use tsweb debugger (#2420)

This PR switches the homegrown debug endpoint to using tsweb.Debugger, a neat toolkit with batteries included for pprof and friends, and making it easy to add additional debug info:

I've started out by adding a bunch of "introspect" endpoints
image

So users can see the acl, filter, config, derpmap and connected nodes as headscale sees them.

hscontrol/debug.go

@@ -0,0 +1,120 @@
+package hscontrol
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+
+	"github.com/arl/statsviz"
+	"github.com/juanfont/headscale/hscontrol/types"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"tailscale.com/tailcfg"
+	"tailscale.com/tsweb"
+)
+
+func (h *Headscale) debugHTTPServer() *http.Server {
+	debugMux := http.NewServeMux()
+	debug := tsweb.Debugger(debugMux)
+	debug.Handle("notifier", "Connected nodes in notifier", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(h.nodeNotifier.String()))
+	}))
+	debug.Handle("config", "Current configuration", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		config, err := json.MarshalIndent(h.cfg, "", "  ")
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain")
+		w.WriteHeader(http.StatusOK)
+		w.Write(config)
+	}))
+	debug.Handle("policy", "Current policy", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		pol, err := h.policyBytes()
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain")
+		w.WriteHeader(http.StatusOK)
+		w.Write(pol)
+	}))
+	debug.Handle("filter", "Current filter", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		filter := h.polMan.Filter()
+
+		filterJSON, err := json.MarshalIndent(filter, "", "  ")
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain")
+		w.WriteHeader(http.StatusOK)
+		w.Write(filterJSON)
+	}))
+	debug.Handle("ssh", "SSH Policy per node", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		nodes, err := h.db.ListNodes()
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+
+		sshPol := make(map[string]*tailcfg.SSHPolicy)
+		for _, node := range nodes {
+			pol, err := h.polMan.SSHPolicy(node)
+			if err != nil {
+				httpError(w, err)
+				return
+			}
+
+			sshPol[fmt.Sprintf("id:%d  hostname:%s givenname:%s", node.ID, node.Hostname, node.GivenName)] = pol
+		}
+
+		sshJSON, err := json.MarshalIndent(sshPol, "", "  ")
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain")
+		w.WriteHeader(http.StatusOK)
+		w.Write(sshJSON)
+	}))
+	debug.Handle("derpmap", "Current DERPMap", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		dm := h.DERPMap
+
+		dmJSON, err := json.MarshalIndent(dm, "", "  ")
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain")
+		w.WriteHeader(http.StatusOK)
+		w.Write(dmJSON)
+	}))
+	debug.Handle("registration-cache", "Pending registrations", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		registrationsJSON, err := json.MarshalIndent(h.registrationCache.Items(), "", "  ")
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain")
+		w.WriteHeader(http.StatusOK)
+		w.Write(registrationsJSON)
+	}))
+
+	err := statsviz.Register(debugMux)
+	if err == nil {
+		debug.URL("/debug/statsviz", "Statsviz (visualise go metrics)")
+	}
+
+	debug.URL("/metrics", "Prometheus metrics")
+	debugMux.Handle("/metrics", promhttp.Handler())
+
+	debugHTTPServer := &http.Server{
+		Addr:         h.cfg.MetricsAddr,
+		Handler:      debugMux,
+		ReadTimeout:  types.HTTPTimeout,
+		WriteTimeout: 0,
+	}
+
+	return debugHTTPServer
+}