iter: fix race instrumentation in Pull2

Pull2 tests are failing with -race, giving false-positive race conditions due to
bad race instrumentation.

No tests for this as it should be caught by the race builders. The only reason
it was not caught is because it is behind GOEXPERIMENT=rangefunc.

Fixes [reserved]

src/iter/iter.go

@@ -122,18 +122,22 @@ func Pull[V any](seq Seq[V]) (next func() (V, bool), stop func()) {
 // simultaneously.
 func Pull2[K, V any](seq Seq2[K, V]) (next func() (K, V, bool), stop func()) {
 	var (
-		k    K
-		v    V
-		ok   bool
-		done bool
+		k     K
+		v     V
+		ok    bool
+		done  bool
+		racer int
 	)
 	c := newcoro(func(c *coro) {
+		race.Acquire(unsafe.Pointer(&racer))
 		yield := func(k1 K, v1 V) bool {
 			if done {
 				return false
 			}
 			k, v, ok = k1, v1, true
+			race.Release(unsafe.Pointer(&racer))
 			coroswitch(c)
+			race.Acquire(unsafe.Pointer(&racer))
 			return !done
 		}
 		seq(yield)
@@ -141,20 +145,25 @@ func Pull2[K, V any](seq Seq2[K, V]) (next func() (K, V, bool), stop func()) {
 		var v0 V
 		k, v, ok = k0, v0, false
 		done = true
+		race.Release(unsafe.Pointer(&racer))
 	})
 	next = func() (k1 K, v1 V, ok1 bool) {
-		race.Write(unsafe.Pointer(&c)) // detect races
+		race.Write(unsafe.Pointer(&racer)) // detect races
 		if done {
 			return
 		}
+		race.Release(unsafe.Pointer(&racer))
 		coroswitch(c)
+		race.Acquire(unsafe.Pointer(&racer))
 		return k, v, ok
 	}
 	stop = func() {
-		race.Write(unsafe.Pointer(&c)) // detect races
+		race.Write(unsafe.Pointer(&racer)) // detect races
 		if !done {
 			done = true
+			race.Release(unsafe.Pointer(&racer))
 			coroswitch(c)
+			race.Acquire(unsafe.Pointer(&racer))
 		}
 	}
 	return next, stop