iter: fix race instrumentation for 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 #64651

Change-Id: I20554da930b0e19594e0e267f01a1e7a9cbc577a
GitHub-Last-Rev: 7c1f19238d
GitHub-Pull-Request: golang/go#64653
Reviewed-on: https://go-review.googlesource.com/c/go/+/548895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/iter/iter.go

@@ -14,8 +14,7 @@ package iter
 import (
 	"internal/race"
 	"unsafe"
-	_ "unsafe"
-) // for linkname
+)
 
 // Seq is an iterator over sequences of individual values.
 // When called as seq(yield), seq calls yield(v) for each value v in the sequence,
@@ -122,18 +121,22 @@ 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 +144,25 @@ 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