Fix nil pointer dereference when WitnessHash is called before HasWitness

Issue: btcsuite/btcd#1543
Related PR: btcsuite/btcutil#156

block_test.go

@@ -145,10 +145,10 @@ func TestBlock(t *testing.T) {
 
 	// Transaction offsets and length for the transaction in Block100000.
 	wantTxLocs := []wire.TxLoc{
-		{TxStart: 81, TxLen: 135},
-		{TxStart: 216, TxLen: 259},
-		{TxStart: 475, TxLen: 257},
-		{TxStart: 732, TxLen: 225},
+		{TxStart: 81, TxLen: 144},
+		{TxStart: 225, TxLen: 259},
+		{TxStart: 484, TxLen: 257},
+		{TxStart: 741, TxLen: 225},
 	}
 
 	// Ensure the transaction location information is accurate.
@@ -333,6 +333,10 @@ var Block100000 = wire.MsgBlock{
 						0x04, 0x4c, 0x86, 0x04, 0x1b, 0x02, 0x06, 0x02,
 					},
 					Sequence: 0xffffffff,
+					Witness: [][]byte{
+						{0x04, 0x31},
+						{0x01, 0x43},
+					},
 				},
 			},
 			TxOut: []*wire.TxOut{

tx.go

@@ -70,7 +70,7 @@ func (t *Tx) WitnessHash() *chainhash.Hash {
 // HasWitness on the underlying wire.MsgTx, however it caches the result so
 // subsequent calls are more efficient.
 func (t *Tx) HasWitness() bool {
-	if t.txHashWitness != nil {
+	if t.txHasWitness != nil {
 		return *t.txHasWitness
 	}
 

tx_test.go

@@ -95,3 +95,42 @@ func TestTxErrors(t *testing.T) {
 			"got %v, want %v", err, io.EOF)
 	}
 }
+
+// TestTxHasWitness tests the HasWitness() method.
+func TestTxHasWitness(t *testing.T) {
+	msgTx := Block100000.Transactions[0] // contains witness data
+	tx := btcutil.NewTx(msgTx)
+
+	tx.WitnessHash() // Populate the witness hash cache
+	tx.HasWitness()  // Should not fail (see btcsuite/btcd#1543)
+
+	if !tx.HasWitness() {
+		t.Errorf("HasWitness: got false, want true")
+	}
+
+	for _, msgTxWithoutWitness := range Block100000.Transactions[1:] {
+		txWithoutWitness := btcutil.NewTx(msgTxWithoutWitness)
+		if txWithoutWitness.HasWitness() {
+			t.Errorf("HasWitness: got false, want true")
+		}
+	}
+}
+
+// TestTxWitnessHash tests the WitnessHash() method.
+func TestTxWitnessHash(t *testing.T) {
+	msgTx := Block100000.Transactions[0] // contains witness data
+	tx := btcutil.NewTx(msgTx)
+
+	if tx.WitnessHash().IsEqual(tx.Hash()) {
+		t.Errorf("WitnessHash: witness hash and tx id must NOT be same - "+
+			"got %v, want %v", tx.WitnessHash(), tx.Hash())
+	}
+
+	for _, msgTxWithoutWitness := range Block100000.Transactions[1:] {
+		txWithoutWitness := btcutil.NewTx(msgTxWithoutWitness)
+		if !txWithoutWitness.WitnessHash().IsEqual(txWithoutWitness.Hash()) {
+			t.Errorf("WitnessHash: witness hash and tx id must be same - "+
+				"got %v, want %v", txWithoutWitness.WitnessHash(), txWithoutWitness.Hash())
+		}
+	}
+}