cmd/containerboot: serve health on local endpoint (#14246)

* cmd/containerboot: serve health on local endpoint

We introduced stable (user) metrics in #14035, and `TS_LOCAL_ADDR_PORT`
with it. Rather than requiring users to specify a new addr/port
combination for each new local endpoint they want the container to
serve, this combines the health check endpoint onto the local addr/port
used by metrics if `TS_ENABLE_HEALTH_CHECK` is used instead of
`TS_HEALTHCHECK_ADDR_PORT`.

`TS_LOCAL_ADDR_PORT` now defaults to binding to all interfaces on 9002
so that it works more seamlessly and with less configuration in
environments other than Kubernetes, where the operator always overrides
the default anyway. In particular, listening on localhost would not be
accessible from outside the container, and many scripted container
environments do not know the IP address of the container before it's
started. Listening on all interfaces allows users to just set one env
var (`TS_ENABLE_METRICS` or `TS_ENABLE_HEALTH_CHECK`) to get a fully
functioning local endpoint they can query from outside the container.

Updates #14035, #12898

Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

cmd/containerboot/healthz.go

@@ -7,7 +7,6 @@ package main
 
 import (
 	"log"
-	"net"
 	"net/http"
 	"sync"
 )
@@ -23,29 +22,29 @@ type healthz struct {
 func (h *healthz) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	h.Lock()
 	defer h.Unlock()
+
 	if h.hasAddrs {
 		w.Write([]byte("ok"))
 	} else {
-		http.Error(w, "node currently has no tailscale IPs", http.StatusInternalServerError)
+		http.Error(w, "node currently has no tailscale IPs", http.StatusServiceUnavailable)
 	}
 }
 
-// runHealthz runs a simple HTTP health endpoint on /healthz, listening on the
-// provided address. A containerized tailscale instance is considered healthy if
+func (h *healthz) update(healthy bool) {
+	h.Lock()
+	defer h.Unlock()
+
+	if h.hasAddrs != healthy {
+		log.Println("Setting healthy", healthy)
+	}
+	h.hasAddrs = healthy
+}
+
+// healthHandlers registers a simple health handler at /healthz.
+// A containerized tailscale instance is considered healthy if
 // it has at least one tailnet IP address.
-func runHealthz(addr string, h *healthz) {
-	lis, err := net.Listen("tcp", addr)
-	if err != nil {
-		log.Fatalf("error listening on the provided health endpoint address %q: %v", addr, err)
-	}
-	mux := http.NewServeMux()
+func healthHandlers(mux *http.ServeMux) *healthz {
+	h := &healthz{}
 	mux.Handle("GET /healthz", h)
-	log.Printf("Running healthcheck endpoint at %s/healthz", addr)
-	hs := &http.Server{Handler: mux}
-
-	go func() {
-		if err := hs.Serve(lis); err != nil {
-			log.Fatalf("failed running health endpoint: %v", err)
-		}
-	}()
+	return h
 }