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.

1 files changed, 84 insertions, 0 deletions

diff --git a/src/rpc/mempool.cpp b/src/rpc/mempool.cpp
index 27080d3881..7ee3528a63 100644
--- a/src/rpc/mempool.cpp
+++ b/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},