cmd/gb: verify the Go install has not changed

cmd/gb/main.go

@@ -2,8 +2,11 @@ package main
 
 import (
 	"flag"
+	"io/ioutil"
 	"os"
 	"path/filepath"
+	"runtime"
+	"strings"
 
 	"github.com/constabulary/gb"
 	"github.com/constabulary/gb/cmd"
@@ -46,6 +49,9 @@ func main() {
 		help(args[2:])
 		return
 	}
+
+	verifyGoVersion()
+
 	command, ok := commands[name]
 	if (command != nil && !command.Runnable()) || !ok {
 		if _, err := lookupPlugin(name); err != nil {
@@ -105,3 +111,19 @@ func main() {
 		gb.Fatalf("command %q failed: %v", name, err)
 	}
 }
+
+// verify that the version of Go that compiled this binary is still correct.
+func verifyGoVersion() {
+	want := runtime.Version()
+	filename := filepath.Join(runtime.GOROOT(), "VERSION")
+	if strings.Contains(want, "devel") {
+		filename += ".cache"
+	}
+	got, err := ioutil.ReadFile(filename)
+	if err != nil {
+		gb.Fatalf("cannot validate Go version: %v", err)
+	}
+	if want != string(got) {
+		gb.Fatalf("Go version does not match: gb was compiled with %q, installed Go version from %v is %q", want, runtime.GOROOT(), got)
+	}
+}