fix(chain/local_v1): prevent double-close panic in block counter watcher

The count() loop could call close(watcher.channel) on consecutive ticks
before the WatchBlocks cleanup goroutine removed the cancelled watcher
from the list, causing a "close of closed channel" panic. Use sync.Once
to guarantee the channel is closed exactly once.

Author: lrsaturnino

pkg/chain/local_v1/blockcounter.go

@@ -16,8 +16,9 @@ type localBlockCounter struct {
 }
 
 type watcher struct {
-	ctx     context.Context
-	channel chan uint64
+	ctx       context.Context
+	channel   chan uint64
+	closeOnce sync.Once
 }
 
 var defaultBlockTime = 500 * time.Millisecond
@@ -120,7 +121,7 @@ func (lbc *localBlockCounter) count(blockTime ...time.Duration) {
 
 		for _, watcher := range watchers {
 			if watcher.ctx.Err() != nil {
-				close(watcher.channel)
+				watcher.closeOnce.Do(func() { close(watcher.channel) })
 				continue
 			}