Define HTTP endpoint for liveness probe

Now, we use tetra status command to report the status of tetragon
agent. This comes with some overheads as tetra binary has a lot of
additional functionality and it seems like an overkill to use that for
status reporting.

On the other hand, k8s supports liveness probes by using an HTTP
endpoint (i.e.
https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#http-probes).
This patch first creates a new HTTP endpoint to report agent status that
can be used for the liveness probe.

Signed-off-by: Anastasios Papagiannis <[email protected]>

cmd/tetragon/main.go

@@ -36,6 +36,7 @@ import (
 	"github.com/cilium/tetragon/pkg/fileutils"
 	"github.com/cilium/tetragon/pkg/filters"
 	tetragonGrpc "github.com/cilium/tetragon/pkg/grpc"
+	"github.com/cilium/tetragon/pkg/health"
 	"github.com/cilium/tetragon/pkg/logger"
 	"github.com/cilium/tetragon/pkg/metrics"
 	"github.com/cilium/tetragon/pkg/metrics/metricsconfig"
@@ -245,6 +246,19 @@ func tetragonExecute() error {
 	bpf.CheckOrMountDebugFS()
 	bpf.CheckOrMountCgroup2()
 
+	// start liveness probe http endpoint
+	go func() {
+		http.HandleFunc("/liveness", func(w http.ResponseWriter, _ *http.Request) {
+			resp, err := health.GetHealth()
+			if err == nil && len(resp.HealthStatus) == 1 && resp.HealthStatus[0].Status == tetragon.HealthStatusResult_HEALTH_STATUS_RUNNING {
+				w.WriteHeader(http.StatusOK)
+			} else {
+				w.WriteHeader(http.StatusServiceUnavailable)
+			}
+		})
+		http.ListenAndServe(":6789", nil)
+	}()
+
 	if option.Config.PprofAddr != "" {
 		go func() {
 			if err := servePprof(option.Config.PprofAddr); err != nil {