runtime: lock mheap_.speciallock when allocating synctest specials

Avoid racing use of mheap_.specialBubbleAlloc.

Fixes #75134

Change-Id: I0c9140c18d2bca1e1c3387cd81230f0e8c9ac23e
Reviewed-on: https://go-review.googlesource.com/c/go/+/699255
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/synctest/synctest_test.go

@@ -779,6 +779,28 @@ func TestWaitGroupHeapAllocated(t *testing.T) {
 	})
 }
 
+// Issue #75134: Many racing bubble associations.
+func TestWaitGroupManyBubbles(t *testing.T) {
+	var wg sync.WaitGroup
+	for range 100 {
+		wg.Go(func() {
+			synctest.Run(func() {
+				cancelc := make(chan struct{})
+				var wg2 sync.WaitGroup
+				for range 100 {
+					wg2.Go(func() {
+						<-cancelc
+					})
+				}
+				synctest.Wait()
+				close(cancelc)
+				wg2.Wait()
+			})
+		})
+	}
+	wg.Wait()
+}
+
 func TestHappensBefore(t *testing.T) {
 	// Use two parallel goroutines accessing different vars to ensure that
 	// we correctly account for multiple goroutines in the bubble.

src/runtime/synctest.go

@@ -410,7 +410,9 @@ func getOrSetBubbleSpecial(p unsafe.Pointer, bubbleid uint64, add bool) (assoc i
 	} else if add {
 		// p is not associated with a bubble,
 		// and we've been asked to add an association.
+		lock(&mheap_.speciallock)
 		s := (*specialBubble)(mheap_.specialBubbleAlloc.alloc())
+		unlock(&mheap_.speciallock)
 		s.bubbleid = bubbleid
 		s.special.kind = _KindSpecialBubble
 		s.special.offset = offset