Implement joinpsbts RPC and tests

Adds a joinpsbts RPC which combines multiple distinct PSBTs into
one PSBT.

src/rpc/rawtransaction.cpp

@@ -1755,6 +1755,80 @@ UniValue utxoupdatepsbt(const JSONRPCRequest& request)
     return EncodeBase64((unsigned char*)ssTx.data(), ssTx.size());
 }
 
+UniValue joinpsbts(const JSONRPCRequest& request)
+{
+    if (request.fHelp || request.params.size() != 1) {
+        throw std::runtime_error(
+            RPCHelpMan{"joinpsbts",
+            "\nJoins multiple distinct PSBTs with different inputs and outputs into one PSBT with inputs and outputs from all of the PSBTs\n"
+            "No input in any of the PSBTs can be in more than one of the PSBTs.\n",
+            {
+                {"txs", RPCArg::Type::ARR, RPCArg::Optional::NO, "A json array of base64 strings of partially signed transactions",
+                    {
+                        {"psbt", RPCArg::Type::STR, RPCArg::Optional::NO, "A base64 string of a PSBT"}
+                    }}
+            },
+            RPCResult {
+                "  \"psbt\"          (string) The base64-encoded partially signed transaction\n"
+            },
+            RPCExamples {
+                HelpExampleCli("joinpsbts", "\"psbt\"")
+            }}.ToString());
+    }
+
+    RPCTypeCheck(request.params, {UniValue::VARR}, true);
+
+    // Unserialize the transactions
+    std::vector<PartiallySignedTransaction> psbtxs;
+    UniValue txs = request.params[0].get_array();
+
+    if (txs.size() <= 1) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "At least two PSBTs are required to join PSBTs.");
+    }
+
+    int32_t best_version = 1;
+    uint32_t best_locktime = 0xffffffff;
+    for (unsigned int i = 0; i < txs.size(); ++i) {
+        PartiallySignedTransaction psbtx;
+        std::string error;
+        if (!DecodeBase64PSBT(psbtx, txs[i].get_str(), error)) {
+            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
+        }
+        psbtxs.push_back(psbtx);
+        // Choose the highest version number
+        if (psbtx.tx->nVersion > best_version) {
+            best_version = psbtx.tx->nVersion;
+        }
+        // Choose the lowest lock time
+        if (psbtx.tx->nLockTime < best_locktime) {
+            best_locktime = psbtx.tx->nLockTime;
+        }
+    }
+
+    // Create a blank psbt where everything will be added
+    PartiallySignedTransaction merged_psbt;
+    merged_psbt.tx = CMutableTransaction();
+    merged_psbt.tx->nVersion = best_version;
+    merged_psbt.tx->nLockTime = best_locktime;
+
+    // Merge
+    for (auto& psbt : psbtxs) {
+        for (unsigned int i = 0; i < psbt.tx->vin.size(); ++i) {
+            if (!merged_psbt.AddInput(psbt.tx->vin[i], psbt.inputs[i])) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Input %s:%d exists in multiple PSBTs", psbt.tx->vin[i].prevout.hash.ToString().c_str(), psbt.tx->vin[i].prevout.n));
+            }
+        }
+        for (unsigned int i = 0; i < psbt.tx->vout.size(); ++i) {
+            merged_psbt.AddOutput(psbt.tx->vout[i], psbt.outputs[i]);
+        }
+        merged_psbt.unknown.insert(psbt.unknown.begin(), psbt.unknown.end());
+    }
+
+    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
+    ssTx << merged_psbt;
+    return EncodeBase64((unsigned char*)ssTx.data(), ssTx.size());
+}
+
 // clang-format off
 static const CRPCCommand commands[] =
 { //  category              name                            actor (function)            argNames
@@ -1774,6 +1848,7 @@ static const CRPCCommand commands[] =
     { "rawtransactions",    "createpsbt",                   &createpsbt,                {"inputs","outputs","locktime","replaceable"} },
     { "rawtransactions",    "converttopsbt",                &converttopsbt,             {"hexstring","permitsigdata","iswitness"} },
     { "rawtransactions",    "utxoupdatepsbt",               &utxoupdatepsbt,            {"psbt"} },
+    { "rawtransactions",    "joinpsbts",                    &joinpsbts,                 {"txs"} },
 
     { "blockchain",         "gettxoutproof",                &gettxoutproof,             {"txids", "blockhash"} },
     { "blockchain",         "verifytxoutproof",             &verifytxoutproof,          {"proof"} },