Add Signal Forwarding to Entrypoint Runner

The Entrypoint process should be notified and signals
received should be propogated as necessary. This signal
forwarding mimics the one in
https://github.com/pablo-ruth/go-init which is an golang
implementation of https://github.com/Yelp/dumb-init .

The cmd.Run() has also been replaced with a cmd.Start()
and cmd.Wait() to systematically start the command
and Wait for it's completion without prematurely exiting.

cmd/entrypoint/runner.go

@@ -3,6 +3,8 @@ package main
 import (
 	"os"
 	"os/exec"
+	"os/signal"
+	"syscall"
 
 	"github.com/tektoncd/pipeline/pkg/entrypoint"
 )
@@ -21,12 +23,38 @@ func (*realRunner) Run(args ...string) error {
 	}
 	name, args := args[0], args[1:]
 
+	// Receive system signals on "signals"
+	signals := make(chan os.Signal, 1)
+	defer close(signals)
+	signal.Notify(signals, syscall.SIGINT, syscall.SIGTERM)
+	defer signal.Reset()
+
 	cmd := exec.Command(name, args...)
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
+	// dedicated PID group used to forward signals to
+	// main process and all children
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+
+	// Goroutine for signals forwarding
+	go func() {
+		for s := range signals {
+			// Forward signal to main process and all children
+			syscall.Kill(-cmd.Process.Pid, s.(syscall.Signal))
+		}
+	}()
+
+	// Start defined command
+	err := cmd.Start()
+	if err != nil {
+		return err
+	}
 
-	if err := cmd.Run(); err != nil {
+	// Wait for command to exit
+	err = cmd.Wait()
+	if err != nil {
 		return err
 	}
+
 	return nil
 }