Get our "best three" peers to announce blocks using cmpctblocks

Author: TheBlueMatt

src/main.cpp

@@ -204,6 +204,9 @@ namespace {
     };
     map<uint256, pair<NodeId, list<QueuedBlock>::iterator> > mapBlocksInFlight;
 
+    /** Stack of nodes which we have set to announce using compact blocks */
+    list<NodeId> lNodesAnnouncingHeaderAndIDs;
+
     /** Number of preferable block download peers. */
     int nPreferredDownload = 0;
 
@@ -456,6 +459,28 @@ void UpdateBlockAvailability(NodeId nodeid, const uint256 &hash) {
     }
 }
 
+void MaybeSetPeerAsAnnouncingHeaderAndIDs(const CNodeState* nodestate, CNode* pfrom) {
+    if (nodestate->fProvidesHeaderAndIDs) {
+        BOOST_FOREACH(const NodeId nodeid, lNodesAnnouncingHeaderAndIDs)
+            if (nodeid == pfrom->GetId())
+                return;
+        bool fAnnounceUsingCMPCTBLOCK = false;
+        uint64_t nCMPCTBLOCKVersion = 1;
+        if (lNodesAnnouncingHeaderAndIDs.size() >= 3) {
+            // As per BIP152, we only get 3 of our peers to announce
+            // blocks using compact encodings.
+            CNode* pnodeStop = FindNode(lNodesAnnouncingHeaderAndIDs.front());
+            if (pnodeStop) {
+                pnodeStop->PushMessage(NetMsgType::SENDCMPCT, fAnnounceUsingCMPCTBLOCK, nCMPCTBLOCKVersion);
+                lNodesAnnouncingHeaderAndIDs.pop_front();
+            }
+        }
+        fAnnounceUsingCMPCTBLOCK = true;
+        pfrom->PushMessage(NetMsgType::SENDCMPCT, fAnnounceUsingCMPCTBLOCK, nCMPCTBLOCKVersion);
+        lNodesAnnouncingHeaderAndIDs.push_back(pfrom->GetId());
+    }
+}
+
 // Requires cs_main
 bool CanDirectFetch(const Consensus::Params &consensusParams)
 {
@@ -5531,6 +5556,10 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                 }
                 if (vGetData.size() > 0) {
                     if (nodestate->fProvidesHeaderAndIDs && vGetData.size() == 1 && mapBlocksInFlight.size() == 1 && pindexLast->pprev->IsValid(BLOCK_VALID_CHAIN)) {
+                        // We seem to be rather well-synced, so it appears pfrom was the first to provide us
+                        // with this block! Let's get them to announce using compact blocks in the future.
+                        MaybeSetPeerAsAnnouncingHeaderAndIDs(nodestate, pfrom);
+                        // In any case, we want to download using a compact block, not a regular one
                         vGetData[0] = CInv(MSG_CMPCT_BLOCK, vGetData[0].hash);
                     }
                     pfrom->PushMessage(NetMsgType::GETDATA, vGetData);