Detect if the host need to be rebooted

And expose it via Prometheus:

     # HELP comin_host_info Info of the host.
     # TYPE comin_host_info gauge
     comin_host_info{need_to_reboot="1"} 1

internal/manager/manager.go

@@ -44,6 +44,7 @@ type Manager struct {
 	// for a first iteration: this needs to be removed
 	isRunning               bool
 	needToBeRestarted       bool
+	needToReboot            bool
 	cominServiceRestartFunc func() error
 
 	evalFunc  generation.EvalFunc
@@ -153,6 +154,8 @@ func (m Manager) onDeployment(ctx context.Context, deploymentResult deployment.D
 	if getsEvicted && evicted.ProfilePath != "" {
 		profile.RemoveProfilePath(evicted.ProfilePath)
 	}
+	m.needToReboot = utils.NeedToReboot()
+	m.prometheus.SetHostInfo(m.needToReboot)
 	return m
 }
 
@@ -211,6 +214,9 @@ func (m Manager) Run() {
 	logrus.Infof("  machineId = %s", m.machineId)
 	logrus.Infof("  repositoryPath = %s", m.repositoryPath)
 
+	m.needToReboot = utils.NeedToReboot()
+	m.prometheus.SetHostInfo(m.needToReboot)
+
 	for {
 		select {
 		case <-m.stateRequestCh:

internal/prometheus/prometheus.go

@@ -12,14 +12,15 @@ type Prometheus struct {
 	buildInfo      *prometheus.GaugeVec
 	deploymentInfo *prometheus.GaugeVec
 	fetchCounter   *prometheus.CounterVec
+	hostInfo       *prometheus.GaugeVec
 }
 
 func New() Prometheus {
 	promReg := prometheus.NewRegistry()
 	buildInfo := prometheus.NewGaugeVec(prometheus.GaugeOpts{
 		Name: "comin_build_info",
 		Help: "Build info for comin.",
-        }, []string{"version"})
+	}, []string{"version"})
 	deploymentInfo := prometheus.NewGaugeVec(prometheus.GaugeOpts{
 		Name: "comin_deployment_info",
 		Help: "Info of the last deployment.",
@@ -28,14 +29,20 @@ func New() Prometheus {
 		Name: "comin_fetch_count",
 		Help: "Number of fetches per status",
 	}, []string{"remote_name", "status"})
+	hostInfo := prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Name: "comin_host_info",
+		Help: "Info of the host.",
+	}, []string{"need_to_reboot"})
 	promReg.MustRegister(buildInfo)
 	promReg.MustRegister(deploymentInfo)
 	promReg.MustRegister(fetchCounter)
+	promReg.MustRegister(hostInfo)
 	return Prometheus{
 		promRegistry:   promReg,
 		buildInfo:      buildInfo,
 		deploymentInfo: deploymentInfo,
 		fetchCounter:   fetchCounter,
+		hostInfo:       hostInfo,
 	}
 }
 
@@ -60,3 +67,14 @@ func (m Prometheus) SetDeploymentInfo(commitId, status string) {
 	m.deploymentInfo.Reset()
 	m.deploymentInfo.With(prometheus.Labels{"commit_id": commitId, "status": status}).Set(1)
 }
+
+func (m Prometheus) SetHostInfo(needToReboot bool) {
+	m.hostInfo.Reset()
+	var value string
+	if needToReboot {
+		value = "1"
+	} else {
+		value = "0"
+	}
+	m.hostInfo.With(prometheus.Labels{"need_to_reboot": value}).Set(1)
+}

internal/utils/reboot.go

@@ -0,0 +1,28 @@
+package utils
+
+import (
+	"os"
+
+	"github.com/sirupsen/logrus"
+)
+
+// NeedToReboot return true when the current deployed kernel is not
+// the booted kernel. Note we should implement something smarter such
+// as described in
+// https://discourse.nixos.org/t/nixos-needsreboot-determine-if-you-need-to-reboot-your-nixos-machine/40790
+func NeedToReboot() (reboot bool) {
+	current, err := os.Readlink("/run/current-system/kernel")
+	if err != nil {
+		logrus.Errorf("Failed to read the symlink /run/current-system/kernel: %s", err)
+		return
+	}
+	booted, err := os.Readlink("/run/booted-system/kernel")
+	if err != nil {
+		logrus.Errorf("Failed to read the symlink /run/booted-system/kernel: %s", err)
+		return
+	}
+	if current != booted {
+		reboot = true
+	}
+	return
+}