util/eventbus: unify Subscriber/SubscriberFunc cores; structural symmetry

Brings Subscriber[T] in line with the same non-generic-core pattern already
applied to SubscriberFunc[T] and Publisher[T]:

  - Renames subscriberFuncCore to subscriberCore and shares it between
    Subscriber[T] and SubscriberFunc[T]. Both typed facades hold a
    *subscriberCore plus their respective per-T delivery state
    (Subscriber: chan T; SubscriberFunc: nothing, the user callback is
    captured in the dispatch closure).

  - The bus's outputs map and subscriber-interface itab key on
    *subscriberCore for both subscriber kinds, so adding a new Subscribe[T]
    call site no longer pays a per-T itab, dictionary, or equality function
    for the subscriber-interface side.

  - Subscribe[T] now hoists the non-generic constructor portion into
    newSubscriberCore (timer setup, core allocation, cached type/typeName,
    unregister method-value), matching SubscribeFunc.

The dispatch loop is intentionally NOT extracted to a non-generic helper for
Subscriber[T], unlike SubscriberFunc[T]. The reason is the typed channel send
'case s.read <- t:' must appear lexically inside the select; the only way to
lift it into a non-generic loop is to bridge typed and untyped via a per-event
goroutine, which costs ~2.7x throughput on BenchmarkBasicThroughput. We keep
dispatchTyped on the generic facade and accept the per-shape stencil cost as
the cheaper alternative.

Symbol-level effect on tailscaled (linux/amd64, measured via
`go tool nm -size`):

  Before:
    (*Subscriber[T]).dispatch
      2 shape stencils:        1,682 + 1,549 = 3,231 B
      3 thin per-T wrappers:   124 B each   =   372 B
      2 deferwrap1 helpers:    62 B each    =   124 B
      total:                                 3,727 B

  After:
    (*Subscriber[T]).dispatchTyped
      2 shape stencils:        1,678 + 1,582 = 3,260 B
      0 per-T wrappers (replaced by closure stored on core)
      2 deferwrap1 helpers:    62 B each    =   124 B
      total:                                 3,384 B

  dispatch path .text delta:                   -343 B (-9.2%)

Per-shape stencils are ~1,600 B (.text body) + ~1,100 B (pclntab) =
~2,700 B each on production tailscaled. The shape count matches before/after
(two distinct GC shapes for the Subscriber[T] event types in this binary).
What changes is that the per-T thin wrappers are eliminated because
Subscriber[T] no longer implements the subscriber interface directly.

Whole-binary section deltas:

  .text:        -2,304 B  (includes the dispatch savings plus other
                            small downstream effects)
  .rodata:        +512 B  (additional closure-type metadata)
  .gopclntab:   -2,981 B  (fewer per-T compiled functions => less metadata)

Stripped tailscaled (linux/amd64): no change at the file level (the savings
fall below the linker's section-alignment boundary). Unstripped builds shrink
by ~2,900 B.

Behavior is unchanged:
  BenchmarkBasicThroughput:       2,161 ns/op,  0 B/op,  0 allocs/op
  BenchmarkBasicFuncThroughput:   2,493 ns/op, 144 B/op, 2 allocs/op
  BenchmarkSubsThroughput:        3,727 ns/op,  0 B/op,  0 allocs/op

Updates #12614

Change-Id: I97918ec68bd2cdb15958bbfd7687592b39663efe
Signed-off-by: James Tucker <james@tailscale.com>

util/eventbus/client.go

@@ -146,7 +146,10 @@ func Subscribe[T any](c *Client) *Subscriber[T] {
 
 	r := c.subscribeStateLocked()
 	s := newSubscriber[T](r, logfForCaller(c.logger()))
-	r.addSubscriber(s)
+	// Register the non-generic core with the bus rather than the typed facade,
+	// mirroring SubscribeFunc and Publish: this keeps the bus's outputs map
+	// and subscriber-interface itab out of per-T cost.
+	r.addSubscriber(s.core)
 	return s
 }
 

util/eventbus/subscribe.go

@@ -184,49 +184,62 @@ func (s *subscribeState) closed() <-chan struct{} {
 // A Subscriber delivers one type of event from a [Client].
 // Events are sent to the [Subscriber.Events] channel.
 type Subscriber[T any] struct {
-	stop       stopFlag
-	read       chan T
-	unregister func()
-	logf       logger.Logf
-	slow       *time.Timer // used to detect slow subscriber service
+	// core holds the non-generic subscriber-interface implementation
+	// (Close, subscribeType, dispatch, slow timer, unregister) shared
+	// with [SubscriberFunc] via [subscriberCore]. The only per-T state
+	// owned by the facade itself is the typed delivery channel; the
+	// dispatch loop, unlike SubscriberFunc, must remain per-T — see
+	// [Subscriber.dispatchTyped].
+	core *subscriberCore
+	read chan T
 }
 
 func newSubscriber[T any](r *subscribeState, logf logger.Logf) *Subscriber[T] {
-	slow := time.NewTimer(0)
-	slow.Stop() // reset in dispatch
-	return &Subscriber[T]{
-		read:       make(chan T),
-		unregister: func() { r.deleteSubscriber(reflect.TypeFor[T]()) },
-		logf:       logf,
-		slow:       slow,
+	core := newSubscriberCore(r, logf, reflect.TypeFor[T]())
+	s := &Subscriber[T]{
+		core: core,
+		read: make(chan T),
 	}
-}
-
-func newMonitor[T any](attach func(fn func(T)) (cancel func())) *Subscriber[T] {
-	ret := &Subscriber[T]{
-		read: make(chan T, 100), // arbitrary, large
+	// Subscriber[T] keeps a per-T dispatch loop; see [Subscriber.dispatchTyped]
+	// for why we don't share the non-generic dispatchFunc that SubscriberFunc
+	// uses.
+	core.dispatchFn = func(
+		ctx context.Context,
+		vals *queue[DeliveredEvent],
+		acceptCh func() chan DeliveredEvent,
+		snapshot chan chan []DeliveredEvent,
+	) bool {
+		return s.dispatchTyped(ctx, vals, acceptCh, snapshot)
 	}
-	ret.unregister = attach(ret.monitor)
-	return ret
+	return s
 }
 
-func (s *Subscriber[T]) subscribeType() reflect.Type {
-	return reflect.TypeFor[T]()
-}
-
-func (s *Subscriber[T]) monitor(debugEvent T) {
-	select {
-	case s.read <- debugEvent:
-	case <-s.stop.Done():
-	}
-}
-
-func (s *Subscriber[T]) dispatch(ctx context.Context, vals *queue[DeliveredEvent], acceptCh func() chan DeliveredEvent, snapshot chan chan []DeliveredEvent) bool {
+// dispatchTyped is the per-T dispatch loop for Subscriber[T]. It has to remain
+// generic because the typed channel send `case s.read <- t:` must appear
+// lexically inside the select. The rest of the cases match the non-generic
+// dispatchFunc body to keep behavior aligned between Subscriber and
+// SubscriberFunc.
+//
+// We don't share dispatchFunc (the way SubscriberFunc does) because bridging
+// the typed channel send and the non-generic select would require running the
+// send on its own goroutine on every event delivery. That bridge was measured
+// at ~2.7x throughput regression on BenchmarkBasicThroughput, so we keep
+// dispatchTyped generic and pay the per-shape stencil cost instead (measured
+// at ~1,600 B body + ~1,100 B pclntab per shape on linux/amd64 tailscaled).
+// Only the typed select lives in the per-shape stencil; the surrounding state
+// (slow timer, log function, type name) is reached through the non-generic
+// core.
+func (s *Subscriber[T]) dispatchTyped(
+	ctx context.Context,
+	vals *queue[DeliveredEvent],
+	acceptCh func() chan DeliveredEvent,
+	snapshot chan chan []DeliveredEvent,
+) bool {
 	t := vals.Peek().Event.(T)
 
 	start := time.Now()
-	s.slow.Reset(slowSubscriberTimeout)
-	defer s.slow.Stop()
+	s.core.slow.Reset(slowSubscriberTimeout)
+	defer s.core.slow.Stop()
 
 	for {
 		// Keep the cases in this select in sync with subscribeState.pump
@@ -242,13 +255,35 @@ func (s *Subscriber[T]) dispatch(ctx context.Context, vals *queue[DeliveredEvent
 			return false
 		case ch := <-snapshot:
 			ch <- vals.Snapshot()
-		case <-s.slow.C:
-			s.logf("subscriber for %T is slow (%v elapsed)", t, time.Since(start))
-			s.slow.Reset(slowSubscriberTimeout)
+		case <-s.core.slow.C:
+			s.core.logf("subscriber for %s is slow (%v elapsed)", s.core.typeName, time.Since(start))
+			s.core.slow.Reset(slowSubscriberTimeout)
 		}
 	}
 }
 
+func newMonitor[T any](attach func(fn func(T)) (cancel func())) *Subscriber[T] {
+	s := &Subscriber[T]{
+		// Monitors don't go through the bus's dispatch path (they
+		// are attached directly to the debug hook), so they don't
+		// need a fully-initialized subscriberCore — only the typed
+		// delivery channel and an unregister callback. We give them
+		// a placeholder core so Close() and Done() work uniformly.
+		core: &subscriberCore{},
+		read: make(chan T, 100), // arbitrary, large
+	}
+	cancel := attach(s.monitor)
+	s.core.unregister = func(reflect.Type) { cancel() }
+	return s
+}
+
+func (s *Subscriber[T]) monitor(debugEvent T) {
+	select {
+	case s.read <- debugEvent:
+	case <-s.core.stop.Done():
+	}
+}
+
 // Events returns a channel on which the subscriber's events are
 // delivered.
 func (s *Subscriber[T]) Events() <-chan T {
@@ -258,7 +293,7 @@ func (s *Subscriber[T]) Events() <-chan T {
 // Done returns a channel that is closed when the subscriber is
 // closed.
 func (s *Subscriber[T]) Done() <-chan struct{} {
-	return s.stop.Done()
+	return s.core.stop.Done()
 }
 
 // Close closes the Subscriber, indicating the caller no longer wishes
@@ -268,28 +303,26 @@ func (s *Subscriber[T]) Done() <-chan struct{} {
 // If the Bus from which the Subscriber was created is closed,
 // the Subscriber is implicitly closed and does not need to be closed
 // separately.
-func (s *Subscriber[T]) Close() {
-	s.stop.Stop() // unblock receivers
-	s.unregister()
-}
+func (s *Subscriber[T]) Close() { s.core.Close() }
 
 // A SubscriberFunc delivers one type of event from a [Client].
 // Events are forwarded synchronously to a function provided at construction.
 type SubscriberFunc[T any] struct {
-	// Implementation note: SubscriberFunc[T] is a thin facade over a
-	// non-generic *subscriberFuncCore. All of the behavior — the
-	// subscriber-interface implementation (Close, subscribeType, dispatch), the
-	// slow-subscriber timer, the type assertion, and the user callback
-	// invocation — lives on the core and is not instantiated per T. The only
-	// per-T cost is the small forwarding Close method below.
-	core *subscriberFuncCore
+	// core holds the non-generic subscriber-interface implementation shared
+	// with [Subscriber] via [subscriberCore]. The user callback is captured
+	// in the dispatchFn closure on the core, so SubscriberFunc[T] itself
+	// carries no per-T state beyond the core pointer; per-T cost is limited
+	// to the small forwarding Close method below.
+	core *subscriberCore
 }
 
-// subscriberFuncCore is the non-generic implementation of a
-// SubscriberFunc. It implements the package-private subscriber
-// interface so that the bus (and the subscribeState map) can store
-// it without per-T itabs or dictionaries.
-type subscriberFuncCore struct {
+// subscriberCore is the non-generic backing for both Subscriber[T] and
+// SubscriberFunc[T]. It implements the package-private subscriber interface
+// so that the bus (and the subscribeState map) can store it without per-T
+// itabs or dictionaries. The per-T behavior (type assertion plus either typed
+// channel send or user callback invocation) is encapsulated in the dispatchFn
+// closure set up by the constructor of the typed facade.
+type subscriberCore struct {
 	stop       stopFlag
 	unregister func(reflect.Type)
 	logf       logger.Logf
@@ -306,13 +339,13 @@ type subscriberFuncCore struct {
 	// of T, so it doesn't contribute to per-T stencil cost.
 	typeName string
 
-	// dispatchFn is the per-T dispatch closure. It performs the
-	// type assertion vals.Peek().Event.(T) and runs the user
-	// callback on the unboxed value. The closure body is
-	// non-generic; its only per-T contribution is the type
-	// assertion and the call through s.read(T), which sit inside
-	// a single small captured closure rather than across a full
-	// select-loop stencil per T.
+	// dispatchFn is the per-T dispatch closure. It performs the type
+	// assertion vals.Peek().Event.(T) and runs the typed delivery (either a
+	// user-callback invocation for SubscriberFunc[T] or a typed channel send
+	// for Subscriber[T]). The closure body is non-generic apart from those
+	// two T-bound operations; the bulk of the dispatch work happens in the
+	// non-generic dispatchFunc helper (used by SubscriberFunc) or in the
+	// Subscriber[T].dispatchTyped per-shape stencil.
 	dispatchFn func(
 		ctx context.Context,
 		vals *queue[DeliveredEvent],
@@ -322,12 +355,12 @@ type subscriberFuncCore struct {
 }
 
 func newSubscriberFunc[T any](r *subscribeState, f func(T), logf logger.Logf) *SubscriberFunc[T] {
-	core := newSubscriberFuncCore(r, logf, reflect.TypeFor[T]())
+	core := newSubscriberCore(r, logf, reflect.TypeFor[T]())
 	// The dispatch closure is the only piece that intrinsically
 	// needs T: it performs the type assertion on the head queue
 	// value and forwards the unboxed value to the user callback.
 	// All non-generic setup (timer, core allocation, unregister
-	// closure) lives in newSubscriberFuncCore so it isn't
+	// closure) lives in newSubscriberCore so it isn't
 	// duplicated per T.
 	core.dispatchFn = func(
 		ctx context.Context,
@@ -347,22 +380,21 @@ func newSubscriberFunc[T any](r *subscribeState, f func(T), logf logger.Logf) *S
 	return &SubscriberFunc[T]{core: core}
 }
 
-// newSubscriberFuncCore performs the non-generic portion of
-// subscriber construction: timer setup, core struct allocation,
-// and creation of the unregister closure that captures only the
-// (non-generic) reflect.Type and *subscribeState. The caller fills
-// in the per-T dispatchFn afterward.
+// newSubscriberCore performs the non-generic portion of subscriber
+// construction: timer setup, core struct allocation, and assignment of the
+// unregister method-value. The caller fills in the per-T dispatchFn
+// afterward.
 //
-// Hoisting this out of newSubscriberFunc[T] eliminates the bulk of
-// the constructor body's per-T stencil cost; the only T-typed
-// instructions left in the generic constructor are the
+// Hoisting this out of the typed constructors (newSubscriber[T] and
+// newSubscriberFunc[T]) eliminates the bulk of their per-T stencil cost; the
+// only T-typed instructions left in each generic constructor are the
 // reflect.TypeFor[T]() call (whose body is shared via the
-// internal/abi.TypeFor[T] dictionary) and the construction of the
-// dispatch closure itself.
-func newSubscriberFuncCore(r *subscribeState, logf logger.Logf, typ reflect.Type) *subscriberFuncCore {
+// internal/abi.TypeFor[T] dictionary) and the construction of the dispatch
+// closure itself.
+func newSubscriberCore(r *subscribeState, logf logger.Logf, typ reflect.Type) *subscriberCore {
 	slow := time.NewTimer(0)
 	slow.Stop() // reset in dispatch
-	core := &subscriberFuncCore{
+	core := &subscriberCore{
 		logf:     logf,
 		slow:     slow,
 		typ:      typ,
@@ -380,19 +412,19 @@ func newSubscriberFuncCore(r *subscribeState, logf logger.Logf, typ reflect.Type
 // does not need to be closed separately.
 func (s *SubscriberFunc[T]) Close() { s.core.Close() }
 
-// Close implements the subscriber interface and the user-facing
-// (*SubscriberFunc[T]).Close.
-func (c *subscriberFuncCore) Close() {
+// Close implements the subscriber interface and the user-facing Close on
+// both Subscriber[T] and SubscriberFunc[T].
+func (c *subscriberCore) Close() {
 	c.stop.Stop()
 	c.unregister(c.typ)
 }
 
 // subscribeType implements the subscriber interface.
-func (c *subscriberFuncCore) subscribeType() reflect.Type { return c.typ }
+func (c *subscriberCore) subscribeType() reflect.Type { return c.typ }
 
 // dispatch implements the subscriber interface by invoking the
 // per-T dispatch closure that was captured at construction time.
-func (c *subscriberFuncCore) dispatch(
+func (c *subscriberCore) dispatch(
 	ctx context.Context,
 	vals *queue[DeliveredEvent],
 	acceptCh func() chan DeliveredEvent,
@@ -412,7 +444,7 @@ func (c *subscriberFuncCore) dispatch(
 // callDone is closed by runFuncCallback when the user callback returns.
 func dispatchFunc(
 	ctx context.Context,
-	core *subscriberFuncCore,
+	core *subscriberCore,
 	vals *queue[DeliveredEvent],
 	acceptCh func() chan DeliveredEvent,
 	snapshot chan chan []DeliveredEvent,