backend/transactions: add test for tx serialization

Recent btcd updates changed the way hashes are serialized. We proposed a
fix (see btcsuite/btcd#2025) to restore backward
compatibility.

This commit adds a specific test case to ensure that the newer and the
legacy serialization methods work.

backend/coins/btc/transactions/transactions_test.go

@@ -15,6 +15,7 @@
 package transactions_test
 
 import (
+	"encoding/json"
 	"os"
 	"testing"
 
@@ -190,6 +191,40 @@ func (s *transactionsSuite) TestUpdateAddressHistorySingleTxReceive() {
 	require.Equal(s.T(), expectedHeight, transactions[0].Height)
 }
 
+// TestTxSerialization checks that Tx marshaling/unmarshaling work with both current and legacy
+// chainhash.Hash marshaling methods.
+// see https://github.com/btcsuite/btcd/pull/2025
+func (s *transactionsSuite) TestTxSerialization() {
+	// create a new Tx
+	addresses, err := s.addressChain.EnsureAddresses()
+	require.NoError(s.T(), err)
+	address := addresses[0]
+	expectedAmount := btcutil.Amount(123)
+	tx1 := newTx(chainhash.HashH(nil), 0, address, expectedAmount)
+
+	// marshal transaction with current method
+	tx1Serialized, err := json.Marshal(tx1)
+	require.NoError(s.T(), err)
+	s.log.Print("Tx marshaled: " + string(tx1Serialized))
+
+	// marshal prevout hash with legacy method
+	hashBytes := [32]byte(tx1.TxIn[0].PreviousOutPoint.Hash.CloneBytes())
+	legacyMarshalledHash, err := json.Marshal(hashBytes)
+	require.NoError(s.T(), err)
+	s.log.Printf("legacy hash: %s", legacyMarshalledHash)
+	var tx2 *wire.MsgTx
+
+	// build a serialized tx string which is identical of the previous tx, except for the prevout hash.
+	tx2serialized := "{\"Version\":1,\"TxIn\":[{\"PreviousOutPoint\":{\"Hash\":" + string(legacyMarshalledHash) + ",\"Index\":0},\"SignatureScript\":null,\"Witness\":null,\"Sequence\":4294967295}],\"TxOut\":[{\"Value\":123,\"PkScript\":\"dqkU+tJNI2oRHKW1g4q4XbLEvaq3w76IrA==\"}],\"LockTime\":0}"
+	err = json.Unmarshal([]byte(tx2serialized), &tx2)
+	require.NoError(s.T(), err)
+	s.log.Print("Tx1 hash: " + tx1.TxIn[0].PreviousOutPoint.Hash.String())
+	s.log.Print("Tx2 hash: " + tx2.TxIn[0].PreviousOutPoint.Hash.String())
+
+	// check prevout hashes
+	require.Equal(s.T(), tx1.TxIn[0].PreviousOutPoint.Hash.String(), tx2.TxIn[0].PreviousOutPoint.Hash.String())
+}
+
 // TestSpendableOutputs checks that the utxo set is correct. Only confirmed (or unconfirmed outputs
 // we own) outputs can be spent.
 func (s *transactionsSuite) TestSpendableOutputs() {