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 255 bytes of payload (default: %u)", 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);
@@ -981,6 +982,7 @@ bool AppInitParameterInteraction(const ArgsManager& args, bool use_syscall_sandb
 
     if (!g_wallet_init_interface.ParameterInteraction()) return false;
 
+    accept_annex_data = args.GetBoolArg("-annexcarrier", DEFAULT_ACCEPT_ANNEXDATA);
     // Option to startup with mocktime set (used for regression testing):
     SetMockTime(args.GetIntArg("-mocktime", 0)); // SetMockTime(0) is a no-op
 

src/policy/policy.cpp

@@ -215,6 +215,7 @@ bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 
 bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 {
+    bool annex_data_already_found = false;
     if (tx.IsCoinBase())
         return true; // Coinbases are skipped
 
@@ -271,8 +272,14 @@ bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
             // 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 std::vector<unsigned char> &annex = stack.back();
+                // For now allow 0 to 126 bytes push to allow drop-in replacement via BIP PR#1381
+                // But only one annex per transaction to avoid witness stuffing.
+                if (!accept_annex_data || annex_data_already_found ||
+                    annex.size() < 2 || annex.size() > 1 + 1 + 126 || (size_t)annex[1] != annex.size() - 2) {
+                    return false;
+                }
+                annex_data_already_found = true;
             }
             if (stack.size() >= 2) {
                 // Script path spend (2 or more stack elements after removing optional annex)

src/script/standard.cpp

@@ -16,6 +16,7 @@
 
 typedef std::vector<unsigned char> valtype;
 
+bool accept_annex_data = DEFAULT_ACCEPT_ANNEXDATA;
 CScriptID::CScriptID(const CScript& in) : BaseHash(Hash160(in)) {}
 CScriptID::CScriptID(const ScriptHash& in) : BaseHash(static_cast<uint160>(in)) {}
 

src/script/standard.h

@@ -17,6 +17,7 @@
 #include <variant>
 
 static const bool DEFAULT_ACCEPT_DATACARRIER = true;
+static const bool DEFAULT_ACCEPT_ANNEXDATA = false;
 
 class CKeyID;
 class CScript;
@@ -38,6 +39,12 @@ 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;
+
 /**
  * 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,

test/functional/mempool_datacarrier.py

@@ -24,7 +24,7 @@ class DataCarrierTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 3
         self.extra_args = [
-            [],
+            ["-annexcarrier=1"],
             ["-datacarrier=0"],
             ["-datacarrier=1", f"-datacarriersize={MAX_OP_RETURN_RELAY - 1}"]
         ]
@@ -42,6 +42,20 @@ def test_null_data_transaction(self, node: TestNode, data: bytes, success: bool)
         else:
             assert_raises_rpc_error(-26, "scriptpubkey", self.wallet.sendrawtransaction, from_node=node, tx_hex=tx_hex)
 
+    def test_annex_data_transaction(self, node: TestNode, data: bytes, success: bool) -> None:
+        tx = self.wallet.create_self_transfer(fee_rate=0)["tx"]
+        tx.wit.vtxinwit[0].scriptWitness.stack = tx.wit.vtxinwit[0].scriptWitness.stack + [bytes.fromhex("50") + len(data).to_bytes(1, 'little') + bytes.fromhex("ff"*len(data))]
+        tx.vout[0].nValue -= tx.get_vsize()  # simply pay 1sat/vbyte fee
+
+        tx_hex = tx.serialize().hex()
+
+        if success:
+            self.wallet.sendrawtransaction(from_node=node, tx_hex=tx_hex)
+            assert tx.rehash() in node.getrawmempool(True), f'{tx_hex} not in mempool'
+        else:
+            assert_raises_rpc_error(-26, "bad-witness-nonstandard", self.wallet.sendrawtransaction, from_node=node, tx_hex=tx_hex)
+
+
     def run_test(self):
         self.wallet = MiniWallet(self.nodes[0])
         self.wallet.rescan_utxos()
@@ -51,6 +65,20 @@ def run_test(self):
         too_long_data = random_bytes(MAX_OP_RETURN_RELAY - 2)
         small_data = random_bytes(MAX_OP_RETURN_RELAY - 4)
 
+        small_annex = random_bytes(0)
+        large_annex = random_bytes(126)
+        oversize_annex = random_bytes(127)
+
+        self.log.info("Testing annex data transaction with true -annexcarrier value.")
+        self.test_annex_data_transaction(node=self.nodes[0], data=small_annex, success=True)
+        self.test_annex_data_transaction(node=self.nodes[0], data=large_annex, success=True)
+        self.test_annex_data_transaction(node=self.nodes[0], data=oversize_annex, success=False)
+
+        self.log.info("Testing annex data transaction with default -annexcarrier value.")
+        self.test_annex_data_transaction(node=self.nodes[1], data=small_annex, success=False)
+        self.test_annex_data_transaction(node=self.nodes[1], data=large_annex, success=False)
+        self.test_annex_data_transaction(node=self.nodes[1], data=oversize_annex, success=False)
+
         self.log.info("Testing null data transaction with default -datacarrier and -datacarriersize values.")
         self.test_null_data_transaction(node=self.nodes[0], data=default_size_data, success=True)