version: add CmdName func for future use by logpolicy

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

Change-Id: I02a7c907844f71242ef06ed097f2a92ece7ae091

go.mod

@@ -22,4 +22,5 @@ require (
 	golang.org/x/sys v0.0.0-20200217220822-9197077df867
 	gortc.io/stun v1.22.1
 	honnef.co/go/tools v0.0.1-2019.2.3 // indirect
+	rsc.io/goversion v1.2.0
 )

go.sum

@@ -114,3 +114,5 @@ gortc.io/stun v1.22.1 h1:96mOdDATYRqhYB+TZdenWBg4CzL2Ye5kPyBXQ8KAB+8=
 gortc.io/stun v1.22.1/go.mod h1:XD5lpONVyjvV3BgOyJFNo0iv6R2oZB4L+weMqxts+zg=
 honnef.co/go/tools v0.0.1-2019.2.3 h1:3JgtbtFHMiCmsznwGVTUWbgGov+pVqnlf1dEJTNAXeM=
 honnef.co/go/tools v0.0.1-2019.2.3/go.mod h1:a3bituU0lyd329TUQxRnasdCoJDkEUEAqEt0JzvZhAg=
+rsc.io/goversion v1.2.0 h1:SPn+NLTiAG7w30IRK/DKp1BjvpWabYgxlLp/+kx5J8w=
+rsc.io/goversion v1.2.0/go.mod h1:Eih9y/uIBS3ulggl7KNJ09xGSLcuNaLgmvvqa07sgfo=

version/cmdname.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package version
+
+import (
+	"os"
+	"path"
+	"strings"
+
+	"rsc.io/goversion/version"
+)
+
+// CmdName returns either the base name of the current binary
+// using os.Executable. If os.Executable fails (it sholdn't), then
+// "cmd" is returned.
+func CmdName() string {
+	e, err := os.Executable()
+	if err != nil {
+		return "cmd"
+	}
+	var ret string
+	v, err := version.ReadExe(e)
+	if err != nil {
+		ret = strings.TrimSuffix(strings.ToLower(e), ".exe")
+	} else {
+		// v is like:
+		// "path\ttailscale.com/cmd/tailscale\nmod\ttailscale.com\t(devel)\t\ndep\tgithub.com/apenwarr/fixconsole\tv0.0.0-20191012055117-5a9f6489cc29\th1:muXWUcay7DDy1/hEQWrYlBy+g0EuwT70sBHg65SeUc4=\ndep\tgithub....
+		for _, line := range strings.Split(v.ModuleInfo, "\n") {
+			if strings.HasPrefix(line, "path\t") {
+				ret = path.Base(strings.TrimPrefix(line, "path\t"))
+				break
+			}
+		}
+	}
+	if ret == "" {
+		return "cmd"
+	}
+	return ret
+}