transaction: microoptimize Hash()

It doesn't cost much, but it's used _a lot_, so optimizing it makes sense.

name      old time/op    new time/op    delta
TxHash-8    4.89ns ± 5%    0.54ns ± 2%  -88.86%  (p=0.008 n=5+5)

name      old alloc/op   new alloc/op   delta
TxHash-8     0.00B          0.00B          ~     (all equal)

name      old allocs/op  new allocs/op  delta
TxHash-8      0.00           0.00          ~     (all equal)

pkg/core/transaction/transaction.go

@@ -68,6 +68,9 @@ type Transaction struct {
 	// Hash of the transaction (double SHA256).
 	hash util.Uint256
 
+	// Whether hash is correct.
+	hashed bool
+
 	// Trimmed indicates this is a transaction from trimmed
 	// data.
 	Trimmed bool
@@ -78,6 +81,7 @@ type Transaction struct {
 func NewTrimmedTX(hash util.Uint256) *Transaction {
 	return &Transaction{
 		hash:    hash,
+		hashed:  true,
 		Trimmed: true,
 	}
 }
@@ -98,7 +102,7 @@ func New(script []byte, gas int64) *Transaction {
 
 // Hash returns the hash of the transaction.
 func (t *Transaction) Hash() util.Uint256 {
-	if t.hash.Equals(util.Uint256{}) {
+	if !t.hashed {
 		if t.createHash() != nil {
 			panic("failed to compute hash!")
 		}
@@ -172,6 +176,7 @@ func (t *Transaction) decodeHashableFields(br *io.BinReader, buf []byte) {
 	if buf != nil {
 		end = len(buf) - br.Len()
 		t.hash = hash.Sha256(buf[start:end])
+		t.hashed = true
 	}
 }
 
@@ -261,6 +266,7 @@ func (t *Transaction) createHash() error {
 	}
 
 	shaHash.Sum(t.hash[:0])
+	t.hashed = true
 	return nil
 }
 

pkg/core/transaction/transaction_test.go

@@ -308,3 +308,17 @@ func TestTransaction_HasSigner(t *testing.T) {
 	require.True(t, tx.HasSigner(u1))
 	require.False(t, tx.HasSigner(util.Uint160{}))
 }
+
+func BenchmarkTxHash(b *testing.B) {
+	script := []byte{0x51}
+	tx := New(script, 1)
+	tx.NetworkFee = 123
+	tx.Signers = []Signer{{Account: util.Uint160{1, 2, 3}}}
+	tx.Scripts = []Witness{{InvocationScript: []byte{}, VerificationScript: []byte{}}}
+
+	// Prime cache.
+	tx.Hash()
+	for i := 0; i < b.N; i++ {
+		_ = tx.Hash()
+	}
+}