[net processing] Only send a getheaders for one block in an INV

Headers-first is the primary method of announcement on the network. If a
node fell back sending blocks by inv, it's probably for a re-org. The
final block hash provided should be the highest, so send a getheaders
and then fetch the blocks we need to catch up.

Author: jnewbery

src/net_processing.cpp

@@ -2420,6 +2420,7 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
 
         uint32_t nFetchFlags = GetFetchFlags(pfrom);
         const auto current_time = GetTime<std::chrono::microseconds>();
+        uint256* best_block{nullptr};
 
         for (CInv &inv : vInv)
         {
@@ -2436,17 +2437,14 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
             if (inv.type == MSG_BLOCK) {
                 UpdateBlockAvailability(pfrom->GetId(), inv.hash);
                 if (!fAlreadyHave && !fImporting && !fReindex && !mapBlocksInFlight.count(inv.hash)) {
-                    // We used to request the full block here, but since headers-announcements are now the
-                    // primary method of announcement on the network, and since, in the case that a node
-                    // fell back to inv we probably have a reorg which we should get the headers for first,
-                    // we now only provide a getheaders response here. When we receive the headers, we will
-                    // then ask for the blocks we need.
-                    connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::GETHEADERS, ::ChainActive().GetLocator(pindexBestHeader), inv.hash));
-                    LogPrint(BCLog::NET, "getheaders (%d) %s to peer=%d\n", pindexBestHeader->nHeight, inv.hash.ToString(), pfrom->GetId());
+                    // Headers-first is the primary method of announcement on
+                    // the network. If a node fell back to sending blocks by inv,
+                    // it's probably for a re-org. The final block hash
+                    // provided should be the highest, so send a getheaders and
+                    // then fetch the blocks we need to catch up.
+                    best_block = &inv.hash;
                 }
-            }
-            else
-            {
+            } else {
                 pfrom->AddInventoryKnown(inv);
                 if (fBlocksOnly) {
                     LogPrint(BCLog::NET, "transaction (%s) inv sent in violation of protocol, disconnecting peer=%d\n", inv.hash.ToString(), pfrom->GetId());
@@ -2457,6 +2455,12 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
                 }
             }
         }
+
+        if (best_block != nullptr) {
+            connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::GETHEADERS, ::ChainActive().GetLocator(pindexBestHeader), *best_block));
+            LogPrint(BCLog::NET, "getheaders (%d) %s to peer=%d\n", pindexBestHeader->nHeight, best_block->ToString(), pfrom->GetId());
+        }
+
         return true;
     }