Add -version flag to the in-VM agent

Investigating issues like #370 will be easier if the agent can report
the version.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

agent/Makefile

@@ -17,14 +17,15 @@ GOMOD := $(shell go env GOMOD)
 GOSUM := $(GOMOD:.mod=.sum)
 SOURCES:=$(shell find . -name '*.go')
 DOCKER_IMAGE_TAG?=latest
+VERSION := $(shell git describe --always)
 
 all: agent
 
 agent: *.go $(GOMOD) $(GOSUM)
 ifneq ($(STATIC_AGENT),)
-	CGO_ENABLED=0 go build -installsuffix cgo -a -ldflags "-s" $(EXTRAGOARGS) -o agent
+	CGO_ENABLED=0 go build -installsuffix cgo -a -ldflags "-s -X main.version=$(VERSION)" $(EXTRAGOARGS) -o agent
 else
-	go build $(EXTRAGOARGS) -o agent
+	go build $(EXTRAGOARGS) -ldflags "-X main.version=$(VERSION)" -o agent
 endif
 
 test:

agent/main.go

@@ -16,6 +16,7 @@ package main
 import (
 	"context"
 	"flag"
+	"fmt"
 	"os"
 	"os/signal"
 	"syscall"
@@ -47,20 +48,31 @@ const (
 	enableSubreaper = 1
 )
 
+var (
+    version string
+)
+
 func main() {
 	var (
 		port  int
 		debug bool
+		version bool
 	)
 
 	flag.IntVar(&port, "port", defaultPort, "Vsock port to listen to")
 	flag.BoolVar(&debug, "debug", false, "Turn on debug mode")
+	flag.BoolVar(&version, "version", false, "Show the version")
 	flag.Parse()
 
 	if debug {
 		logrus.SetLevel(logrus.DebugLevel)
 	}
 
+	if version {
+		showVersion()
+		return
+	}
+
 	signals := make(chan os.Signal, 32)
 	signal.Notify(signals, syscall.SIGINT, syscall.SIGTERM, unix.SIGCHLD)
 
@@ -155,3 +167,9 @@ func main() {
 		panic(err)
 	}
 }
+
+func showVersion() {
+	// After Go 1.12, We can use runtime/debug.BuildInfo instead.
+	// https://github.com/golang/go/issues/22147
+	fmt.Printf("containerd Firecracker agent %s\n", version)
+}