I'm writing a little program to manage restarts to other processes.
Basically when an app process starts (call it A), it spawns a new process (call it D), which has a simple HTTP server. When D receives an http request, it kills A and restarts it.
Problem is, A now doesn't respond to CTRL-C, and I'm not sure why. It may be something simple or maybe I don't really understand the relationship between processes, the terminal, and signals. But it's running in the same terminal with the same stdin/stdout/stderr. Below is a full program demonstrating this behaviour.
package main
import (
"flag"
"log"
"net/http"
"os"
"os/exec"
"strconv"
"time"
)
/*
Running this program starts an app (repeatdly prints 'hi') and spawns a new process running a simple HTTP server
When the server receives a request, it kills the other process and restarts it.
All three processes use the same stdin/stdout/stderr.
The restarted process does not respond to CTRL-C :(
*/
var serv = flag.Bool("serv", false, "run server")
// run the app or run the server
func main() {
flag.Parse()
if *serv {
runServer()
} else {
runApp()
}
}
// handle request to server
// url should contain pid of process to restart
func handler(w http.ResponseWriter, r *http.Request) {
pid, err := strconv.Atoi(r.URL.Path[1:])
if err != nil {
log.Println("send a number...")
}
// find the process
proc, err := os.FindProcess(pid)
if err != nil {
log.Println("can't find proc", pid)
return
}
// terminate the process
log.Println("Terminating the process...")
err = proc.Signal(os.Interrupt)
if err != nil {
log.Println("failed to signal interupt")
return
}
// restart the process
cmd := exec.Command("restarter")
cmd.Stdin = os.Stdin
cmd.Stdout = os.Stdout
cmd.Stderr = os.Stderr
if err := cmd.Start(); err != nil {
log.Println("Failed to restart app")
return
}
log.Println("Process restarted")
}
// run the server.
// this will only work the first time and that's fine
func runServer() {
http.HandleFunc("/", handler)
if err := http.ListenAndServe(":9999", nil); err != nil {
log.Println(err)
}
}
// the app prints 'hi' in a loop
// but first it spawns a child process which runs the server
func runApp() {
cmd := exec.Command("restarter", "-serv")
cmd.Stdin = os.Stdin
cmd.Stdout = os.Stdout
cmd.Stderr = os.Stderr
if err := cmd.Start(); err != nil {
log.Println(err)
}
log.Println("This is my process. It goes like this")
log.Println("PID:", os.Getpid())
for {
time.Sleep(time.Second)
log.Println("hi again")
}
}
The program expects to be installed. For convenience you can fetch it with go get github.com/ebuchman/restarter.
Run the program with restarter. It should print its process id. Then curl http://localhost:9999/<procid> to initiate the restart. The new process will now not respond to CTRL-C. Why? What am I missing?
You can check out the approach taken by two http server frameworks in order to listen and intercept signals (including SIGINT, or even SIGTERM)
kornel661/nserv, where the ZeroDowntime-example/server.go uses a channel:
// catch signals:
signals := make(chan os.Signal)
signal.Notify(signals, os.Interrupt, os.Kill)
zenazn/goji, where graceful/signal.go uses a similar approach:
var stdSignals = []os.Signal{syscall.SIGINT, syscall.SIGTERM}
var sigchan = make(chan os.Signal, 1)
func init() {
go waitForSignal()
}
This doesn't really have anything to do with Go. You start process A from your terminal shell. Process A starts process D (not sure what happened to B, but never mind). Process D kills process A. Now your shell sees that the process it started has exited, so the shell prepares to listen to another command. Process D starts another copy of process A, but the shell doesn't know anything about it. When you type ^C, the shell will handle it. If you run another program, the shell will arrange so that ^C goes to that program. The shell knows nothing about your copy of process A, so it's never going to direct a ^C to that process.