[rpc] query node for syncing calls (#4309)

* [rpc] query node for syncing calls

Resolves #3966

* fix(downloader): do not underflow

hmy/downloader/downloader.go

@@ -120,11 +120,13 @@ func (d *Downloader) NumPeers() int {
 
 // IsSyncing return the current sync status
 func (d *Downloader) SyncStatus() (bool, uint64, uint64) {
+	current := d.bc.CurrentBlock().NumberU64()
 	syncing, target := d.status.get()
-	if !syncing {
-		target = d.bc.CurrentBlock().NumberU64()
+	if !syncing { // means synced
+		target = current
 	}
-	return syncing, target, 0
+	// isSyncing, target, blocks to target
+	return syncing, target, target - current
 }
 
 // SubscribeDownloadStarted subscribe download started

rpc/harmony.go

@@ -42,18 +42,21 @@ func (s *PublicHarmonyService) ProtocolVersion(
 	}
 }
 
-// Syncing returns false in case the node is currently not syncing with the network. It can be up to date or has not
-// yet received the latest block headers from its pears. In case it is synchronizing:
-// - startingBlock: block number this node started to synchronise from
-// - currentBlock:  block number this node is currently importing
-// - highestBlock:  block number of the highest block header this node has received from peers
-// - pulledStates:  number of state entries processed until now
-// - knownStates:   number of known state entries that still need to be pulled
+// Syncing returns false in case the node is in sync with the network
+// With the current network height as well as the difference
 func (s *PublicHarmonyService) Syncing(
 	ctx context.Context,
 ) (interface{}, error) {
-	// TODO(dm): find our Downloader module for syncing blocks
-	return false, nil
+	inSync, target, difference := s.hmy.NodeAPI.SyncStatus(s.hmy.ShardID)
+	return struct {
+		InSync bool   `json:"in-sync"`
+		Target uint64 `json:"network-height"`
+		Delta  uint64 `json:"difference"`
+	}{
+		InSync: inSync,
+		Target: target,
+		Delta:  difference,
+	}, nil
 }
 
 // GasPrice returns a suggestion for a gas price.