Implement joinpsbts RPC and tests

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

src/psbt.cpp

@@ -42,6 +42,26 @@ bool PartiallySignedTransaction::IsSane() const
     return true;
 }
 
+bool PartiallySignedTransaction::AddInput(const CTxIn& txin, PSBTInput& psbtin)
+{
+    if (std::find(tx->vin.begin(), tx->vin.end(), txin) != tx->vin.end()) {
+        return false;
+    }
+    tx->vin.push_back(txin);
+    psbtin.partial_sigs.clear();
+    psbtin.final_script_sig.clear();
+    psbtin.final_script_witness.SetNull();
+    inputs.push_back(psbtin);
+    return true;
+}
+
+bool PartiallySignedTransaction::AddOutput(const CTxOut& txout, const PSBTOutput& psbtout)
+{
+    tx->vout.push_back(txout);
+    outputs.push_back(psbtout);
+    return true;
+}
+
 bool PSBTInput::IsNull() const
 {
     return !non_witness_utxo && witness_utxo.IsNull() && partial_sigs.empty() && unknown.empty() && hd_keypaths.empty() && redeem_script.empty() && witness_script.empty();

src/psbt.h

@@ -389,6 +389,8 @@ struct PartiallySignedTransaction
       * same actual Bitcoin transaction.) Returns true if the merge succeeded, false otherwise. */
     NODISCARD bool Merge(const PartiallySignedTransaction& psbt);
     bool IsSane() const;
+    bool AddInput(const CTxIn& txin, PSBTInput& psbtin);
+    bool AddOutput(const CTxOut& txout, const PSBTOutput& psbtout);
     PartiallySignedTransaction() {}
     PartiallySignedTransaction(const PartiallySignedTransaction& psbt_in) : tx(psbt_in.tx), inputs(psbt_in.inputs), outputs(psbt_in.outputs), unknown(psbt_in.unknown) {}
     explicit PartiallySignedTransaction(const CMutableTransaction& tx);

src/rpc/client.cpp

@@ -112,6 +112,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "createpsbt", 2, "locktime" },
     { "createpsbt", 3, "replaceable" },
     { "combinepsbt", 0, "txs"},
+    { "joinpsbts", 0, "txs"},
     { "finalizepsbt", 1, "extract"},
     { "converttopsbt", 1, "permitsigdata"},
     { "converttopsbt", 2, "iswitness"},

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"} },

test/functional/rpc_psbt.py

@@ -321,6 +321,23 @@ def run_test(self):
         assert "witness_utxo" not in decoded['inputs'][1] and "non_witness_utxo" not in decoded['inputs'][1]
         assert "witness_utxo" not in decoded['inputs'][2] and "non_witness_utxo" not in decoded['inputs'][2]
 
+        # Two PSBTs with a common input should not be joinable
+        psbt1 = self.nodes[1].createpsbt([{"txid":txid1, "vout":vout1}], {self.nodes[0].getnewaddress():Decimal('10.999')})
+        assert_raises_rpc_error(-8, "exists in multiple PSBTs", self.nodes[1].joinpsbts, [psbt1, updated])
+
+        # Join two distinct PSBTs
+        addr4 = self.nodes[1].getnewaddress("", "p2sh-segwit")
+        txid4 = self.nodes[0].sendtoaddress(addr4, 5)
+        vout4 = find_output(self.nodes[0], txid4, 5)
+        self.nodes[0].generate(6)
+        self.sync_all()
+        psbt2 = self.nodes[1].createpsbt([{"txid":txid4, "vout":vout4}], {self.nodes[0].getnewaddress():Decimal('4.999')})
+        psbt2 = self.nodes[1].walletprocesspsbt(psbt2)['psbt']
+        psbt2_decoded = self.nodes[0].decodepsbt(psbt2)
+        assert "final_scriptwitness" in psbt2_decoded['inputs'][0] and "final_scriptSig" in psbt2_decoded['inputs'][0]
+        joined = self.nodes[0].joinpsbts([psbt, psbt2])
+        joined_decoded = self.nodes[0].decodepsbt(joined)
+        assert len(joined_decoded['inputs']) == 4 and len(joined_decoded['outputs']) == 2 and "final_scriptwitness" not in joined_decoded['inputs'][3] and "final_scriptSig" not in joined_decoded['inputs'][3]
 
 
 if __name__ == '__main__':