support for better combining and finalising psbts

src/main/java/com/sparrowwallet/drongo/protocol/Transaction.java

@@ -153,6 +153,16 @@ public class Transaction extends ChildMessage {
         }
     }
 
+    public boolean hasScriptSigs() {
+        for(TransactionInput in : inputs) {
+            if(in.getScriptBytes().length > 0) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     public boolean hasWitnesses() {
         for(TransactionInput in : inputs) {
             if(in.hasWitness()) {

src/main/java/com/sparrowwallet/drongo/psbt/PSBT.java

@@ -393,6 +393,16 @@ public class PSBT {
         return true;
     }
 
+    public boolean isFinalized() {
+        for(PSBTInput psbtInput : getPsbtInputs()) {
+            if(psbtInput.getFinalScriptSig() != null || psbtInput.getFinalScriptWitness() != null) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     private List<PSBTEntry> getGlobalEntries() {
         List<PSBTEntry> entries = new ArrayList<>();
 
@@ -462,10 +472,8 @@ public class PSBT {
             throw new IllegalArgumentException("Provided PSBT does contain a matching global transaction");
         }
 
-        for(PSBTInput psbtInput : psbt.getPsbtInputs()) {
-            if(psbtInput.getFinalScriptSig() != null || psbtInput.getFinalScriptWitness() != null) {
-                throw new IllegalArgumentException("Cannot combine an already finalised PSBT");
-            }
+        if(isFinalized() || psbt.isFinalized()) {
+            throw new IllegalArgumentException("Cannot combine an already finalised PSBT");
         }
 
         if(psbt.getVersion() != null) {
@@ -499,12 +507,14 @@ public class PSBT {
             }
         }
 
-        if(hasWitness && !transaction.isSegwit()) {
-            transaction.setSegwitVersion(1);
+        Transaction finalTransaction = new Transaction(transaction.bitcoinSerialize());
+
+        if(hasWitness && !finalTransaction.isSegwit()) {
+            finalTransaction.setSegwitVersion(1);
         }
 
-        for(int i = 0; i < transaction.getInputs().size(); i++) {
-            TransactionInput txInput = transaction.getInputs().get(i);
+        for(int i = 0; i < finalTransaction.getInputs().size(); i++) {
+            TransactionInput txInput = finalTransaction.getInputs().get(i);
             PSBTInput psbtInput = getPsbtInputs().get(i);
             txInput.setScriptBytes(psbtInput.getFinalScriptSig().getProgram());
 
@@ -512,12 +522,12 @@ public class PSBT {
                 if(psbtInput.getFinalScriptWitness() != null) {
                     txInput.setWitness(psbtInput.getFinalScriptWitness());
                 } else {
-                    txInput.setWitness(new TransactionWitness(transaction));
+                    txInput.setWitness(new TransactionWitness(finalTransaction));
                 }
             }
         }
 
-        return transaction;
+        return finalTransaction;
     }
 
     public List<PSBTInput> getPsbtInputs() {

src/main/java/com/sparrowwallet/drongo/psbt/PSBTInput.java

@@ -395,6 +395,16 @@ public class PSBTInput {
         }
     }
 
+    public Collection<TransactionSignature> getSignatures() {
+        if(getFinalScriptWitness() != null) {
+            return getFinalScriptWitness().getSignatures();
+        } else if(getFinalScriptSig() != null) {
+            return getFinalScriptSig().getSignatures();
+        } else {
+            return getPartialSignatures().values();
+        }
+    }
+
     public boolean sign(ECKey privKey) {
         SigHash localSigHash = getSigHash();
         if(localSigHash == null) {

src/main/java/com/sparrowwallet/drongo/wallet/FinalizingPSBTWallet.java

@@ -0,0 +1,76 @@
+package com.sparrowwallet.drongo.wallet;
+
+import com.sparrowwallet.drongo.crypto.ECKey;
+import com.sparrowwallet.drongo.policy.Miniscript;
+import com.sparrowwallet.drongo.policy.Policy;
+import com.sparrowwallet.drongo.policy.PolicyType;
+import com.sparrowwallet.drongo.protocol.ScriptType;
+import com.sparrowwallet.drongo.protocol.TransactionSignature;
+import com.sparrowwallet.drongo.psbt.PSBT;
+import com.sparrowwallet.drongo.psbt.PSBTInput;
+
+import java.util.*;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+/**
+ * This is a special wallet that is used solely to finalize a fully signed PSBT by reading from the partial signatures and UTXO scriptPubKey
+ *
+ */
+public class FinalizingPSBTWallet extends Wallet {
+    private final Map<PSBTInput, WalletNode> signedInputNodes = new LinkedHashMap<>();
+    private final Map<WalletNode, List<ECKey>> signedNodeKeys = new LinkedHashMap<>();
+    private int numSignatures;
+
+    public FinalizingPSBTWallet(PSBT psbt) {
+        super("Finalizing PSBT Wallet");
+
+        Map<PSBTInput, WalletNode> signingNodes = new LinkedHashMap<>();
+        for(int i = 0; i < psbt.getPsbtInputs().size(); i++) {
+            PSBTInput psbtInput = psbt.getPsbtInputs().get(i);
+            Set<ECKey> keys = psbtInput.getPartialSignatures().keySet();
+            WalletNode signedNode = new WalletNode("m/" + i);
+            signedInputNodes.put(psbtInput, signedNode);
+            signedNodeKeys.put(signedNode, new ArrayList<>(keys));
+            numSignatures = keys.size();
+            setScriptType(ScriptType.getType(psbtInput.getUtxo().getScript()));
+        }
+
+        setPolicyType(numSignatures == 1 ? PolicyType.SINGLE : PolicyType.MULTI);
+    }
+
+    @Override
+    public Map<PSBTInput, List<Keystore>> getSignedKeystores(PSBT psbt) {
+        Map<PSBTInput, List<Keystore>> signedKeystores = new LinkedHashMap<>();
+        for(PSBTInput psbtInput : psbt.getPsbtInputs()) {
+            Collection<TransactionSignature> signatures = psbtInput.getSignatures();
+            signedKeystores.put(psbtInput, IntStream.range(1, signatures.size() + 1).mapToObj(i -> new Keystore("Keystore " + i)).collect(Collectors.toList()));
+        }
+        return signedKeystores;
+    }
+
+    @Override
+    public Policy getDefaultPolicy() {
+        return new Policy(new Miniscript("")) {
+            @Override
+            public int getNumSignaturesRequired() {
+                return numSignatures;
+            }
+        };
+    }
+
+    @Override
+    public Map<PSBTInput, WalletNode> getSigningNodes(PSBT psbt) {
+        return signedInputNodes;
+    }
+
+    @Override
+    public ECKey getPubKey(WalletNode node) {
+        return signedNodeKeys.get(node).get(0);
+    }
+
+    @Override
+    public List<ECKey> getPubKeys(WalletNode node) {
+        return signedNodeKeys.get(node);
+    }
+}