Delay getdata requests from peers using txid-based relay

Using both txid and wtxid-based relay with peers means that we could sometimes
download the same transaction twice, if announced via two different hashes from
different peers.

Use a heuristic of delaying txid-peer-getdata requests by 2 seconds, if we have
at least one wtxid-based peer.

src/net_processing.cpp

@@ -75,6 +75,8 @@ static const unsigned int MAX_INV_SZ = 50000;
 static constexpr int32_t MAX_PEER_TX_IN_FLIGHT = 100;
 /** Maximum number of announced transactions from a peer */
 static constexpr int32_t MAX_PEER_TX_ANNOUNCEMENTS = 2 * MAX_INV_SZ;
+/** How many microseconds to delay requesting transactions via txids, if we have wtxid-relaying peers */
+static constexpr std::chrono::microseconds TXID_RELAY_DELAY{std::chrono::seconds{2}};
 /** How many microseconds to delay requesting transactions from inbound peers */
 static constexpr std::chrono::microseconds INBOUND_PEER_TX_DELAY{std::chrono::seconds{2}};
 /** How long to wait (in microseconds) before downloading a transaction from an additional peer */
@@ -219,6 +221,9 @@ namespace {
     /** Number of peers from which we're downloading blocks. */
     int nPeersWithValidatedDownloads GUARDED_BY(cs_main) = 0;
 
+    /** Number of peers with wtxid relay. */
+    int g_wtxid_relay_peers GUARDED_BY(cs_main) = 0;
+
     /** Number of outbound peers with m_chain_sync.m_protect. */
     int g_outbound_peers_with_protect_from_disconnect GUARDED_BY(cs_main) = 0;
 
@@ -764,7 +769,7 @@ void UpdateTxRequestTime(const uint256& txid, std::chrono::microseconds request_
     }
 }
 
-std::chrono::microseconds CalculateTxGetDataTime(const uint256& txid, std::chrono::microseconds current_time, bool use_inbound_delay) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+std::chrono::microseconds CalculateTxGetDataTime(const uint256& txid, std::chrono::microseconds current_time, bool use_inbound_delay, bool use_txid_delay) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     std::chrono::microseconds process_time;
     const auto last_request_time = GetTxRequestTime(txid);
@@ -780,6 +785,9 @@ std::chrono::microseconds CalculateTxGetDataTime(const uint256& txid, std::chron
     // We delay processing announcements from inbound peers
     if (use_inbound_delay) process_time += INBOUND_PEER_TX_DELAY;
 
+    // We delay processing announcements from peers that use txid-relay (instead of wtxid)
+    if (use_txid_delay) process_time += TXID_RELAY_DELAY;
+
     return process_time;
 }
 
@@ -797,7 +805,7 @@ void RequestTx(CNodeState* state, const uint256& txid, std::chrono::microseconds
 
     // Calculate the time to try requesting this transaction. Use
     // fPreferredDownload as a proxy for outbound peers.
-    const auto process_time = CalculateTxGetDataTime(txid, current_time, !state->fPreferredDownload);
+    const auto process_time = CalculateTxGetDataTime(txid, current_time, !state->fPreferredDownload, !state->m_wtxid_relay && g_wtxid_relay_peers > 0);
 
     peer_download_state.m_tx_process_time.emplace(process_time, txid);
 }
@@ -874,6 +882,8 @@ void PeerLogicValidation::FinalizeNode(NodeId nodeid, bool& fUpdateConnectionTim
     assert(nPeersWithValidatedDownloads >= 0);
     g_outbound_peers_with_protect_from_disconnect -= state->m_chain_sync.m_protect;
     assert(g_outbound_peers_with_protect_from_disconnect >= 0);
+    g_wtxid_relay_peers -= state->m_wtxid_relay;
+    assert(g_wtxid_relay_peers >= 0);
 
     mapNodeState.erase(nodeid);
 
@@ -883,6 +893,7 @@ void PeerLogicValidation::FinalizeNode(NodeId nodeid, bool& fUpdateConnectionTim
         assert(nPreferredDownload == 0);
         assert(nPeersWithValidatedDownloads == 0);
         assert(g_outbound_peers_with_protect_from_disconnect == 0);
+        assert(g_wtxid_relay_peers == 0);
     }
     LogPrint(BCLog::NET, "Cleared nodestate for peer=%d\n", nodeid);
 }
@@ -2489,6 +2500,7 @@ void ProcessMessage(
             LOCK(cs_main);
             if (!State(pfrom.GetId())->m_wtxid_relay) {
                 State(pfrom.GetId())->m_wtxid_relay = true;
+                g_wtxid_relay_peers++;
             }
         }
         return;
@@ -4490,7 +4502,15 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                     // up processing to happen after the download times out
                     // (with a slight delay for inbound peers, to prefer
                     // requests to outbound peers).
-                    const auto next_process_time = CalculateTxGetDataTime(txid, current_time, !state.fPreferredDownload);
+                    // Don't apply the txid-delay to re-requests of a
+                    // transaction; the heuristic of delaying requests to
+                    // txid-relay peers is to save bandwidth on initial
+                    // announcement of a transaction, and doesn't make sense
+                    // for a followup request if our first peer times out (and
+                    // would open us up to an attacker using inbound
+                    // wtxid-relay to prevent us from requesting transactions
+                    // from outbound txid-relay peers).
+                    const auto next_process_time = CalculateTxGetDataTime(txid, current_time, !state.fPreferredDownload, false);
                     tx_process_time.emplace(next_process_time, txid);
                 }
             } else {

test/functional/p2p_tx_download.py

@@ -44,12 +44,13 @@ class TestP2PConn(P2PInterface):
 GETDATA_TX_INTERVAL = 60  # seconds
 MAX_GETDATA_RANDOM_DELAY = 2  # seconds
 INBOUND_PEER_TX_DELAY = 2  # seconds
+TXID_RELAY_DELAY = 2 # seconds
 MAX_GETDATA_IN_FLIGHT = 100
 TX_EXPIRY_INTERVAL = GETDATA_TX_INTERVAL * 10
 
 # Python test constants
 NUM_INBOUND = 10
-MAX_GETDATA_INBOUND_WAIT = GETDATA_TX_INTERVAL + MAX_GETDATA_RANDOM_DELAY + INBOUND_PEER_TX_DELAY
+MAX_GETDATA_INBOUND_WAIT = GETDATA_TX_INTERVAL + MAX_GETDATA_RANDOM_DELAY + INBOUND_PEER_TX_DELAY + TXID_RELAY_DELAY
 
 
 class TxDownloadTest(BitcoinTestFramework):