eth: broadcast blocks to at least 4 peers (#17725)

eth/handler.go

@@ -49,6 +49,9 @@ const (
 	// txChanSize is the size of channel listening to NewTxsEvent.
 	// The number is referenced from the size of tx pool.
 	txChanSize = 4096
+
+	// minimim number of peers to broadcast new blocks to
+	minBroadcastPeers = 4
 )
 
 var (
@@ -705,7 +708,14 @@ func (pm *ProtocolManager) BroadcastBlock(block *types.Block, propagate bool) {
 			return
 		}
 		// Send the block to a subset of our peers
-		transfer := peers[:int(math.Sqrt(float64(len(peers))))]
+		transferLen := int(math.Sqrt(float64(len(peers))))
+		if transferLen < minBroadcastPeers {
+			transferLen = minBroadcastPeers
+		}
+		if transferLen > len(peers) {
+			transferLen = len(peers)
+		}
+		transfer := peers[:transferLen]
 		for _, peer := range transfer {
 			peer.AsyncSendNewBlock(block, td)
 		}