not just exec runsvdir?**
+
+`docker stop` issues SIGTERM (or, in a future version of Docker, perhaps another custom signal)
+but if runsvdir receives a signal,
+it doesn't wait for its supervised processes to exit before returning.
+If you don't care about graceful shutdown of your daemons, no problem, you don't need this tool.
+
+**Why not wrap runsvdir in a simple shell script?**
+
+This works great:
+
+```sh
+#!/bin/sh
+
+sv_stop() {
+	for s in $(ls -d /etc/service/*)
+	do
+		/sbin/sv stop $s
+	done
+}
+
+trap "sv_stop; exit" SIGTERM
+/sbin/runsvdir /etc/service &
+wait
+```
+
+...except it doesn't [reap orphaned child processes](https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem/)
+and is therefore unsuitable for being PID 1.
+
+**Why not use my_init from phusion/baseimage-docker?**
+
+That works great — if you're willing to add python3 to your Docker images :)
+
+**So this is just a stripped-down my_init in Go?**
+
+Basically, yes.
+

bar

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+#!/bin/sh
+
+handle() {
+	echo "got signal"
+	exit
+}
+
+trap handle SIGINT
+
+while true
+do
+	echo bar: `date`
+	sleep 1
+done
+

foo

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+#!/bin/sh
+
+handle() {
+	echo "got signal"
+	exit
+}
+
+trap "handle" SIGINT
+
+for i in `seq 1 5`
+do
+	echo foo: $i
+	sleep 1
+done
+
+echo foo: terminating!
+

main.go

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+package main
+
+import (
+	"log"
+	"os"
+	"os/exec"
+	"os/signal"
+	"path/filepath"
+	"strings"
+	"syscall"
+)
+
+const etcService = "/etc/service"
+
+func main() {
+	log.SetFlags(0)
+
+	runsvdir, err := exec.LookPath("runsvdir")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	sv, err := exec.LookPath("sv")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if fi, err := os.Stat(etcService); err != nil {
+		log.Fatal(err)
+	} else if !fi.IsDir() {
+		log.Fatalf("%s is not a directory", etcService)
+	}
+
+	if pid := os.Getpid(); pid != 1 {
+		log.Printf("warning: I'm not PID 1, I'm PID %d", pid)
+	}
+
+	go reapAll()
+
+	supervisor := cmd(runsvdir, etcService)
+	if err := supervisor.Start(); err != nil {
+		log.Fatal(err)
+	}
+
+	log.Printf("%s started", runsvdir)
+
+	go gracefulShutdown(sv, supervisor.Process)
+
+	if err := supervisor.Wait(); err != nil {
+		log.Printf("%s exited with error: %v", runsvdir, err)
+	} else {
+		log.Printf("%s exited cleanly", runsvdir)
+	}
+}
+
+func reapAll() {
+	c := make(chan os.Signal)
+	signal.Notify(c, syscall.SIGCHLD)
+	for range c {
+		go reapOne()
+	}
+}
+
+// From https://github.com/ramr/go-reaper/blob/master/reaper.go
+func reapOne() {
+	var (
+		ws  syscall.WaitStatus
+		pid int
+		err error
+	)
+	for {
+		pid, err = syscall.Wait4(-1, &ws, 0, nil)
+		if err != syscall.EINTR {
+			break
+		}
+	}
+	if err == syscall.ECHILD {
+		return
+	}
+	log.Printf("reaped child process %d (%+v)", pid, ws)
+}
+
+type signaler interface {
+	Signal(os.Signal) error
+}
+
+func gracefulShutdown(sv string, s signaler) {
+	c := make(chan os.Signal)
+	signal.Notify(c, syscall.SIGTERM, syscall.SIGINT)
+	log.Printf("received %s", <-c)
+
+	matches, err := filepath.Glob(filepath.Join(etcService, "*"))
+	if err != nil {
+		log.Printf("when shutting down services: %v", err)
+		return
+	}
+
+	var stopped []string
+	for _, match := range matches {
+		fi, err := os.Stat(match)
+		if err != nil {
+			log.Printf("%s: %v", match, err)
+			continue
+		}
+		if !fi.IsDir() {
+			log.Printf("%s: not a directory", match)
+			continue
+		}
+		service := filepath.Base(match)
+		stop := cmd(sv, "stop", service)
+		if err := stop.Run(); err != nil {
+			log.Printf("%s: %v", strings.Join(stop.Args, " "), err)
+			continue
+		}
+		stopped = append(stopped, service)
+	}
+
+	log.Printf("stopped %d: %s", len(stopped), strings.Join(stopped, ", "))
+	log.Printf("stopping supervisor...")
+	if err := s.Signal(syscall.SIGTERM); err != nil {
+		log.Print(err)
+	}
+	log.Printf("graceful SIGTERM handler exiting")
+}
+
+func cmd(path string, args ...string) *exec.Cmd {
+	return &exec.Cmd{
+		Path:   path,
+		Args:   append([]string{path}, args...),
+		Env:    os.Environ(),
+		Stdin:  os.Stdin,
+		Stdout: os.Stdout,
+		Stderr: os.Stderr,
+	}
+}

run-bar

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+#!/bin/sh
+
+exec /bar

run-foo

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+#!/bin/sh
+
+exec /foo