control/controlclient: introduce eventbus messages instead of callbacks (#16956)

This is a small introduction of the eventbus into controlclient that
communicates with mainly ipnlocal. While ipnlocal is a complicated part
of the codebase, the subscribers here are from the perspective of
ipnlocal already called async.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"math/rand/v2"
 	"net/netip"
 	"strings"
 	"sync"
@@ -39,6 +40,7 @@ import (
 	"tailscale.com/types/persist"
 	"tailscale.com/types/preftype"
 	"tailscale.com/util/dnsname"
+	"tailscale.com/util/eventbus/eventbustest"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/must"
 	"tailscale.com/wgengine"
@@ -113,10 +115,11 @@ func (nt *notifyThrottler) drain(count int) []ipn.Notify {
 // in the controlclient.Client, so by controlling it, we can check that
 // the state machine works as expected.
 type mockControl struct {
-	tb     testing.TB
-	logf   logger.Logf
-	opts   controlclient.Options
-	paused atomic.Bool
+	tb              testing.TB
+	logf            logger.Logf
+	opts            controlclient.Options
+	paused          atomic.Bool
+	controlClientID int64
 
 	mu          sync.Mutex
 	persist     *persist.Persist
@@ -127,12 +130,13 @@ type mockControl struct {
 
 func newClient(tb testing.TB, opts controlclient.Options) *mockControl {
 	return &mockControl{
-		tb:          tb,
-		authBlocked: true,
-		logf:        opts.Logf,
-		opts:        opts,
-		shutdown:    make(chan struct{}),
-		persist:     opts.Persist.Clone(),
+		tb:              tb,
+		authBlocked:     true,
+		logf:            opts.Logf,
+		opts:            opts,
+		shutdown:        make(chan struct{}),
+		persist:         opts.Persist.Clone(),
+		controlClientID: rand.Int64(),
 	}
 }
 
@@ -287,6 +291,10 @@ func (cc *mockControl) UpdateEndpoints(endpoints []tailcfg.Endpoint) {
 	cc.called("UpdateEndpoints")
 }
 
+func (cc *mockControl) ClientID() int64 {
+	return cc.controlClientID
+}
+
 func (b *LocalBackend) nonInteractiveLoginForStateTest() {
 	b.mu.Lock()
 	if b.cc == nil {
@@ -1507,7 +1515,8 @@ func newLocalBackendWithMockEngineAndControl(t *testing.T, enableLogging bool) (
 	dialer := &tsdial.Dialer{Logf: logf}
 	dialer.SetNetMon(netmon.NewStatic())
 
-	sys := tsd.NewSystem()
+	bus := eventbustest.NewBus(t)
+	sys := tsd.NewSystemWithBus(bus)
 	sys.Set(dialer)
 	sys.Set(dialer.NetMon())