Introduce constant for mempool-based relay separate from mapRelay caching

This constant is set to 2 minutes, rather than 15. This is still many times
larger than the transaction broadcast interval (2s for outbound, 5s for
inbound), so it should be acceptable for peers to know what our contents of
the mempool was that long ago.

Author: sipa

src/net_processing.cpp

@@ -38,7 +38,9 @@ static constexpr int64_t ORPHAN_TX_EXPIRE_TIME = 20 * 60;
 /** Minimum time between orphan transactions expire time checks in seconds */
 static constexpr int64_t ORPHAN_TX_EXPIRE_INTERVAL = 5 * 60;
 /** How long to cache transactions in mapRelay for normal relay */
-static constexpr std::chrono::seconds RELAY_TX_CACHE_TIME{15 * 60};
+static constexpr std::chrono::seconds RELAY_TX_CACHE_TIME = std::chrono::minutes{15};
+/** How long a transaction has to be in the mempool before it can unconditionally be relayed (even when not in mapRelay). */
+static constexpr std::chrono::seconds UNCONDITIONAL_RELAY_DELAY = std::chrono::minutes{2};
 /** Headers download timeout expressed in microseconds
  *  Timeout = base + per_header * (expected number of headers) */
 static constexpr int64_t HEADERS_DOWNLOAD_TIMEOUT_BASE = 15 * 60 * 1000000; // 15 minutes
@@ -1617,7 +1619,7 @@ void static ProcessGetBlockData(CNode& pfrom, const CChainParams& chainparams, c
 }
 
 //! Determine whether or not a peer can request a transaction, and return it (or nullptr if not found or not allowed).
-CTransactionRef static FindTxForGetData(CNode& peer, const uint256& txid, const std::chrono::seconds mempool_req, const std::chrono::seconds longlived_mempool_time) LOCKS_EXCLUDED(cs_main)
+CTransactionRef static FindTxForGetData(const CNode& peer, const uint256& txid, const std::chrono::seconds mempool_req, const std::chrono::seconds now) LOCKS_EXCLUDED(cs_main)
 {
     // Check if the requested transaction is so recent that we're just
     // about to announce it to the peer; if so, they certainly shouldn't
@@ -1631,8 +1633,8 @@ CTransactionRef static FindTxForGetData(CNode& peer, const uint256& txid, const
     if (txinfo.tx) {
         // To protect privacy, do not answer getdata using the mempool when
         // that TX couldn't have been INVed in reply to a MEMPOOL request,
-        // or when it's too recent to have expired from mapRelay.
-        if ((mempool_req.count() && txinfo.m_time <= mempool_req) || txinfo.m_time <= longlived_mempool_time) {
+        // and it's more recent than UNCONDITIONAL_RELAY_DELAY.
+        if ((mempool_req.count() && txinfo.m_time <= mempool_req) || txinfo.m_time <= now - UNCONDITIONAL_RELAY_DELAY) {
             return txinfo.tx;
         }
     }
@@ -1655,8 +1657,7 @@ void static ProcessGetData(CNode& pfrom, const CChainParams& chainparams, CConnm
     std::vector<CInv> vNotFound;
     const CNetMsgMaker msgMaker(pfrom.GetSendVersion());
 
-    // mempool entries added before this time have likely expired from mapRelay
-    const std::chrono::seconds longlived_mempool_time = GetTime<std::chrono::seconds>() - RELAY_TX_CACHE_TIME;
+    const std::chrono::seconds now = GetTime<std::chrono::seconds>();
     // Get last mempool request time
     const std::chrono::seconds mempool_req = pfrom.m_tx_relay != nullptr ? pfrom.m_tx_relay->m_last_mempool_req.load()
                                                                           : std::chrono::seconds::min();
@@ -1677,7 +1678,7 @@ void static ProcessGetData(CNode& pfrom, const CChainParams& chainparams, CConnm
             continue;
         }
 
-        CTransactionRef tx = FindTxForGetData(pfrom, inv.hash, mempool_req, longlived_mempool_time);
+        CTransactionRef tx = FindTxForGetData(pfrom, inv.hash, mempool_req, now);
         if (tx) {
             int nSendFlags = (inv.type == MSG_TX ? SERIALIZE_TRANSACTION_NO_WITNESS : 0);
             connman->PushMessage(&pfrom, msgMaker.Make(nSendFlags, NetMsgType::TX, *tx));