Supress header download when queue is full

Author: foriequal0

sync/src/block/downloader/header.rs

@@ -25,6 +25,7 @@ use primitives::{H256, U256};
 use super::super::message::RequestMessage;
 
 const MAX_HEADER_REQUEST_LENGTH: u64 = 128;
+const MAX_HEADER_QUEUE_LENGTH: usize = 1024;
 const MAX_RETRY: usize = 3;
 const MAX_WAIT: u64 = 15;
 
@@ -128,6 +129,9 @@ impl HeaderDownloader {
         if !self.is_idle() {
             return None
         }
+        if self.queued.len() > MAX_HEADER_QUEUE_LENGTH {
+            return None
+        }
 
         let pivot_number = self.pivot_header().number();