Export the Praefect version running

Praefect didn't expose the version running, which makes debugging
harder. This change follows conventions set by Gitaly.

A few ways of exposing the version are implemented:
1. Through the `-version` flag
2. Logging the version at boot of the server
3. A prometheus metric, the metric is stale, but the labels are not

Closes https://gitlab.com/gitlab-org/gitaly/issues/1499

3 files changed, 57 insertions, 2 deletions

diff --git a/changelogs/unreleased/zj-praefect-version.yml b/changelogs/unreleased/zj-praefect-version.yml
new file mode 100644
index 000000000..c5c5259c9
--- /dev/null
+++ b/changelogs/unreleased/zj-praefect-version.yml
@@ -0,0 +1,5 @@
+---
+title: Expose the Praefect server version
+merge_request: 1358
+author:
+type: added
diff --git a/cmd/praefect/main.go b/cmd/praefect/main.go
index b4efeeae9..37fdddb22 100644
--- a/cmd/praefect/main.go
+++ b/cmd/praefect/main.go
@@ -13,16 +13,19 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/sirupsen/logrus"
 	"gitlab.com/gitlab-org/gitaly/internal/praefect"
 	"gitlab.com/gitlab-org/gitaly/internal/praefect/config"
+	"gitlab.com/gitlab-org/gitaly/internal/version"
 	"gitlab.com/gitlab-org/labkit/tracing"
 )
 
 var (
-	flagConfig = flag.String("config", "", "Location for the config.toml")
-	logger     = logrus.New()
+	flagConfig  = flag.String("config", "", "Location for the config.toml")
+	flagVersion = flag.Bool("version", false, "Print version and exit")
+	logger      = logrus.New()
 
 	errNoConfigFile = errors.New("the config flag must be passed")
 )
@@ -30,6 +33,12 @@ var (
 func main() {
 	flag.Parse()
 
+	// If invoked with -version
+	if *flagVersion {
+		fmt.Println(version.GetVersionString())
+		os.Exit(0)
+	}
+
 	conf, err := configure()
 	if err != nil {
 		logger.Fatal(err)
@@ -74,6 +83,9 @@ func configure() (config.Config, error) {
 		}()
 	}
 
+	registerServerVersionPromGauge()
+	logger.WithField("version", praefect.GetVersionString()).Info("Starting Praefect")
+
 	return conf, nil
 }
 
@@ -161,3 +173,19 @@ func getListeners(socketPath, listenAddr string) ([]net.Listener, error) {
 
 	return listeners, nil
 }
+
+// registerServerVersionPromGauge registers a label with the current server version
+// making it easy to see what versions of Gitaly are running across a cluster
+func registerServerVersionPromGauge() {
+	gitlabBuildInfoGauge := prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "gitlab_build_info",
+		Help: "Current build info for this GitLab Service",
+		ConstLabels: prometheus.Labels{
+			"version": praefect.GetVersion(),
+			"built":   praefect.GetBuildTime(),
+		},
+	})
+
+	prometheus.MustRegister(gitlabBuildInfoGauge)
+	gitlabBuildInfoGauge.Set(1)
+}
diff --git a/internal/praefect/version.go b/internal/praefect/version.go
new file mode 100644
index 000000000..25758f35a
--- /dev/null
+++ b/internal/praefect/version.go
@@ -0,0 +1,22 @@
+package praefect
+
+import (
+	"fmt"
+
+	"gitlab.com/gitlab-org/gitaly/internal/version"
+)
+
+// GetVersionString returns a standard version header
+func GetVersionString() string {
+	return fmt.Sprintf("Praefect, version %v", version.GetVersion())
+}
+
+// GetVersion returns the semver compatible version number
+func GetVersion() string {
+	return version.GetVersion()
+}
+
+// GetBuildTime returns the time at which the build took place
+func GetBuildTime() string {
+	return version.GetBuildTime()
+}