rest: add interface for gettxspendingprevout rpc

This creates a REST interface for the gettxspendingprevout rpc and also
includes functional tests

Author: kevkevinpal

doc/REST-interface.md

@@ -150,6 +150,30 @@ Refer to the `getrawmempool` RPC help for details. Defaults to setting
 
 *Query parameters for `verbose` and `mempool_sequence` available in 25.0 and up.*
 
+#### Transaction Spending Previous Outputs
+`GET /rest/txspendingprevout/<TXID>-<N>/<TXID>-<N>/.../<TXID>-<N>.json`
+
+Scans the mempool to find transactions spending any of the given outputs.
+Only supports JSON as output format.
+Refer to the `gettxspendingprevout` RPC help for details.
+
+For each queried output, returns an object containing:
+- `txid`: the transaction id of the checked output
+- `vout`: the output number
+- `spendingtxid`: (optional) the transaction id of the mempool transaction spending this output (omitted if unspent)
+
+Example:
+```
+$ curl localhost:8332/rest/txspendingprevout/a08e6907dbbd3d809776dbfc5d82e371b764ed838b5655e72f463568df1aadf0-3.json
+[
+  {
+    "txid": "a08e6907dbbd3d809776dbfc5d82e371b764ed838b5655e72f463568df1aadf0",
+    "vout": 3,
+    "spendingtxid": "b2cdfd7b89def827ff8af7cd9bff7627ff72e5e8b0f71210f92ea7a4000c5d75"
+  }
+]
+```
+
 
 Risks
 -------------

src/rest.cpp

@@ -810,6 +810,73 @@ static bool rest_mempool(const std::any& context, HTTPRequest* req, const std::s
     }
 }
 
+static bool rest_txspendingprevout(const std::any& context, HTTPRequest* req, const std::string& uri_part)
+{
+    if (!CheckWarmup(req))
+        return false;
+
+    std::string param;
+    const RESTResponseFormat rf = ParseDataFormat(param, uri_part);
+
+    std::vector<std::string> uriParts = SplitString(param, '/');
+
+    // Check that we have at least one outpoint
+    if (uriParts.size() == 0 || (uriParts.size() == 1 && uriParts[0].empty()))
+        return RESTERR(req, HTTP_BAD_REQUEST, "Error: empty request");
+
+    std::vector<COutPoint> prevouts;
+
+    // Parse outpoints from URI in the format txid-vout
+    for (size_t i = 0; i < uriParts.size(); i++)
+    {
+        const auto txid_out{util::Split<std::string_view>(uriParts[i], '-')};
+        if (txid_out.size() != 2) {
+            return RESTERR(req, HTTP_BAD_REQUEST, "Parse error");
+        }
+        auto txid{Txid::FromHex(txid_out.at(0))};
+        auto output{ToIntegral<uint32_t>(txid_out.at(1))};
+
+        if (!txid || !output.has_value()) {
+            return RESTERR(req, HTTP_BAD_REQUEST, "Parse error");
+        }
+
+        prevouts.emplace_back(*txid, *output);
+    }
+
+    switch (rf) {
+    case RESTResponseFormat::JSON: {
+        const CTxMemPool* mempool = GetMemPool(context, req);
+        if (!mempool) return false;
+
+        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(std::move(o));
+        }
+
+        std::string strJSON = result.write() + "\n";
+        req->WriteHeader("Content-Type", "application/json");
+        req->WriteReply(HTTP_OK, strJSON);
+        return true;
+    }
+
+    default: {
+        return RESTERR(req, HTTP_NOT_FOUND, "output format not found (available: json)");
+    }
+    }
+}
+
 static bool rest_tx(const std::any& context, HTTPRequest* req, const std::string& uri_part)
 {
     if (!CheckWarmup(req))
@@ -1130,6 +1197,7 @@ static const struct {
       {"/rest/deploymentinfo", rest_deploymentinfo},
       {"/rest/blockhashbyheight/", rest_blockhash_by_height},
       {"/rest/spenttxouts/", rest_spent_txouts},
+      {"/rest/txspendingprevout/", rest_txspendingprevout},
 };
 
 void StartREST(const std::any& context)

test/functional/interface_rest.py

@@ -412,6 +412,59 @@ def run_test(self):
         for tx in txs:
             assert tx in json_obj['tx']
 
+        self.log.info("Test the /txspendingprevout URI")
+
+        # Create two new transactions - one to spend and one spending transaction
+        # First, create a transaction that will have an output to spend
+        utxo_for_new_tx = self.wallet.get_utxo()
+        base_tx = self.wallet.send_self_transfer(from_node=self.nodes[0], utxo_to_spend=utxo_for_new_tx)
+        base_txid = base_tx['txid']
+        base_vout = 0  # The change output
+        self.sync_all()
+
+        # Now create a transaction that spends from base_tx
+        spending_tx = self.wallet.send_self_transfer(from_node=self.nodes[0], utxo_to_spend=self.wallet.get_utxo(txid=base_txid))
+        spending_txid = spending_tx['txid']
+        self.sync_all()
+
+        # Test with single outpoint - should find the spending transaction
+        json_obj = self.test_rest_request(f"/txspendingprevout/{base_txid}-{base_vout}")
+        assert_equal(len(json_obj), 1)
+        assert_equal(json_obj[0]['txid'], base_txid)
+        assert_equal(json_obj[0]['vout'], base_vout)
+        assert_equal(json_obj[0]['spendingtxid'], spending_txid)
+
+        # Test with unspent output - should not have spendingtxid
+        utxo_unspent = self.wallet.get_utxo()
+        json_obj = self.test_rest_request(f"/txspendingprevout/{utxo_unspent['txid']}-{utxo_unspent['vout']}")
+        assert_equal(len(json_obj), 1)
+        assert_equal(json_obj[0]['txid'], utxo_unspent['txid'])
+        assert_equal(json_obj[0]['vout'], utxo_unspent['vout'])
+        assert 'spendingtxid' not in json_obj[0]
+
+        # Test with multiple outpoints
+        json_obj = self.test_rest_request(f"/txspendingprevout/{base_txid}-{base_vout}/{utxo_unspent['txid']}-{utxo_unspent['vout']}")
+        assert_equal(len(json_obj), 2)
+        # First output is spent
+        assert_equal(json_obj[0]['txid'], base_txid)
+        assert_equal(json_obj[0]['vout'], base_vout)
+        assert_equal(json_obj[0]['spendingtxid'], spending_txid)
+        # Second output is unspent
+        assert_equal(json_obj[1]['txid'], utxo_unspent['txid'])
+        assert_equal(json_obj[1]['vout'], utxo_unspent['vout'])
+        assert 'spendingtxid' not in json_obj[1]
+
+        # Test with invalid outpoint format
+        resp = self.test_rest_request(f"/txspendingprevout/{INVALID_PARAM}", ret_type=RetType.OBJ, status=400)
+        assert_equal(resp.read().decode('utf-8').strip(), 'Parse error')
+
+        # Test with empty request
+        resp = self.test_rest_request("/txspendingprevout/", ret_type=RetType.OBJ, status=400)
+        assert_equal(resp.read().decode('utf-8').strip(), 'Error: empty request')
+
+        # Mine the transactions to clean up mempool
+        self.generate(self.nodes[0], 1)
+
         self.log.info("Test the /chaininfo URI")
 
         bb_hash = self.nodes[0].getbestblockhash()