multi: use prev output fetcher where possible

input/taproot.go

@@ -1,6 +1,8 @@
 package input
 
 import (
+	"fmt"
+
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 )
@@ -18,3 +20,25 @@ func NewTxSigHashesV0Only(tx *wire.MsgTx) *txscript.TxSigHashes {
 	nilFetcher := txscript.NewCannedPrevOutputFetcher(nil, 0)
 	return txscript.NewTxSigHashes(tx, nilFetcher)
 }
+
+// MultiPrevOutFetcher returns a txscript.MultiPrevOutFetcher for the given set
+// of inputs.
+func MultiPrevOutFetcher(inputs []Input) (*txscript.MultiPrevOutFetcher, error) {
+	fetcher := txscript.NewMultiPrevOutFetcher(nil)
+	for _, inp := range inputs {
+		op := inp.OutPoint()
+		desc := inp.SignDesc()
+
+		if op == nil {
+			return nil, fmt.Errorf("missing input outpoint")
+		}
+
+		if desc == nil || desc.Output == nil {
+			return nil, fmt.Errorf("missing input utxo information")
+		}
+
+		fetcher.AddPrevOut(*op, desc.Output)
+	}
+
+	return fetcher, nil
+}