Factor out combine / finalize / extract PSBT helpers

Refactor the new CombinePSBT, FinalizePSBT, and FinalizeAndExtractPSBT
general-purpose functions out of the combinepsbt and finalizepsbt RPCs,
for use in the GUI code.

src/node/transaction.cpp

@@ -29,6 +29,8 @@ const char* TransactionErrorString(const TransactionError err)
             return "AcceptToMemoryPool failed";
         case TransactionError::INVALID_PSBT:
             return "PSBT is not sane";
+        case TransactionError::PSBT_MISMATCH:
+            return "PSBTs not compatible (different transactions)";
         case TransactionError::SIGHASH_MISMATCH:
             return "Specified sighash value does not match existing value";
 

src/node/transaction.h

@@ -18,6 +18,7 @@ enum class TransactionError {
     MEMPOOL_REJECTED,
     MEMPOOL_ERROR,
     INVALID_PSBT,
+    PSBT_MISMATCH,
     SIGHASH_MISMATCH,
 
     ERROR_COUNT

src/psbt.cpp

@@ -232,3 +232,52 @@ bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction&
 
     return sig_complete;
 }
+
+bool FinalizePSBT(PartiallySignedTransaction& psbtx)
+{
+    // Finalize input signatures -- in case we have partial signatures that add up to a complete
+    //   signature, but have not combined them yet (e.g. because the combiner that created this
+    //   PartiallySignedTransaction did not understand them), this will combine them into a final
+    //   script.
+    bool complete = true;
+    for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
+        complete &= SignPSBTInput(DUMMY_SIGNING_PROVIDER, psbtx, i, SIGHASH_ALL);
+    }
+
+    return complete;
+}
+
+bool FinalizeAndExtractPSBT(PartiallySignedTransaction& psbtx, CMutableTransaction& result)
+{
+    // It's not safe to extract a PSBT that isn't finalized, and there's no easy way to check
+    //   whether a PSBT is finalized without finalizing it, so we just do this.
+    if (!FinalizePSBT(psbtx)) {
+        return false;
+    }
+
+    result = *psbtx.tx;
+    for (unsigned int i = 0; i < result.vin.size(); ++i) {
+        result.vin[i].scriptSig = psbtx.inputs[i].final_script_sig;
+        result.vin[i].scriptWitness = psbtx.inputs[i].final_script_witness;
+    }
+    return true;
+}
+
+bool CombinePSBTs(PartiallySignedTransaction& out, TransactionError& error, const std::vector<PartiallySignedTransaction>& psbtxs)
+{
+    out = psbtxs[0]; // Copy the first one
+
+    // Merge
+    for (auto it = std::next(psbtxs.begin()); it != psbtxs.end(); ++it) {
+        if (!out.Merge(*it)) {
+            error = TransactionError::PSBT_MISMATCH;
+            return false;
+        }
+    }
+    if (!out.IsSane()) {
+        error = TransactionError::INVALID_PSBT;
+        return false;
+    }
+
+    return true;
+}

src/psbt.h

@@ -544,4 +544,31 @@ bool PSBTInputSigned(PSBTInput& input);
 /** Signs a PSBTInput, verifying that all provided data matches what is being signed. */
 bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction& psbt, int index, int sighash = SIGHASH_ALL);
 
+/**
+ * Finalizes a PSBT if possible, combining partial signatures.
+ *
+ * @param[in,out] &psbtx reference to PartiallySignedTransaction to finalize
+ * return True if the PSBT is now complete, false otherwise
+ */
+bool FinalizePSBT(PartiallySignedTransaction& psbtx);
+
+/**
+ * Finalizes a PSBT if possible, and extracts it to a CMutableTransaction if it could be finalized.
+ *
+ * @param[in]  &psbtx reference to PartiallySignedTransaction
+ * @param[out] result CMutableTransaction representing the complete transaction, if successful
+ * @return True if we successfully extracted the transaction, false otherwise
+ */
+bool FinalizeAndExtractPSBT(PartiallySignedTransaction& psbtx, CMutableTransaction& result);
+
+/**
+ * Combines PSBTs with the same underlying transaction, resulting in a single PSBT with all partial signatures from each input.
+ *
+ * @param[out] &out   the combined PSBT, if successful
+ * @param[out] &error reference to TransactionError to fill with error info on failure
+ * @param[in]  psbtxs the PSBTs to combine
+ * @return True if we successfully combined the transactions, false if they were not compatible
+ */
+bool CombinePSBTs(PartiallySignedTransaction& out, TransactionError& error, const std::vector<PartiallySignedTransaction>& psbtxs);
+
 #endif // BITCOIN_PSBT_H

src/rpc/rawtransaction.cpp

@@ -1477,16 +1477,10 @@ UniValue combinepsbt(const JSONRPCRequest& request)
         psbtxs.push_back(psbtx);
     }
 
-    PartiallySignedTransaction merged_psbt(psbtxs[0]); // Copy the first one
-
-    // Merge
-    for (auto it = std::next(psbtxs.begin()); it != psbtxs.end(); ++it) {
-        if (!merged_psbt.Merge(*it)) {
-            throw JSONRPCError(RPC_INVALID_PARAMETER, "PSBTs do not refer to the same transactions.");
-        }
-    }
-    if (!merged_psbt.IsSane()) {
-        throw JSONRPCError(RPC_INVALID_PARAMETER, "Merged PSBT is inconsistent");
+    PartiallySignedTransaction merged_psbt;
+    TransactionError error;
+    if (!CombinePSBTs(merged_psbt, error, psbtxs)) {
+        throw JSONRPCTransactionError(error);
     }
 
     UniValue result(UniValue::VOBJ);
@@ -1531,29 +1525,23 @@ UniValue finalizepsbt(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
     }
 
-    // Finalize input signatures -- in case we have partial signatures that add up to a complete
-    //   signature, but have not combined them yet (e.g. because the combiner that created this
-    //   PartiallySignedTransaction did not understand them), this will combine them into a final
-    //   script.
-    bool complete = true;
-    for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
-        complete &= SignPSBTInput(DUMMY_SIGNING_PROVIDER, psbtx, i, SIGHASH_ALL);
-    }
+    bool extract = request.params[1].isNull() || (!request.params[1].isNull() && request.params[1].get_bool());
+
+    CMutableTransaction mtx;
+    bool complete = FinalizeAndExtractPSBT(psbtx, mtx);
 
     UniValue result(UniValue::VOBJ);
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
-    bool extract = request.params[1].isNull() || (!request.params[1].isNull() && request.params[1].get_bool());
+    std::string result_str;
+
     if (complete && extract) {
-        CMutableTransaction mtx(*psbtx.tx);
-        for (unsigned int i = 0; i < mtx.vin.size(); ++i) {
-            mtx.vin[i].scriptSig = psbtx.inputs[i].final_script_sig;
-            mtx.vin[i].scriptWitness = psbtx.inputs[i].final_script_witness;
-        }
         ssTx << mtx;
-        result.pushKV("hex", HexStr(ssTx.str()));
+        result_str = HexStr(ssTx.str());
+        result.pushKV("hex", result_str);
     } else {
         ssTx << psbtx;
-        result.pushKV("psbt", EncodeBase64(ssTx.str()));
+        result_str = EncodeBase64(ssTx.str());
+        result.pushKV("psbt", result_str);
     }
     result.pushKV("complete", complete);
 

src/rpc/util.cpp

@@ -151,6 +151,8 @@ RPCErrorCode RPCErrorFromTransactionError(TransactionError terr)
         case TransactionError::P2P_DISABLED:
             return RPC_CLIENT_P2P_DISABLED;
         case TransactionError::INVALID_PSBT:
+        case TransactionError::PSBT_MISMATCH:
+            return RPC_INVALID_PARAMETER;
         case TransactionError::SIGHASH_MISMATCH:
             return RPC_DESERIALIZATION_ERROR;
         default: break;