write .cache/keybase/service.version on service startup

This provides an easy way for updated clients to find out what version
of the service is currently running, so that they can bounce it if need
be.

Closes keybase#415.

go/service/main.go

@@ -2,8 +2,10 @@ package service
 
 import (
 	"fmt"
+	"io/ioutil"
 	"net"
 	"os"
+	"path"
 
 	"github.com/codegangsta/cli"
 	"github.com/keybase/client/go/libcmdline"
@@ -56,6 +58,11 @@ func (d *Service) RunClient() (err error) {
 func (d *Service) Run() (err error) {
 	G.Service = true
 
+	err = d.writeVersionFile()
+	if err != nil {
+		return
+	}
+
 	if len(d.chdirTo) != 0 {
 		e_tmp := os.Chdir(d.chdirTo)
 		if e_tmp != nil {
@@ -81,6 +88,13 @@ func (d *Service) Run() (err error) {
 	return
 }
 
+// If the daemon is already running, we need to be able to check what version
+// it is, in case the client has been updated.
+func (d *Service) writeVersionFile() error {
+	versionFilePath := path.Join(G.Env.GetCacheDir(), "service.version")
+	return ioutil.WriteFile(versionFilePath, []byte(libkb.CLIENT_VERSION), 0644)
+}
+
 func (d *Service) ReleaseLock() error {
 	return d.lockPid.Close()
 }