gpython: leverage runtime/debug.ReadBuildInfo to infer version

[sbinet]

starting with Go-1.12, there's a nifty function runtime/debug.ReadBuildInfo to extract build informations out of a Go binary.

we could leverage this to display:

$> gpython
Python 3.4.0 (none, unknown)
[Gpython (devel)]
- os/arch: linux/amd64
- go version: devel +4f13a9c5b1 Tue Oct 1 07:16:47 2019 +0000
>>> 

(notice the (devel) string that comes out of debug.ReadBuildInfo().Main.Version

this could be factored into a gpython/vm.Version() function:

// Copyright 2019 The go-python Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build go1.12

package vm

import (
	"fmt"
	"runtime/debug"
)

// Version returns the version of Gpython and its checksum. The returned
// values are only valid in binaries built with module support.
//
// If a replace directive exists in the Gpython go.mod, the replace will
// be reported in the version in the following format:
//  "version=>[replace-path] [replace-version]"
// and the replace sum will be returned in place of the original sum.
//
// The exact version format returned by Version may change in future.
func Version() (version, sum string) {
	b, ok := debug.ReadBuildInfo()
	if !ok {
		return "", ""
	}

	const root = "github.com/go-python/gpython"

	modules := append([]*debug.Module{&b.Main}, b.Deps...)

	for _, m := range modules {
		if m.Path == root {
			if m.Replace != nil {
				switch {
				case m.Replace.Version != "" && m.Replace.Path != "":
					return fmt.Sprintf("%s=>%s %s", m.Version, m.Replace.Path, m.Replace.Version), m.Replace.Sum
				case m.Replace.Version != "":
					return fmt.Sprintf("%s=>%s", m.Version, m.Replace.Version), m.Replace.Sum
				case m.Replace.Path != "":
					return fmt.Sprintf("%s=>%s", m.Version, m.Replace.Path), m.Replace.Sum
				default:
					return m.Version + "*", m.Sum + "*"
				}
			}
			return m.Version, m.Sum
		}
	}

	return "", ""
}

[ncw]

That is a nice idea :-) I hadn't seen that function before.

Do you want to submit that as a PR?

Do we need the replace part of the above? How likely is that to happen?