cmd/tailscale: add event bus queue depth debugging

Under extremely high load it appears we may have some retention issues
as a result of queue depth build up, but there is currently no direct
way to observe this. The scenario does not trigger the slow subscriber
log message, and the event stream debugging endpoint produces a
saturating volume of information.

Updates tailscale/corp#36904

Signed-off-by: James Tucker <james@tailscale.com>

client/local/local.go

@@ -446,6 +446,11 @@ func (lc *Client) EventBusGraph(ctx context.Context) ([]byte, error) {
 	return lc.get200(ctx, "/localapi/v0/debug-bus-graph")
 }
 
+// EventBusQueues returns a JSON snapshot of event bus queue depths per client.
+func (lc *Client) EventBusQueues(ctx context.Context) ([]byte, error) {
+	return lc.get200(ctx, "/localapi/v0/debug-bus-queues")
+}
+
 // StreamBusEvents returns an iterator of Tailscale bus events as they arrive.
 // Each pair is a valid event and a nil error, or a zero event a non-nil error.
 // In case of error, the iterator ends after the pair reporting the error.

cmd/tailscale/cli/debug.go

@@ -124,6 +124,12 @@ func debugCmd() *ffcli.Command {
 					return fs
 				})(),
 			},
+			{
+				Name:       "daemon-bus-queues",
+				ShortUsage: "tailscale debug daemon-bus-queues",
+				Exec:       runDaemonBusQueues,
+				ShortHelp:  "Print event bus queue depths per client",
+			},
 			{
 				Name:       "metrics",
 				ShortUsage: "tailscale debug metrics",
@@ -840,6 +846,15 @@ func runDaemonBusGraph(ctx context.Context, args []string) error {
 	return nil
 }
 
+func runDaemonBusQueues(ctx context.Context, args []string) error {
+	data, err := localClient.EventBusQueues(ctx)
+	if err != nil {
+		return err
+	}
+	fmt.Print(string(data))
+	return nil
+}
+
 // generateDOTGraph generates the DOT graph format based on the events
 func generateDOTGraph(topics []eventbus.DebugTopic) string {
 	var sb strings.Builder

ipn/localapi/debug.go

@@ -6,6 +6,7 @@
 package localapi
 
 import (
+	"cmp"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -35,6 +36,7 @@ func init() {
 	Register("dev-set-state-store", (*Handler).serveDevSetStateStore)
 	Register("debug-bus-events", (*Handler).serveDebugBusEvents)
 	Register("debug-bus-graph", (*Handler).serveEventBusGraph)
+	Register("debug-bus-queues", (*Handler).serveDebugBusQueues)
 	Register("debug-derp-region", (*Handler).serveDebugDERPRegion)
 	Register("debug-dial-types", (*Handler).serveDebugDialTypes)
 	Register("debug-log", (*Handler).serveDebugLog)
@@ -424,6 +426,62 @@ func (h *Handler) serveEventBusGraph(w http.ResponseWriter, r *http.Request) {
 	json.NewEncoder(w).Encode(topics)
 }
 
+func (h *Handler) serveDebugBusQueues(w http.ResponseWriter, r *http.Request) {
+	if r.Method != httpm.GET {
+		http.Error(w, "GET required", http.StatusMethodNotAllowed)
+		return
+	}
+
+	bus, ok := h.LocalBackend().Sys().Bus.GetOK()
+	if !ok {
+		http.Error(w, "event bus not running", http.StatusPreconditionFailed)
+		return
+	}
+
+	debugger := bus.Debugger()
+
+	type clientQueue struct {
+		Name           string   `json:"name"`
+		SubscribeDepth int      `json:"subscribeDepth"`
+		SubscribeTypes []string `json:"subscribeTypes,omitempty"`
+		PublishTypes   []string `json:"publishTypes,omitempty"`
+	}
+
+	publishQueue := debugger.PublishQueue()
+	clients := debugger.Clients()
+	result := struct {
+		PublishQueueDepth int           `json:"publishQueueDepth"`
+		Clients           []clientQueue `json:"clients"`
+	}{
+		PublishQueueDepth: len(publishQueue),
+	}
+
+	for _, c := range clients {
+		sq := debugger.SubscribeQueue(c)
+		cq := clientQueue{
+			Name:           c.Name(),
+			SubscribeDepth: len(sq),
+		}
+		for _, t := range debugger.SubscribeTypes(c) {
+			cq.SubscribeTypes = append(cq.SubscribeTypes, t.String())
+		}
+		for _, t := range debugger.PublishTypes(c) {
+			cq.PublishTypes = append(cq.PublishTypes, t.String())
+		}
+		result.Clients = append(result.Clients, cq)
+	}
+
+	slices.SortFunc(result.Clients, func(a, b clientQueue) int {
+		if a.SubscribeDepth != b.SubscribeDepth {
+			return b.SubscribeDepth - a.SubscribeDepth
+		}
+		return cmp.Compare(a.Name, b.Name)
+	})
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(result)
+}
+
 func (h *Handler) serveDebugLog(w http.ResponseWriter, r *http.Request) {
 	if !buildfeatures.HasLogTail {
 		http.Error(w, feature.ErrUnavailable.Error(), http.StatusNotImplemented)