If -spkreuse=0, ensure transactions in mempool always have unique scriptPubKeys

Exceptions:
- Multiple inputs in the same transaction are allowed to spend against the same scriptPubKey
- The same scriptPubKey may be used in the mempool as both first an output, and then spent in a later transaction's input

src/init.cpp

@@ -589,6 +589,7 @@ void SetupServerArgs(ArgsManager& argsman)
                    OptionsCategory::NODE_RELAY);
     argsman.AddArg("-minrelaytxfee=<amt>", strprintf("Fees (in %s/kvB) smaller than this are considered zero fee for relaying, mining and transaction creation (default: %s)",
         CURRENCY_UNIT, FormatMoney(DEFAULT_MIN_RELAY_TX_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
+    argsman.AddArg("-spkreuse", strprintf("Accept transactions reusing addresses or other pubkey scripts (default: %s)", DEFAULT_SPKREUSE), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-whitelistforcerelay", strprintf("Add 'forcerelay' permission to whitelisted peers with default permissions. This will relay transactions even if the transactions were already in the mempool. (default: %d)", DEFAULT_WHITELISTFORCERELAY), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-whitelistrelay", strprintf("Add 'relay' permission to whitelisted peers with default permissions. This will accept relayed transactions even when not relaying transactions (default: %d)", DEFAULT_WHITELISTRELAY), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
 
@@ -969,6 +970,16 @@ bool AppInitParameterInteraction(const ArgsManager& args, bool use_syscall_sandb
 
     if (!g_wallet_init_interface.ParameterInteraction()) return false;
 
+    {
+        std::string strSpkReuse = gArgs.GetArg("-spkreuse", DEFAULT_SPKREUSE);
+        // Uses string values so future versions can implement other modes
+        if (strSpkReuse == "allow" || gArgs.GetBoolArg("-spkreuse", false)) {
+            SpkReuseMode = SRM_ALLOW;
+        } else {
+            SpkReuseMode = SRM_REJECT;
+        }
+    }
+
     // Option to startup with mocktime set (used for regression testing):
     SetMockTime(args.GetIntArg("-mocktime", 0)); // SetMockTime(0) is a no-op
 

src/kernel/mempool_entry.h

@@ -35,6 +35,15 @@ struct LockPoints {
     CBlockIndex* maxInputBlock{nullptr};
 };
 
+enum MemPool_SPK_State {
+    MSS_UNSEEN  = 0,
+    MSS_SPENT   = 1,
+    MSS_CREATED = 2,
+    MSS_BOTH    = 3,
+};
+
+typedef std::map<uint160, enum MemPool_SPK_State> SPKStates_t;
+
 struct CompareIteratorByHash {
     // SFINAE for T where T is either a pointer type (e.g., a txiter) or a reference_wrapper<T>
     // (e.g. a wrapped CTxMemPoolEntry&)
@@ -169,6 +178,8 @@ public:
 
     mutable size_t vTxHashesIdx; //!< Index in mempool's vTxHashes
     mutable Epoch::Marker m_epoch_marker; //!< epoch when last touched, useful for graph algorithms
+
+    SPKStates_t mapSPK;
 };
 
 #endif // BITCOIN_KERNEL_MEMPOOL_ENTRY_H

src/policy/policy.h

@@ -54,6 +54,7 @@ static constexpr unsigned int MAX_STANDARD_SCRIPTSIG_SIZE{1650};
  * only increase the dust limit after prior releases were already not creating
  * outputs below the new threshold */
 static constexpr unsigned int DUST_RELAY_TX_FEE{3000};
+static const std::string DEFAULT_SPKREUSE{"allow"};
 /** Default for -minrelaytxfee, minimum relay fee for transactions */
 static constexpr unsigned int DEFAULT_MIN_RELAY_TX_FEE{1000};
 /** Default for -limitancestorcount, max number of in-mempool ancestors */

src/txmempool.cpp

@@ -10,10 +10,12 @@
 #include <consensus/consensus.h>
 #include <consensus/tx_verify.h>
 #include <consensus/validation.h>
+#include <crypto/ripemd160.h>
 #include <policy/fees.h>
 #include <policy/policy.h>
 #include <policy/settings.h>
 #include <reverse_iterator.h>
+#include <script/script.h>
 #include <util/check.h>
 #include <util/moneystr.h>
 #include <util/overflow.h>
@@ -46,6 +48,13 @@ bool TestLockPointValidity(CChain& active_chain, const LockPoints& lp)
     return true;
 }
 
+uint160 ScriptHashkey(const CScript& script)
+{
+    uint160 hash;
+    CRIPEMD160().Write(script.data(), script.size()).Finalize(hash.begin());
+    return hash;
+}
+
 void CTxMemPool::UpdateForDescendants(txiter updateIt, cacheMap& cachedDescendants,
                                       const std::set<uint256>& setExclude, std::set<uint256>& descendants_to_remove)
 {
@@ -482,6 +491,10 @@ void CTxMemPool::addUnchecked(const CTxMemPoolEntry &entry, setEntries &setAnces
     vTxHashes.emplace_back(tx.GetWitnessHash(), newit);
     newit->vTxHashesIdx = vTxHashes.size() - 1;
 
+    for (auto& vSPK : entry.mapSPK) {
+        mapUsedSPK[vSPK.first] = MemPool_SPK_State(mapUsedSPK[vSPK.first] | vSPK.second);
+    }
+
     TRACE3(mempool, added,
         entry.GetTx().GetHash().data(),
         entry.GetTxSize(),
@@ -525,6 +538,14 @@ void CTxMemPool::removeUnchecked(txiter it, MemPoolRemovalReason reason)
     } else
         vTxHashes.clear();
 
+    for (auto& vSPK : it->mapSPK) {
+        if (vSPK.second == mapUsedSPK.find(vSPK.first)->second) {
+            mapUsedSPK.erase(vSPK.first);
+        } else {
+            mapUsedSPK[vSPK.first] = MemPool_SPK_State(mapUsedSPK[vSPK.first] & ~vSPK.second);
+        }
+    }
+
     totalTxSize -= it->GetTxSize();
     m_total_fee -= it->GetFee();
     cachedInnerUsage -= it->DynamicMemoryUsage();

src/txmempool.h

@@ -26,6 +26,7 @@
 #include <policy/feerate.h>
 #include <policy/packages.h>
 #include <primitives/transaction.h>
+#include <script/script.h>
 #include <random.h>
 #include <sync.h>
 #include <util/epochguard.h>
@@ -196,6 +197,8 @@ public:
     }
 };
 
+uint160 ScriptHashkey(const CScript& script);
+
 // Multi_index tag names
 struct descendant_score {};
 struct entry_time {};
@@ -413,6 +416,9 @@ public:
     using Limits = kernel::MemPoolLimits;
 
     uint64_t CalculateDescendantMaximum(txiter entry) const EXCLUSIVE_LOCKS_REQUIRED(cs);
+
+    SPKStates_t mapUsedSPK;
+
 private:
     typedef std::map<txiter, setEntries, CompareIteratorByHash> cacheMap;
 

src/validation.cpp

@@ -109,6 +109,7 @@ static constexpr int PRUNE_LOCK_BUFFER{10};
 GlobalMutex g_best_block_mutex;
 std::condition_variable g_best_block_cv;
 uint256 g_best_block;
+SpkReuseModes SpkReuseMode;
 
 const CBlockIndex* Chainstate::FindForkInGlobalIndex(const CBlockLocator& locator) const
 {
@@ -748,6 +749,9 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         return state.Invalid(TxValidationResult::TX_CONFLICT, "txn-same-nonwitness-data-in-mempool");
     }
 
+    auto spk_reuse_mode = SpkReuseMode;
+    SPKStates_t mapSPK;
+
     // Check for conflicts with in-memory transactions
     for (const CTxIn &txin : tx.vin)
     {
@@ -778,6 +782,19 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         }
     }
 
+    if (spk_reuse_mode != SRM_ALLOW) {
+        for (const CTxOut& txout : tx.vout) {
+            uint160 hashSPK = ScriptHashkey(txout.scriptPubKey);
+            if (m_pool.mapUsedSPK.find(hashSPK) != m_pool.mapUsedSPK.end()) {
+                return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "txn-mempool-spk-reused");
+            }
+            if (mapSPK.find(hashSPK) != mapSPK.end()) {
+                return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "txn-spk-reused-twinoutputs");
+            }
+            mapSPK[hashSPK] = MemPool_SPK_State(mapSPK[hashSPK] | MSS_CREATED);
+        }
+    }
+
     m_view.SetBackend(m_viewmempool);
 
     const CCoinsViewCache& coins_cache = m_active_chainstate.CoinsTip();
@@ -830,6 +847,27 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         return false; // state filled in by CheckTxInputs
     }
 
+    if (spk_reuse_mode != SRM_ALLOW) {
+        for (const CTxIn& txin : tx.vin) {
+            const Coin &coin = m_view.AccessCoin(txin.prevout);
+            uint160 hashSPK = ScriptHashkey(coin.out.scriptPubKey);
+
+            SPKStates_t::iterator mssit = mapSPK.find(hashSPK);
+            if (mssit != mapSPK.end()) {
+                if (mssit->second & MSS_CREATED) {
+                    return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "txn-spk-reused-change");
+                }
+            }
+            mssit = m_pool.mapUsedSPK.find(hashSPK);
+            if (mssit != m_pool.mapUsedSPK.end()) {
+                if (mssit->second & MSS_SPENT) {
+                    return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "txn-mempool-spk-reused-spend");
+                }
+            }
+            mapSPK[hashSPK] = MemPool_SPK_State(mapSPK[hashSPK] | MSS_SPENT);
+        }
+    }
+
     if (m_pool.m_require_standard && !AreInputsStandard(tx, m_view, "bad-txns-input-", reason, ignore_rejects)) {
         return state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, reason);
     }
@@ -859,6 +897,7 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     entry.reset(new CTxMemPoolEntry(ptx, ws.m_base_fees, nAcceptTime, m_active_chainstate.m_chain.Height(),
                                     fSpendsCoinbase, nSigOpsCost, lock_points.value()));
     ws.m_vsize = entry->GetTxSize();
+    entry->mapSPK = mapSPK;
 
     if (nSigOpsCost > MAX_STANDARD_TX_SIGOPS_COST)
         MaybeRejectDbg(TxValidationResult::TX_NOT_STANDARD, "bad-txns-too-many-sigops",

src/validation.h

@@ -93,6 +93,13 @@ extern std::condition_variable g_best_block_cv;
 /** Used to notify getblocktemplate RPC of new tips. */
 extern uint256 g_best_block;
 
+enum SpkReuseModes {
+    SRM_ALLOW,
+    SRM_REJECT,
+};
+
+extern SpkReuseModes SpkReuseMode;
+
 /** Documentation for argument 'checklevel'. */
 extern const std::vector<std::string> CHECKLEVEL_DOC;