Add RPC to get mempool txs spending outputs

We add an RPC to fetch the mempool transactions spending given outpoints.
Without this RPC, application developers would need to first call
`getrawmempool` which returns a long list of `txid`, then fetch each of
these txs individually to check whether they spend the given outpoint(s).

This RPC can later be enriched to also find confirmed transactions instead
of being restricted to mempool transactions.

doc/release-notes/release-notes-24408.md

@@ -0,0 +1,5 @@
+New RPCs
+--------
+
+- A new `gettxspendingprevout` RPC has been added, which scans the mempool to find
+  transactions spending any of the given outpoints. (#24408)

src/rpc/client.cpp

@@ -173,6 +173,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "setwalletflag", 1, "value" },
     { "getmempoolancestors", 1, "verbose" },
     { "getmempooldescendants", 1, "verbose" },
+    { "gettxspendingprevout", 0, "outputs" },
     { "bumpfee", 1, "options" },
     { "psbtbumpfee", 1, "options" },
     { "logging", 0, "include" },

src/rpc/mempool.cpp

@@ -587,6 +587,89 @@ static RPCHelpMan getmempoolentry()
     };
 }
 
+static RPCHelpMan gettxspendingprevout()
+{
+    return RPCHelpMan{"gettxspendingprevout",
+        "Scans the mempool to find transactions spending any of the given outputs",
+        {
+            {"outputs", RPCArg::Type::ARR, RPCArg::Optional::NO, "The transaction outputs that we want to check, and within each, the txid (string) vout (numeric).",
+                {
+                    {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
+                        {
+                            {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
+                            {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
+                        },
+                    },
+                },
+            },
+        },
+        RPCResult{
+            RPCResult::Type::ARR, "", "",
+            {
+                {RPCResult::Type::OBJ, "", "",
+                {
+                    {RPCResult::Type::STR_HEX, "txid", "the transaction id of the checked output"},
+                    {RPCResult::Type::NUM, "vout", "the vout value of the checked output"},
+                    {RPCResult::Type::STR_HEX, "spendingtxid", /*optional=*/true, "the transaction id of the mempool transaction spending this output (omitted if unspent)"},
+                }},
+            }
+        },
+        RPCExamples{
+            HelpExampleCli("gettxspendingprevout", "\"[{\\\"txid\\\":\\\"a08e6907dbbd3d809776dbfc5d82e371b764ed838b5655e72f463568df1aadf0\\\",\\\"vout\\\":3}]\"")
+            + HelpExampleRpc("gettxspendingprevout", "\"[{\\\"txid\\\":\\\"a08e6907dbbd3d809776dbfc5d82e371b764ed838b5655e72f463568df1aadf0\\\",\\\"vout\\\":3}]\"")
+        },
+        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
+        {
+            RPCTypeCheckArgument(request.params[0], UniValue::VARR);
+            const UniValue& output_params = request.params[0];
+            if (output_params.empty()) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, outputs are missing");
+            }
+
+            std::vector<COutPoint> prevouts;
+            prevouts.reserve(output_params.size());
+
+            for (unsigned int idx = 0; idx < output_params.size(); idx++) {
+                const UniValue& o = output_params[idx].get_obj();
+
+                RPCTypeCheckObj(o,
+                                {
+                                    {"txid", UniValueType(UniValue::VSTR)},
+                                    {"vout", UniValueType(UniValue::VNUM)},
+                                }, /*fAllowNull=*/false, /*fStrict=*/true);
+
+                const uint256 txid(ParseHashO(o, "txid"));
+                const int nOutput = find_value(o, "vout").get_int();
+                if (nOutput < 0) {
+                    throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout cannot be negative");
+                }
+
+                prevouts.emplace_back(txid, nOutput);
+            }
+
+            const CTxMemPool& mempool = EnsureAnyMemPool(request.context);
+            LOCK(mempool.cs);
+
+            UniValue result{UniValue::VARR};
+
+            for (const COutPoint& prevout : prevouts) {
+                UniValue o(UniValue::VOBJ);
+                o.pushKV("txid", prevout.hash.ToString());
+                o.pushKV("vout", (uint64_t)prevout.n);
+
+                const CTransaction* spendingTx = mempool.GetConflictTx(prevout);
+                if (spendingTx != nullptr) {
+                    o.pushKV("spendingtxid", spendingTx->GetHash().ToString());
+                }
+
+                result.push_back(o);
+            }
+
+            return result;
+        },
+    };
+}
+
 UniValue MempoolInfoToJSON(const CTxMemPool& pool)
 {
     // Make sure this call is atomic in the pool.
@@ -677,6 +760,7 @@ void RegisterMempoolRPCCommands(CRPCTable& t)
         {"blockchain", &getmempoolancestors},
         {"blockchain", &getmempooldescendants},
         {"blockchain", &getmempoolentry},
+        {"blockchain", &gettxspendingprevout},
         {"blockchain", &getmempoolinfo},
         {"blockchain", &getrawmempool},
         {"blockchain", &savemempool},

src/test/fuzz/rpc.cpp

@@ -128,6 +128,7 @@ const std::vector<std::string> RPC_COMMANDS_SAFE_FOR_FUZZING{
     "getmempoolancestors",
     "getmempooldescendants",
     "getmempoolentry",
+    "gettxspendingprevout",
     "getmempoolinfo",
     "getmininginfo",
     "getnettotals",

test/functional/mempool_packages.py

@@ -100,6 +100,12 @@ class MempoolPackagesTest(BitcoinTestFramework):
             entry = self.nodes[0].getmempoolentry(x)
             assert_equal(entry, mempool[x])
 
+            # Check that gettxspendingprevout is consistent with getrawmempool
+            witnesstx = self.nodes[0].gettransaction(txid=x, verbose=True)['decoded']
+            for tx_in in witnesstx["vin"]:
+                spending_result = self.nodes[0].gettxspendingprevout([ {'txid' : tx_in["txid"], 'vout' : tx_in["vout"]} ])
+                assert_equal(spending_result, [ {'txid' : tx_in["txid"], 'vout' : tx_in["vout"], 'spendingtxid' : x} ])
+
             # Check that the descendant calculations are correct
             assert_equal(entry['descendantcount'], descendant_count)
             descendant_fees += entry['fees']['base']

test/functional/rpc_mempool_info.py

@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+# Copyright (c) 2014-2022 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test RPCs that retrieve information from the mempool."""
+
+from test_framework.blocktools import COINBASE_MATURITY
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+    assert_raises_rpc_error,
+)
+from test_framework.wallet import MiniWallet
+
+
+class RPCMempoolInfoTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+
+    def run_test(self):
+        self.wallet = MiniWallet(self.nodes[0])
+        self.generate(self.wallet, COINBASE_MATURITY + 1)
+        self.wallet.rescan_utxos()
+        confirmed_utxo = self.wallet.get_utxo()
+
+        # Create a tree of unconfirmed transactions in the mempool:
+        #             txA
+        #             / \
+        #            /   \
+        #           /     \
+        #          /       \
+        #         /         \
+        #       txB         txC
+        #       / \         / \
+        #      /   \       /   \
+        #    txD   txE   txF   txG
+        #            \   /
+        #             \ /
+        #             txH
+
+        def create_tx(**kwargs):
+            return self.wallet.send_self_transfer_multi(
+                from_node=self.nodes[0],
+                **kwargs,
+            )
+
+        txA = create_tx(utxos_to_spend=[confirmed_utxo], num_outputs=2)
+        txB = create_tx(utxos_to_spend=[txA["new_utxos"][0]], num_outputs=2)
+        txC = create_tx(utxos_to_spend=[txA["new_utxos"][1]], num_outputs=2)
+        txD = create_tx(utxos_to_spend=[txB["new_utxos"][0]], num_outputs=1)
+        txE = create_tx(utxos_to_spend=[txB["new_utxos"][1]], num_outputs=1)
+        txF = create_tx(utxos_to_spend=[txC["new_utxos"][0]], num_outputs=2)
+        txG = create_tx(utxos_to_spend=[txC["new_utxos"][1]], num_outputs=1)
+        txH = create_tx(utxos_to_spend=[txE["new_utxos"][0],txF["new_utxos"][0]], num_outputs=1)
+        txidA, txidB, txidC, txidD, txidE, txidF, txidG, txidH = [
+            tx["txid"] for tx in [txA, txB, txC, txD, txE, txF, txG, txH]
+        ]
+
+        mempool = self.nodes[0].getrawmempool()
+        assert_equal(len(mempool), 8)
+        for txid in [txidA, txidB, txidC, txidD, txidE, txidF, txidG, txidH]:
+            assert_equal(txid in mempool, True)
+
+        self.log.info("Find transactions spending outputs")
+        result = self.nodes[0].gettxspendingprevout([ {'txid' : confirmed_utxo['txid'], 'vout' : 0}, {'txid' : txidA, 'vout' : 1} ])
+        assert_equal(result, [ {'txid' : confirmed_utxo['txid'], 'vout' : 0, 'spendingtxid' : txidA}, {'txid' : txidA, 'vout' : 1, 'spendingtxid' : txidC} ])
+
+        self.log.info("Find transaction spending multiple outputs")
+        result = self.nodes[0].gettxspendingprevout([ {'txid' : txidE, 'vout' : 0}, {'txid' : txidF, 'vout' : 0} ])
+        assert_equal(result, [ {'txid' : txidE, 'vout' : 0, 'spendingtxid' : txidH}, {'txid' : txidF, 'vout' : 0, 'spendingtxid' : txidH} ])
+
+        self.log.info("Find no transaction when output is unspent")
+        result = self.nodes[0].gettxspendingprevout([ {'txid' : txidH, 'vout' : 0} ])
+        assert_equal(result, [ {'txid' : txidH, 'vout' : 0} ])
+        result = self.nodes[0].gettxspendingprevout([ {'txid' : txidA, 'vout' : 5} ])
+        assert_equal(result, [ {'txid' : txidA, 'vout' : 5} ])
+
+        self.log.info("Mixed spent and unspent outputs")
+        result = self.nodes[0].gettxspendingprevout([ {'txid' : txidB, 'vout' : 0}, {'txid' : txidG, 'vout' : 3} ])
+        assert_equal(result, [ {'txid' : txidB, 'vout' : 0, 'spendingtxid' : txidD}, {'txid' : txidG, 'vout' : 3} ])
+
+        self.log.info("Unknown input fields")
+        assert_raises_rpc_error(-3, "Unexpected key unknown", self.nodes[0].gettxspendingprevout, [{'txid' : txidC, 'vout' : 1, 'unknown' : 42}])
+
+        self.log.info("Invalid vout provided")
+        assert_raises_rpc_error(-8, "Invalid parameter, vout cannot be negative", self.nodes[0].gettxspendingprevout, [{'txid' : txidA, 'vout' : -1}])
+
+        self.log.info("Invalid txid provided")
+        assert_raises_rpc_error(-3, "Expected type string for txid, got number", self.nodes[0].gettxspendingprevout, [{'txid' : 42, 'vout' : 0}])
+
+        self.log.info("Missing outputs")
+        assert_raises_rpc_error(-8, "Invalid parameter, outputs are missing", self.nodes[0].gettxspendingprevout, [])
+
+        self.log.info("Missing vout")
+        assert_raises_rpc_error(-3, "Missing vout", self.nodes[0].gettxspendingprevout, [{'txid' : txidA}])
+
+        self.log.info("Missing txid")
+        assert_raises_rpc_error(-3, "Missing txid", self.nodes[0].gettxspendingprevout, [{'vout' : 3}])
+
+
+if __name__ == '__main__':
+    RPCMempoolInfoTest().main()

test/functional/test_runner.py

@@ -329,6 +329,7 @@ BASE_SCRIPTS = [
     'feature_settings.py',
     'rpc_getdescriptorinfo.py',
     'rpc_mempool_entry_fee_fields_deprecation.py',
+    'rpc_mempool_info.py',
     'rpc_help.py',
     'feature_dirsymlinks.py',
     'feature_help.py',