util/eventbus: add a LogAllEvents helper for testing (#17187)

When developing (and debugging) tests, it is useful to be able to see all the
traffic that transits the event bus during the execution of a test.

Updates #15160

Change-Id: I929aee62ccf13bdd4bd07d786924ce9a74acd17a
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

util/eventbus/eventbustest/eventbustest.go

@@ -263,3 +263,25 @@ func EqualTo[T any](want T) func(T) error {
 		return nil
 	}
 }
+
+// LogAllEvents logs summaries of all the events routed via the specified bus
+// during the execution of the test governed by t. This is intended to support
+// development and debugging of tests.
+func LogAllEvents(t testing.TB, bus *eventbus.Bus) {
+	dw := bus.Debugger().WatchBus()
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		var i int
+		for {
+			select {
+			case <-dw.Done():
+				return
+			case re := <-dw.Events():
+				i++
+				t.Logf("[eventbus] #%[1]d: %[2]T | %+[2]v", i, re.Event)
+			}
+		}
+	}()
+	t.Cleanup(func() { dw.Close(); <-done })
+}

util/eventbus/eventbustest/eventbustest_test.go

@@ -4,6 +4,7 @@
 package eventbustest_test
 
 import (
+	"flag"
 	"fmt"
 	"strings"
 	"testing"
@@ -13,6 +14,8 @@ import (
 	"tailscale.com/util/eventbus/eventbustest"
 )
 
+var doDebug = flag.Bool("debug", false, "Enable debug logging")
+
 type EventFoo struct {
 	Value int
 }
@@ -109,7 +112,11 @@ func TestExpectFilter(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			if *doDebug {
+				eventbustest.LogAllEvents(t, bus)
+			}
 			tw := eventbustest.NewWatcher(t, bus)
+
 			// TODO(cmol): When synctest is out of experimental, use that instead:
 			// https://go.dev/blog/synctest
 			tw.TimeOut = 10 * time.Millisecond