Only gracefully exit rpcchainvm server after Shutdown

[StephenButtolph]

Why this should be merged

This gives us the following nice properties:

1. This solution doesn't require user intervention (systemd will just start working correctly for newer plugin versions)
2. We do not ungracefully kill the subprocess (as it is able to catch the signal we send and gracefully exit with code 0)
3. We still use a signal designed for immediately terminating a process to terminate the subprocess

How this works

1. Upon startup register a signal handler to SIGINT and SIGTERM on the plugin side to avoid systemd from unexpectedly killing the subprocess.
2. During the shutdown flow avalanchego will call Shutdown over the gRPC.
3. During the handling of the Shutdown call - update the signal handler to now honor SIGTERM signals.
4. After Shutdown has been called send the SIGTERM call as we did previously.

How this was tested

CI

[patrick-ogrady]

I think we may be able to unify the stopper logic as well:

avalanchego/vms/rpcchainvm/runtime/subprocess/non_linux_stopper.go

Lines 4 to 31 in b820b46

	//go:build !linux
	// +build !linux
	package subprocess
	import (
	"context"
	"os/exec"
	"go.uber.org/zap"
	"github.com/ava-labs/avalanchego/utils/logging"
	)
	func NewCmd(path string, args ...string) *exec.Cmd {
	return exec.Command(path, args...)
	}
	func stop(_ context.Context, log logging.Logger, cmd *exec.Cmd) {
	err := cmd.Process.Kill()
	if err == nil {
	log.Debug("subprocess was killed")
	} else {
	log.Error("subprocess was killed",
	zap.Error(err),
	)
	}
	}

Both SIGTERM/SIGINT are supported on Windows/Darwin:

• https://cs.opensource.google/go/go/+/master:src/runtime/defs_windows.go;l=21-22
• https://cs.opensource.google/go/go/+/master:src/runtime/defs_darwin.go;l=69

[hexfusion]

I think they exist but are not implemented as expected in Windows more details on Interrupt[1] .

os/signal docs[2] are a little hard to grok.

On Windows a ^C (Control-C) or ^BREAK (Control-Break) normally cause the program to exit. If Notify is called for os.Interrupt, ^C or ^BREAK will cause os.Interrupt to be sent on the channel, and the program will not exit. If Reset is called, or Stop is called on all channels passed to Notify, then the default behavior will be restored.

I believe that is a direct reference to "console process group" which would not be our intended use.

[1] golang/go#46345 (comment)
[2] https://pkg.go.dev/os/signal