Introduce preferred download peers

src/main.cpp

@@ -128,6 +128,8 @@ namespace {
     };
     map<uint256, pair<NodeId, list<QueuedBlock>::iterator> > mapBlocksInFlight;
 
+    // Number of preferrable block download peers.
+    int nPreferredDownload = 0;
 } // anon namespace
 
 //////////////////////////////////////////////////////////////////////////////
@@ -225,6 +227,8 @@ struct CNodeState {
     int64_t nStallingSince;
     list<QueuedBlock> vBlocksInFlight;
     int nBlocksInFlight;
+    // Whether we consider this a preferred download peer.
+    bool fPreferredDownload;
 
     CNodeState() {
         nMisbehavior = 0;
@@ -235,6 +239,7 @@ struct CNodeState {
         fSyncStarted = false;
         nStallingSince = 0;
         nBlocksInFlight = 0;
+        fPreferredDownload = false;
     }
 };
 
@@ -255,6 +260,16 @@ int GetHeight()
     return chainActive.Height();
 }
 
+void UpdatePreferredDownload(CNode* node, CNodeState* state)
+{
+    nPreferredDownload -= state->fPreferredDownload;
+
+    // Whether this node should be marked as a preferred download node.
+    state->fPreferredDownload = (!node->fInbound || node->fWhitelisted) && !node->fOneShot && !node->fClient;
+
+    nPreferredDownload += state->fPreferredDownload;
+}
+
 void InitializeNode(NodeId nodeid, const CNode *pnode) {
     LOCK(cs_main);
     CNodeState &state = mapNodeState.insert(std::make_pair(nodeid, CNodeState())).first->second;
@@ -271,6 +286,7 @@ void FinalizeNode(NodeId nodeid) {
     BOOST_FOREACH(const QueuedBlock& entry, state->vBlocksInFlight)
         mapBlocksInFlight.erase(entry.hash);
     EraseOrphansFor(nodeid);
+    nPreferredDownload -= state->fPreferredDownload;
 
     mapNodeState.erase(nodeid);
 }
@@ -3471,6 +3487,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
         pfrom->fClient = !(pfrom->nServices & NODE_NETWORK);
 
+        // Potentially mark this peer as a preferred download peer.
+        UpdatePreferredDownload(pfrom, State(pfrom->GetId()));
 
         // Change version
         pfrom->PushMessage("verack");
@@ -4415,7 +4433,7 @@ bool SendMessages(CNode* pto, bool fSendTrickle)
         // Start block sync
         if (pindexBestHeader == NULL)
             pindexBestHeader = chainActive.Tip();
-        bool fFetch = !pto->fInbound || (pindexBestHeader && (state.pindexLastCommonBlock ? state.pindexLastCommonBlock->nHeight : 0) + 144 > pindexBestHeader->nHeight);
+        bool fFetch = state.fPreferredDownload || (nPreferredDownload == 0 && !pto->fClient && !pto->fOneShot); // Download if this is a nice peer, or we have no nice peers and this one might do.
         if (!state.fSyncStarted && !pto->fClient && fFetch && !fImporting && !fReindex) {
             // Only actively request headers from a single peer, unless we're close to today.
             if (nSyncStarted == 0 || pindexBestHeader->GetBlockTime() > GetAdjustedTime() - 24 * 60 * 60) {