Go runtime Signal 23 (SIGURG) before child process is running causes preflight to silently exit

[dnwe]

The Go runtime scheduling uses signal 23 (SIGURG) since Go 1.14 (see 24543-non-cooperative-preemption.md). Because the Go runtime does not know if the process expects external SIGURG signals, the signal is not filtered out from signal.Notify, but always reported to the process (see golang/go#37942 for discussion). Preflight should filter this signal out itself before forwarding and when the child process isn't up yet it should also not exit when it is received.

envoy-preflight/main.go

Lines 46 to 53 in eb1500e

	signal.Notify(stop)
	for sig := range stop {
	if proc != nil {
	proc.Signal(sig)
	} else {
	// Signal received before the process even started. Let's just exit.
	os.Exit(1)
	}