Add annex data carrier option behind -annexcarrier option

Allow 0 to 126 byte pushes in annex of the format:

0x50 <1 byte data len> <len bytes>

Author: instagibbs

src/init.cpp

@@ -567,6 +567,7 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-mempoolfullrbf", strprintf("Accept transaction replace-by-fee without requiring replaceability signaling (default: %u)", DEFAULT_MEMPOOL_FULL_RBF), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-permitbaremultisig", strprintf("Relay non-P2SH multisig (default: %u)", DEFAULT_PERMIT_BAREMULTISIG), ArgsManager::ALLOW_ANY,
                    OptionsCategory::NODE_RELAY);
+    argsman.AddArg("-annexcarrier", strprintf("Relay and mine transactions with annex data, up to %u bytes of payload (default: %u)", MAX_ANNEX_DATA, DEFAULT_ACCEPT_ANNEXDATA), ArgsManager::ALLOW_ANY, 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("-whitelistforcerelay", strprintf("Add 'forcerelay' permission to whitelisted inbound 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);

src/kernel/mempool_options.h

@@ -50,6 +50,7 @@ struct MemPoolOptions {
      * If nullopt, any size is nonstandard.
      */
     std::optional<unsigned> max_datacarrier_bytes{DEFAULT_ACCEPT_DATACARRIER ? std::optional{MAX_OP_RETURN_RELAY} : std::nullopt};
+    bool annex_datacarrier{DEFAULT_ACCEPT_ANNEXDATA};
     bool permit_bare_multisig{DEFAULT_PERMIT_BAREMULTISIG};
     bool require_standard{true};
     bool full_rbf{DEFAULT_MEMPOOL_FULL_RBF};

src/node/mempool_args.cpp

@@ -87,6 +87,8 @@ std::optional<bilingual_str> ApplyArgsManOptions(const ArgsManager& argsman, con
         mempool_opts.max_datacarrier_bytes = std::nullopt;
     }
 
+    mempool_opts.annex_datacarrier = argsman.GetBoolArg("-annexcarrier", DEFAULT_ACCEPT_ANNEXDATA);
+
     mempool_opts.require_standard = !argsman.GetBoolArg("-acceptnonstdtxn", !chainparams.RequireStandard());
     if (!chainparams.IsTestChain() && !mempool_opts.require_standard) {
         return strprintf(Untranslated("acceptnonstdtxn is not currently supported for %s chain"), chainparams.NetworkIDString());

src/policy/policy.cpp

@@ -213,8 +213,15 @@ bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
     return true;
 }
 
-bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
+bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs, bool allow_annex_data)
 {
+    // For now allow 0 to MAX_ANNEX_DATA bytes push to allow drop-in replacement via BIP PR#1381
+    // but we allow it spread over any number of inputs based on this budget:
+    // 1(witness stack element size) + 1(annex marker) + 1(payload len) + MAX_ANNEX_DATA(payload)
+    size_t annex_bytes_left{1 + 1 + 1 + MAX_ANNEX_DATA};
+
+    if (!allow_annex_data) annex_bytes_left = 0;
+
     if (tx.IsCoinBase())
         return true; // Coinbases are skipped
 
@@ -266,13 +273,18 @@ bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 
         // Check policy limits for Taproot spends:
         // - MAX_STANDARD_TAPSCRIPT_STACK_ITEM_SIZE limit for stack item size
-        // - No annexes
+        // - Limited annex format
         if (witnessversion == 1 && witnessprogram.size() == WITNESS_V1_TAPROOT_SIZE && !p2sh) {
             // Taproot spend (non-P2SH-wrapped, version 1, witness program size 32; see BIP 341)
             Span stack{tx.vin[i].scriptWitness.stack};
             if (stack.size() >= 2 && !stack.back().empty() && stack.back()[0] == ANNEX_TAG) {
-                // Annexes are nonstandard as long as no semantics are defined for them.
-                return false;
+                const auto& annex = stack.back();
+
+                if (annex.size() >= annex_bytes_left) return false;
+                annex_bytes_left -= annex.size() + 1; // bip141 witness stack element size included
+
+                // limit annex format to allow for future expansion
+                if (annex.size() < 2 || annex.size() != static_cast<size_t>(annex[1]) + 2) return false;
             }
             if (stack.size() >= 2) {
                 // Script path spend (2 or more stack elements after removing optional annex)

src/policy/policy.h

@@ -132,9 +132,9 @@ bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 * 3600bytes witnessScript size, 80bytes per witness stack element, 100 witness stack elements
 * These limits are adequate for multisignatures up to n-of-100 using OP_CHECKSIG, OP_ADD, and OP_EQUAL.
 *
-* Also enforce a maximum stack item size limit and no annexes for tapscript spends.
+* Also enforce a maximum stack item size limit and limited annexes for tapscript spends.
 */
-bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs);
+bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs, bool allow_annex_data);
 
 /** Compute the virtual transaction size (weight reinterpreted as bytes). */
 int64_t GetVirtualTransactionSize(int64_t nWeight, int64_t nSigOpCost, unsigned int bytes_per_sigop);

src/script/standard.h

@@ -17,6 +17,7 @@
 #include <variant>
 
 static const bool DEFAULT_ACCEPT_DATACARRIER = true;
+static const bool DEFAULT_ACCEPT_ANNEXDATA = true;
 
 class CKeyID;
 class CScript;
@@ -38,6 +39,17 @@ class CScriptID : public BaseHash<uint160>
  */
 static const unsigned int MAX_OP_RETURN_RELAY = 83;
 
+/**
+ * Allows up to one annex data to be embedded in a transaction, counting towards the
+ * same data carrier limit as OP_RETURN values
+ */
+extern bool accept_annex_data;
+
+/**
+ * Static maximum of annex data per transaction
+ */
+static const unsigned int MAX_ANNEX_DATA = 126;
+
 /**
  * Mandatory script verification flags that all new blocks must comply with for
  * them to be valid. (but old blocks may not comply with) Currently just P2SH,

src/test/fuzz/coins_view.cpp

@@ -265,7 +265,7 @@ FUZZ_TARGET_INIT(coins_view, initialize_coins_view)
                 (void)GetTransactionSigOpCost(transaction, coins_view_cache, flags);
             },
             [&] {
-                (void)IsWitnessStandard(CTransaction{random_mutable_transaction}, coins_view_cache);
+                (void)IsWitnessStandard(CTransaction{random_mutable_transaction}, coins_view_cache, /* allow_annex_data= */ true);
             });
     }
 }

src/test/fuzz/transaction.cpp

@@ -98,7 +98,7 @@ FUZZ_TARGET_INIT(transaction, initialize_transaction)
     CCoinsView coins_view;
     const CCoinsViewCache coins_view_cache(&coins_view);
     (void)AreInputsStandard(tx, coins_view_cache);
-    (void)IsWitnessStandard(tx, coins_view_cache);
+    (void)IsWitnessStandard(tx, coins_view_cache, /* allow_annex_data= */true);
 
     UniValue u(UniValue::VOBJ);
     TxToUniv(tx, /*block_hash=*/uint256::ZERO, /*entry=*/u);

src/txmempool.cpp

@@ -433,6 +433,7 @@ CTxMemPool::CTxMemPool(const Options& opts)
       m_dust_relay_feerate{opts.dust_relay_feerate},
       m_permit_bare_multisig{opts.permit_bare_multisig},
       m_max_datacarrier_bytes{opts.max_datacarrier_bytes},
+      m_annex_datacarrier{opts.annex_datacarrier},
       m_require_standard{opts.require_standard},
       m_full_rbf{opts.full_rbf},
       m_limits{opts.limits}

src/txmempool.h

@@ -573,6 +573,7 @@ class CTxMemPool
     const CFeeRate m_dust_relay_feerate;
     const bool m_permit_bare_multisig;
     const std::optional<unsigned> m_max_datacarrier_bytes;
+    const bool m_annex_datacarrier;
     const bool m_require_standard;
     const bool m_full_rbf;