p2p: sync from limited peer, only request blocks below threshold

Requesting historical blocks from network limited peers is a
direct disconnection cause.
The node must only request the blocks who know for sure the
limited peer can provide.

Author: furszy

src/net_processing.cpp

@@ -1451,6 +1451,7 @@ void PeerManagerImpl::FindNextBlocks(std::vector<const CBlockIndex*>& vBlocks, c
 {
     std::vector<const CBlockIndex*> vToFetch;
     int nMaxHeight = std::min<int>(state->pindexBestKnownBlock->nHeight, nWindowEnd + 1);
+    bool is_limited_peer = IsLimitedPeer(peer);
     NodeId waitingfor = -1;
     while (pindexWalk->nHeight < nMaxHeight) {
         // Read up to 128 (or more, if more blocks than that are needed) successors of pindexWalk (towards
@@ -1505,6 +1506,11 @@ void PeerManagerImpl::FindNextBlocks(std::vector<const CBlockIndex*>& vBlocks, c
                 return;
             }
 
+            // Don't request blocks that go further than what limited peers can provide
+            if (is_limited_peer && (state->pindexBestKnownBlock->nHeight - pindex->nHeight >= static_cast<int>(NODE_NETWORK_LIMITED_MIN_BLOCKS) - 2 /* two blocks buffer for possible races */)) {
+                continue;
+            }
+
             vBlocks.push_back(pindex);
             if (vBlocks.size() == count) {
                 return;