Do not use mempool for GETDATA for tx accepted after the last mempool req.

The ability to GETDATA a transaction which has not (yet) been relayed
 is a privacy loss vector.

The use of the mempool for this was added as part of the mempool p2p
 message and is only needed to fetch transactions returned by it.

src/main.cpp

@@ -4503,7 +4503,10 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
                 }
                 if (!pushed && inv.type == MSG_TX) {
                     CTransaction tx;
-                    if (mempool.lookup(inv.hash, tx)) {
+                    int64_t txtime;
+                    // To protect privacy, do not answer getdata using the mempool when
+                    // that TX couldn't have been INVed in reply to a MEMPOOL request.
+                    if (mempool.lookup(inv.hash, tx, txtime) && txtime <= pfrom->timeLastMempoolReq) {
                         pfrom->PushMessage(NetMsgType::TX, tx);
                         pushed = true;
                     }
@@ -5902,6 +5905,7 @@ bool SendMessages(CNode* pto)
                         vInv.clear();
                     }
                 }
+                pto->timeLastMempoolReq = GetTime();
             }
 
             // Determine transactions to relay

src/net.cpp

@@ -2396,6 +2396,7 @@ CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNa
     fRelayTxes = false;
     fSentAddr = false;
     pfilter = new CBloomFilter();
+    timeLastMempoolReq = 0;
     nPingNonceSent = 0;
     nPingUsecStart = 0;
     nPingUsecTime = 0;

src/net.h

@@ -17,6 +17,7 @@
 #include "sync.h"
 #include "uint256.h"
 
+#include <atomic>
 #include <deque>
 #include <stdint.h>
 
@@ -413,6 +414,8 @@ public:
     // Used for BIP35 mempool sending, also protected by cs_inventory
     bool fSendMempool;
 
+    // Last time a "MEMPOOL" request was serviced.
+    std::atomic<int64_t> timeLastMempoolReq;
     // Ping time measurement:
     // The pong reply we're expecting, or 0 if no pong expected.
     uint64_t nPingNonceSent;

src/txmempool.cpp

@@ -789,15 +789,23 @@ void CTxMemPool::queryHashes(vector<uint256>& vtxid)
     std::sort(vtxid.begin(), vtxid.end(), DepthAndScoreComparator(this));
 }
 
-bool CTxMemPool::lookup(uint256 hash, CTransaction& result) const
+
+bool CTxMemPool::lookup(uint256 hash, CTransaction& result, int64_t& time) const
 {
     LOCK(cs);
     indexed_transaction_set::const_iterator i = mapTx.find(hash);
     if (i == mapTx.end()) return false;
     result = i->GetTx();
+    time = i->GetTime();
     return true;
 }
 
+bool CTxMemPool::lookup(uint256 hash, CTransaction& result) const
+{
+    int64_t time;
+    return CTxMemPool::lookup(hash, result, time);
+}
+
 bool CTxMemPool::lookupFeeRate(const uint256& hash, CFeeRate& feeRate) const
 {
     LOCK(cs);

src/txmempool.h

@@ -602,6 +602,7 @@ public:
     }
 
     bool lookup(uint256 hash, CTransaction& result) const;
+    bool lookup(uint256 hash, CTransaction& result, int64_t& time) const;
     bool lookupFeeRate(const uint256& hash, CFeeRate& feeRate) const;
 
     /** Estimate fee rate needed to get into the next nBlocks