Implement FillPSBT in DescriptorScriptPubKeyMan

FillPSBT will add our own scripts to the PSBT if those inputs are ours.
If an input also lists pubkeys that we happen to know the private keys
for, we will sign those inputs too.

src/wallet/scriptpubkeyman.cpp

@@ -1974,9 +1974,71 @@ SigningResult DescriptorScriptPubKeyMan::SignMessage(const std::string& message,
     return SigningResult::OK;
 }
 
-TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction& psbt, int sighash_type, bool sign, bool bip32derivs) const
+TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction& psbtx, int sighash_type, bool sign, bool bip32derivs) const
 {
-    return TransactionError::INVALID_PSBT;
+    for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
+        const CTxIn& txin = psbtx.tx->vin[i];
+        PSBTInput& input = psbtx.inputs.at(i);
+
+        if (PSBTInputSigned(input)) {
+            continue;
+        }
+
+        // Verify input looks sane. This will check that we have at most one uxto, witness or non-witness.
+        if (!input.IsSane()) {
+            return TransactionError::INVALID_PSBT;
+        }
+
+        // Get the Sighash type
+        if (sign && input.sighash_type > 0 && input.sighash_type != sighash_type) {
+            return TransactionError::SIGHASH_MISMATCH;
+        }
+
+        // Get the scriptPubKey to know which SigningProvider to use
+        CScript script;
+        if (!input.witness_utxo.IsNull()) {
+            script = input.witness_utxo.scriptPubKey;
+        } else if (input.non_witness_utxo) {
+            if (txin.prevout.n >= input.non_witness_utxo->vout.size()) {
+                return TransactionError::MISSING_INPUTS;
+            }
+            script = input.non_witness_utxo->vout[txin.prevout.n].scriptPubKey;
+        } else {
+            // There's no UTXO so we can just skip this now
+            continue;
+        }
+        SignatureData sigdata;
+        input.FillSignatureData(sigdata);
+
+        std::unique_ptr<FlatSigningProvider> keys = MakeUnique<FlatSigningProvider>();
+        std::unique_ptr<FlatSigningProvider> script_keys = GetSigningProvider(script, sign);
+        if (script_keys) {
+            *keys = Merge(*keys, *script_keys);
+        } else {
+            // Maybe there are pubkeys listed that we can sign for
+            script_keys = MakeUnique<FlatSigningProvider>();
+            for (const auto& pk_pair : input.hd_keypaths) {
+                const CPubKey& pubkey = pk_pair.first;
+                std::unique_ptr<FlatSigningProvider> pk_keys = GetSigningProvider(pubkey);
+                if (pk_keys) {
+                    *keys = Merge(*keys, *pk_keys);
+                }
+            }
+        }
+
+        SignPSBTInput(HidingSigningProvider(keys.get(), !sign, !bip32derivs), psbtx, i, sighash_type);
+    }
+
+    // Fill in the bip32 keypaths and redeemscripts for the outputs so that hardware wallets can identify change
+    for (unsigned int i = 0; i < psbtx.tx->vout.size(); ++i) {
+        std::unique_ptr<SigningProvider> keys = GetSolvingProvider(psbtx.tx->vout.at(i).scriptPubKey);
+        if (!keys) {
+            continue;
+        }
+        UpdatePSBTOutput(HidingSigningProvider(keys.get(), true, !bip32derivs), psbtx, i);
+    }
+
+    return TransactionError::OK;
 }
 
 const CKeyMetadata* DescriptorScriptPubKeyMan::GetMetadata(const CTxDestination& dest) const