Add support for tx-relay via wtxid

This adds a field to CNodeState that tracks whether to relay transactions with
that peer via wtxid, instead of txid. As of this commit the field will always
be false, but in a later commit we will add a way to negotiate turning this on
via p2p messages exchanged with the peer.

6 files changed, 98 insertions, 39 deletions

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index f8b0f98e23..106f061d03 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -362,6 +362,9 @@ struct CNodeState {
     //! Whether this peer is a manual connection
     bool m_is_manual_connection;
 
+    //! Whether this peer relays txs via wtxid
+    bool m_wtxid_relay{false};
+
     CNodeState(CAddress addrIn, std::string addrNameIn, bool is_inbound, bool is_manual) :
         address(addrIn), name(std::move(addrNameIn)), m_is_inbound(is_inbound),
         m_is_manual_connection (is_manual)
@@ -1332,6 +1335,7 @@ bool static AlreadyHave(const CInv& inv, const CTxMemPool& mempool) EXCLUSIVE_LO
     {
     case MSG_TX:
     case MSG_WITNESS_TX:
+    case MSG_WTX:
         {
             assert(recentRejects);
             if (::ChainActive().Tip()->GetBlockHash() != hashRecentRejectsChainTip)
@@ -1346,7 +1350,11 @@ bool static AlreadyHave(const CInv& inv, const CTxMemPool& mempool) EXCLUSIVE_LO
 
             {
                 LOCK(g_cs_orphans);
-                if (mapOrphanTransactions.count(inv.hash)) return true;
+                if (inv.type != MSG_WTX && mapOrphanTransactions.count(inv.hash)) {
+                    return true;
+                } else if (inv.type == MSG_WTX && g_orphans_by_wtxid.count(inv.hash)) {
+                    return true;
+                }
             }
 
             {
@@ -1354,8 +1362,8 @@ bool static AlreadyHave(const CInv& inv, const CTxMemPool& mempool) EXCLUSIVE_LO
                 if (g_recent_confirmed_transactions->contains(inv.hash)) return true;
             }
 
-            return recentRejects->contains(inv.hash) ||
-                   mempool.exists(inv.hash);
+            const bool by_wtxid = (inv.type == MSG_WTX);
+            return recentRejects->contains(inv.hash) || mempool.exists(inv.hash, by_wtxid);
         }
     case MSG_BLOCK:
     case MSG_WITNESS_BLOCK:
@@ -1365,12 +1373,17 @@ bool static AlreadyHave(const CInv& inv, const CTxMemPool& mempool) EXCLUSIVE_LO
     return true;
 }
 
-void RelayTransaction(const uint256& txid, const CConnman& connman)
+void RelayTransaction(const uint256& txid, const uint256& wtxid, const CConnman& connman)
 {
-    CInv inv(MSG_TX, txid);
-    connman.ForEachNode([&inv](CNode* pnode)
+    connman.ForEachNode([&txid, &wtxid](CNode* pnode)
     {
-        pnode->PushInventory(inv);
+        AssertLockHeld(cs_main);
+        CNodeState &state = *State(pnode->GetId());
+        if (state.m_wtxid_relay) {
+            pnode->PushInventory({MSG_TX, wtxid});  // inv type is MSG_TX even for wtxid relay
+        } else {
+            pnode->PushInventory({MSG_TX, txid});
+        }
     });
 }
 
@@ -1585,7 +1598,7 @@ void static ProcessGetData(CNode* pfrom, const CChainParams& chainparams, CConnm
         // Process as many TX items from the front of the getdata queue as
         // possible, since they're common and it's efficient to batch process
         // them.
-        while (it != pfrom->vRecvGetData.end() && (it->type == MSG_TX || it->type == MSG_WITNESS_TX)) {
+        while (it != pfrom->vRecvGetData.end() && (it->type == MSG_TX || it->type == MSG_WITNESS_TX || it->type == MSG_WTX)) {
             if (interruptMsgProc)
                 return;
             // The send buffer provides backpressure. If there's no space in
@@ -1608,7 +1621,7 @@ void static ProcessGetData(CNode* pfrom, const CChainParams& chainparams, CConnm
                 connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::TX, *mi->second));
                 push = true;
             } else {
-                auto txinfo = mempool.info(inv.hash);
+                auto txinfo = mempool.info(inv.hash, inv.type == MSG_WTX);
                 // 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.
@@ -1888,7 +1901,7 @@ void static ProcessOrphanTx(CConnman* connman, CTxMemPool& mempool, std::set<uin
         if (setMisbehaving.count(fromPeer)) continue;
         if (AcceptToMemoryPool(mempool, orphan_state, porphanTx, &removed_txn, false /* bypass_limits */, 0 /* nAbsurdFee */)) {
             LogPrint(BCLog::MEMPOOL, "   accepted orphan tx %s\n", orphanHash.ToString());
-            RelayTransaction(orphanHash, *connman);
+            RelayTransaction(orphanHash, porphanTx->GetWitnessHash(), *connman);
             for (unsigned int i = 0; i < orphanTx.vout.size(); i++) {
                 auto it_by_prev = mapOrphanTransactionsByPrev.find(COutPoint(orphanHash, i));
                 if (it_by_prev != mapOrphanTransactionsByPrev.end()) {
@@ -2559,23 +2572,47 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
         const CTransaction& tx = *ptx;
 
         const uint256& txid = ptx->GetHash();
-        pfrom->AddInventoryKnown(txid);
+        const uint256& wtxid = ptx->GetWitnessHash();
 
         LOCK2(cs_main, g_cs_orphans);
 
+        CNodeState* nodestate = State(pfrom->GetId());
+
+        const uint256& hash = nodestate->m_wtxid_relay ? wtxid : txid;
+        pfrom->AddInventoryKnown(hash);
+        if (nodestate->m_wtxid_relay && txid != wtxid) {
+            // Insert txid into filterInventoryKnown, even for
+            // wtxidrelay peers. This prevents re-adding of
+            // unconfirmed parents to the recently_announced
+            // filter, when a child tx is requested. See
+            // ProcessGetData().
+            pfrom->AddInventoryKnown(txid);
+        }
+
         TxValidationState state;
 
-        CNodeState* nodestate = State(pfrom->GetId());
-        nodestate->m_tx_download.m_tx_announced.erase(txid);
-        nodestate->m_tx_download.m_tx_in_flight.erase(txid);
-        EraseTxRequest(txid);
+        nodestate->m_tx_download.m_tx_announced.erase(hash);
+        nodestate->m_tx_download.m_tx_in_flight.erase(hash);
+        EraseTxRequest(hash);
 
         std::list<CTransactionRef> lRemovedTxn;
 
-        if (!AlreadyHave(CInv(MSG_TX, txid), mempool) &&
+        // We do the AlreadyHave() check using wtxid, rather than txid - in the
+        // absence of witness malleation, this is strictly better, because the
+        // recent rejects filter may contain the wtxid but will never contain
+        // the txid of a segwit transaction that has been rejected.
+        // In the presence of witness malleation, it's possible that by only
+        // doing the check with wtxid, we could overlook a transaction which
+        // was confirmed with a different witness, or exists in our mempool
+        // with a different witness, but this has limited downside:
+        // mempool validation does its own lookup of whether we have the txid
+        // already; and an adversary can already relay us old transactions
+        // (older than our recency filter) if trying to DoS us, without any need
+        // for witness malleation.
+        if (!AlreadyHave(CInv(MSG_WTX, wtxid), mempool) &&
             AcceptToMemoryPool(mempool, state, ptx, &lRemovedTxn, false /* bypass_limits */, 0 /* nAbsurdFee */)) {
             mempool.check(&::ChainstateActive().CoinsTip());
-            RelayTransaction(tx.GetHash(), *connman);
+            RelayTransaction(tx.GetHash(), tx.GetWitnessHash(), *connman);
             for (unsigned int i = 0; i < tx.vout.size(); i++) {
                 auto it_by_prev = mapOrphanTransactionsByPrev.find(COutPoint(txid, i));
                 if (it_by_prev != mapOrphanTransactionsByPrev.end()) {
@@ -2608,10 +2645,17 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
                 uint32_t nFetchFlags = GetFetchFlags(pfrom);
                 const auto current_time = GetTime<std::chrono::microseconds>();
 
-                for (const CTxIn& txin : tx.vin) {
-                    CInv _inv(MSG_TX | nFetchFlags, txin.prevout.hash);
-                    pfrom->AddInventoryKnown(txin.prevout.hash);
-                    if (!AlreadyHave(_inv, mempool)) RequestTx(State(pfrom->GetId()), _inv.hash, current_time);
+                if (!State(pfrom->GetId())->m_wtxid_relay) {
+                    for (const CTxIn& txin : tx.vin) {
+                        // Here, we only have the txid (and not wtxid) of the
+                        // inputs, so we only request parents from
+                        // non-wtxid-relay peers.
+                        // Eventually we should replace this with an improved
+                        // protocol for getting all unconfirmed parents.
+                        CInv _inv(MSG_TX | nFetchFlags, txin.prevout.hash);
+                        pfrom->AddInventoryKnown(txin.prevout.hash);
+                        if (!AlreadyHave(_inv, mempool)) RequestTx(State(pfrom->GetId()), _inv.hash, current_time);
+                    }
                 }
                 AddOrphanTx(ptx, pfrom->GetId());
 
@@ -2672,7 +2716,7 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
                     LogPrintf("Not relaying non-mempool transaction %s from whitelisted peer=%d\n", tx.GetHash().ToString(), pfrom->GetId());
                 } else {
                     LogPrintf("Force relaying tx %s from whitelisted peer=%d\n", tx.GetHash().ToString(), pfrom->GetId());
-                    RelayTransaction(tx.GetHash(), *connman);
+                    RelayTransaction(tx.GetHash(), tx.GetWitnessHash(), *connman);
                 }
             }
         }
@@ -3288,7 +3332,7 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
         vRecv >> vInv;
         if (vInv.size() <= MAX_PEER_TX_IN_FLIGHT + MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
             for (CInv &inv : vInv) {
-                if (inv.type == MSG_TX || inv.type == MSG_WITNESS_TX) {
+                if (inv.type == MSG_TX || inv.type == MSG_WITNESS_TX || inv.type == MSG_WTX) {
                     // If we receive a NOTFOUND message for a txid we requested, erase
                     // it from our data structures for this peer.
                     auto in_flight_it = state->m_tx_download.m_tx_in_flight.find(inv.hash);
@@ -3580,17 +3624,19 @@ namespace {
 class CompareInvMempoolOrder
 {
     CTxMemPool *mp;
+    bool m_wtxid_relay;
 public:
-    explicit CompareInvMempoolOrder(CTxMemPool *_mempool)
+    explicit CompareInvMempoolOrder(CTxMemPool *_mempool, bool use_wtxid)
     {
         mp = _mempool;
+        m_wtxid_relay = use_wtxid;
     }
 
     bool operator()(std::set<uint256>::iterator a, std::set<uint256>::iterator b)
     {
         /* As std::make_heap produces a max-heap, we want the entries with the
          * fewest ancestors/highest fee to sort later. */
-        return mp->CompareDepthAndScore(*b, *a);
+        return mp->CompareDepthAndScore(*b, *a, m_wtxid_relay);
     }
 };
 }
@@ -3897,8 +3943,8 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                     LOCK(pto->m_tx_relay->cs_filter);
 
                     for (const auto& txinfo : vtxinfo) {
-                        const uint256& hash = txinfo.tx->GetHash();
-                        CInv inv(MSG_TX, hash);
+                        const uint256& hash = state.m_wtxid_relay ? txinfo.tx->GetWitnessHash() : txinfo.tx->GetHash();
+                        CInv inv(state.m_wtxid_relay ? MSG_WTX : MSG_TX, hash);
                         pto->m_tx_relay->setInventoryTxToSend.erase(hash);
                         // Don't send transactions that peers will not put into their mempool
                         if (txinfo.fee < filterrate.GetFee(txinfo.vsize)) {
@@ -3932,7 +3978,7 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                     }
                     // Topologically and fee-rate sort the inventory we send for privacy and priority reasons.
                     // A heap is used so that not all items need sorting if only a few are being sent.
-                    CompareInvMempoolOrder compareInvMempoolOrder(&m_mempool);
+                    CompareInvMempoolOrder compareInvMempoolOrder(&m_mempool, state.m_wtxid_relay);
                     std::make_heap(vInvTx.begin(), vInvTx.end(), compareInvMempoolOrder);
                     // No reason to drain out at many times the network's capacity,
                     // especially since we have many peers and some will draw much shorter delays.
@@ -3951,17 +3997,19 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                             continue;
                         }
                         // Not in the mempool anymore? don't bother sending it.
-                        auto txinfo = m_mempool.info(hash);
+                        auto txinfo = m_mempool.info(hash, state.m_wtxid_relay);
                         if (!txinfo.tx) {
                             continue;
                         }
+                        auto txid = txinfo.tx->GetHash();
+                        auto wtxid = txinfo.tx->GetWitnessHash();
                         // Peer told you to not send transactions at that feerate? Don't bother sending it.
                         if (txinfo.fee < filterrate.GetFee(txinfo.vsize)) {
                             continue;
                         }
                         if (pto->m_tx_relay->pfilter && !pto->m_tx_relay->pfilter->IsRelevantAndUpdate(*txinfo.tx)) continue;
                         // Send
-                        vInv.push_back(CInv(MSG_TX, hash));
+                        vInv.push_back(CInv(state.m_wtxid_relay ? MSG_WTX : MSG_TX, hash));
                         nRelayedTransactions++;
                         {
                             // Expire old relay messages
@@ -3971,12 +4019,12 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                                 vRelayExpiration.pop_front();
                             }
 
-                            auto ret = mapRelay.insert(std::make_pair(hash, std::move(txinfo.tx)));
+                            auto ret = mapRelay.emplace(txid, std::move(txinfo.tx));
                             if (ret.second) {
-                                vRelayExpiration.push_back(std::make_pair(nNow + std::chrono::microseconds{RELAY_TX_CACHE_TIME}.count(), ret.first));
+                                vRelayExpiration.emplace_back(nNow + std::chrono::microseconds{RELAY_TX_CACHE_TIME}.count(), ret.first);
                             }
                             // Add wtxid-based lookup into mapRelay as well, so that peers can request by wtxid
-                            auto ret2 = mapRelay.emplace(ret.first->second->GetWitnessHash(), ret.first->second);
+                            auto ret2 = mapRelay.emplace(wtxid, ret.first->second);
                             if (ret2.second) {
                                 vRelayExpiration.emplace_back(nNow + std::chrono::microseconds{RELAY_TX_CACHE_TIME}.count(), ret2.first);
                             }
@@ -3986,6 +4034,14 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                             vInv.clear();
                         }
                         pto->m_tx_relay->filterInventoryKnown.insert(hash);
+                        if (hash != txid) {
+                            // Insert txid into filterInventoryKnown, even for
+                            // wtxidrelay peers. This prevents re-adding of
+                            // unconfirmed parents to the recently_announced
+                            // filter, when a child tx is requested. See
+                            // ProcessGetData().
+                            pto->m_tx_relay->filterInventoryKnown.insert(txid);
+                        }
                     }
                 }
             }
@@ -4110,7 +4166,7 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
             // Erase this entry from tx_process_time (it may be added back for
             // processing at a later time, see below)
             tx_process_time.erase(tx_process_time.begin());
-            CInv inv(MSG_TX | GetFetchFlags(pto), txid);
+            CInv inv(state.m_wtxid_relay ? MSG_WTX : (MSG_TX | GetFetchFlags(pto)), txid);
             if (!AlreadyHave(inv, m_mempool)) {
                 // If this transaction was last requested more than 1 minute ago,
                 // then request.
diff --git a/src/net_processing.h b/src/net_processing.h
index ddb1780148..79a3ccd4ed 100644
--- a/src/net_processing.h
+++ b/src/net_processing.h
@@ -92,6 +92,6 @@ struct CNodeStateStats {
 bool GetNodeStateStats(NodeId nodeid, CNodeStateStats &stats);
 
 /** Relay transaction to every node */
-void RelayTransaction(const uint256&, const CConnman& connman);
+void RelayTransaction(const uint256& txid, const uint256& wtxid, const CConnman& connman) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
 #endif // BITCOIN_NET_PROCESSING_H
diff --git a/src/node/transaction.cpp b/src/node/transaction.cpp
index 201406ce3b..17878eeb17 100644
--- a/src/node/transaction.cpp
+++ b/src/node/transaction.cpp
@@ -78,7 +78,8 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
     }
 
     if (relay) {
-        RelayTransaction(hashTx, *node.connman);
+        LOCK(cs_main);
+        RelayTransaction(hashTx, tx->GetWitnessHash(), *node.connman);
     }
 
     return TransactionError::OK;
diff --git a/src/protocol.cpp b/src/protocol.cpp
index bd3ed25a8a..f5cf9e6de8 100644
--- a/src/protocol.cpp
+++ b/src/protocol.cpp
@@ -183,6 +183,8 @@ std::string CInv::GetCommand() const
     switch (masked)
     {
     case MSG_TX:             return cmd.append(NetMsgType::TX);
+    // WTX is not a message type, just an inv type
+    case MSG_WTX:            return cmd.append("wtx");
     case MSG_BLOCK:          return cmd.append(NetMsgType::BLOCK);
     case MSG_FILTERED_BLOCK: return cmd.append(NetMsgType::MERKLEBLOCK);
     case MSG_CMPCT_BLOCK:    return cmd.append(NetMsgType::CMPCTBLOCK);
diff --git a/src/protocol.h b/src/protocol.h
index 6639ae2aac..1d14585f97 100644
--- a/src/protocol.h
+++ b/src/protocol.h
@@ -362,11 +362,11 @@ const uint32_t MSG_TYPE_MASK    = 0xffffffff >> 2;
  * These numbers are defined by the protocol. When adding a new value, be sure
  * to mention it in the respective BIP.
  */
-enum GetDataMsg
-{
+enum GetDataMsg : uint32_t {
     UNDEFINED = 0,
     MSG_TX = 1,
     MSG_BLOCK = 2,
+    MSG_WTX = 5,                                      //!< Defined in BIP 339
     // The following can only occur in getdata. Invs always use TX or BLOCK.
     MSG_FILTERED_BLOCK = 3,  //!< Defined in BIP37
     MSG_CMPCT_BLOCK = 4,     //!< Defined in BIP152
diff --git a/test/functional/p2p_segwit.py b/test/functional/p2p_segwit.py
index a7cfefc485..f7255d1911 100755
--- a/test/functional/p2p_segwit.py
+++ b/test/functional/p2p_segwit.py
@@ -1296,7 +1296,7 @@ class SegWitTest(BitcoinTestFramework):
         self.std_node.announce_tx_and_wait_for_getdata(tx3)
         test_transaction_acceptance(self.nodes[1], self.std_node, tx3, True, False, 'tx-size')
         self.std_node.announce_tx_and_wait_for_getdata(tx3)
-        test_transaction_acceptance(self.nodes[1], self.std_node, tx3, True, False, 'tx-size')
+        test_transaction_acceptance(self.nodes[1], self.std_node, tx3, True, False)
 
         # Remove witness stuffing, instead add extra witness push on stack
         tx3.vout[0] = CTxOut(tx2.vout[0].nValue - 1000, CScript([OP_TRUE, OP_DROP] * 15 + [OP_TRUE]))