runtime: use small struct TestSynctest to ensure cleanups run

Finalizers and cleanup funcs weren't running on the windows-arm64
builder. Put finalizers/cleanups on a small struct containing a pointer
rather than an *int, which fixes the problem.

Also uncomment a synctest.Wait that was accidentally commented out.

Fixes #73977

Change-Id: Ia6f18d74d6fccf2c5a9222317977c7458d67f158
Reviewed-on: https://go-review.googlesource.com/c/go/+/679696
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/testdata/testsynctest/main.go

@@ -9,6 +9,7 @@ import (
 	"runtime"
 	"runtime/metrics"
 	"sync/atomic"
+	"unsafe"
 )
 
 // This program ensures system goroutines (GC workers, finalizer goroutine)
@@ -27,6 +28,11 @@ func numGCCycles() uint64 {
 	return samples[0].Value.Uint64()
 }
 
+type T struct {
+	v int
+	p unsafe.Pointer
+}
+
 func main() {
 	// Channels created by a finalizer and cleanup func registered within the bubble.
 	var (
@@ -36,8 +42,8 @@ func main() {
 	synctest.Run(func() {
 		// Start the finalizer and cleanup goroutines.
 		{
-			p := new(int)
-			runtime.SetFinalizer(p, func(*int) {
+			p := new(T)
+			runtime.SetFinalizer(p, func(*T) {
 				ch := make(chan struct{})
 				finalizerCh.Store(&ch)
 			})
@@ -47,35 +53,35 @@ func main() {
 			}, struct{}{})
 		}
 		startingCycles := numGCCycles()
-		ch1 := make(chan *int)
-		ch2 := make(chan *int)
+		ch1 := make(chan *T)
+		ch2 := make(chan *T)
 		defer close(ch1)
 		go func() {
-			for i := range ch1 {
-				v := *i + 1
-				ch2 <- &v
+			for range ch1 {
+				ch2 <- &T{}
 			}
 		}()
-		for {
+		const iterations = 1000
+		for range iterations {
 			// Make a lot of short-lived allocations to get the GC working.
-			for i := 0; i < 1000; i++ {
-				v := new(int)
-				*v = i
+			for range 1000 {
+				v := new(T)
 				// Set finalizers on these values, just for added stress.
-				runtime.SetFinalizer(v, func(*int) {})
+				runtime.SetFinalizer(v, func(*T) {})
 				ch1 <- v
 				<-ch2
 			}
 
 			// If we've improperly put a GC goroutine into the synctest group,
 			// this Wait is going to hang.
-			//synctest.Wait()
+			synctest.Wait()
 
 			// End the test after a couple of GC cycles have passed.
 			if numGCCycles()-startingCycles > 1 && finalizerCh.Load() != nil && cleanupCh.Load() != nil {
-				break
+				return
 			}
 		}
+		println("finalizers/cleanups failed to run after", iterations, "cycles")
 	})
 	// Close the channels created by the finalizer and cleanup func.
 	// If the funcs improperly ran inside the bubble, these channels are bubbled