eth/downloader: ensure PendingNodeData returns 0 inside callback

eth/downloader/queue.go

@@ -1096,21 +1096,21 @@ func (q *queue) DeliverNodeData(id string, data [][]byte, callback func(int, boo
 	progressed, nproc, procerr := q.stateScheduler.Process(process, batch)
 	q.stateWriters += 1
 	go func() {
-		defer func() {
-			q.lock.Lock()
-			q.stateWriters -= 1
-			q.lock.Unlock()
-			// Wake up WaitResults after the state has been written because it might be
-			// waiting for completion of the pivot block's state download.
-			q.active.Signal()
-		}()
-		if procerr != nil {
-			// Return processing errors through the callback so the sync gets canceled.
-			callback(nproc, progressed, procerr)
-			return
+		// Wake up WaitResults after the state has been written because it might be
+		// waiting for completion of the pivot block's state download.
+		defer q.active.Signal()
+
+		if procerr == nil {
+			nproc = len(process)
+			procerr = batch.Write()
 		}
-		err := batch.Write()
-		callback(len(process), progressed, err)
+		// Return processing errors through the callback so the sync gets canceled. The
+		// number of writers is decremented prior to the call so PendingNodeData will
+		// return zero when the callback runs.
+		q.lock.Lock()
+		q.stateWriters -= 1
+		q.lock.Unlock()
+		callback(nproc, progressed, procerr)
 	}()
 
 	// If none of the data items were good, it's a stale delivery