mempool: correctly handle tx with oracle response

If tx with the same oracle response ID is already in mempool,
replace it if network fee of added transaction is higher and
return error otherwise.

pkg/core/mempool/mem_pool_test.go

@@ -351,6 +351,59 @@ func TestMempoolItemsOrder(t *testing.T) {
 	require.True(t, item4.CompareTo(item3) < 0)
 }
 
+func TestMempoolAddRemoveOracleResponse(t *testing.T) {
+	mp := New(5)
+	nonce := uint32(0)
+	fs := &FeerStub{}
+	newTx := func(netFee int64, id uint64) *transaction.Transaction {
+		tx := transaction.New(netmode.UnitTestNet, []byte{byte(opcode.PUSH1)}, 0)
+		tx.NetworkFee = netFee
+		tx.Nonce = nonce
+		nonce++
+		tx.Signers = []transaction.Signer{{Account: util.Uint160{1, 2, 3}}}
+		tx.Attributes = []transaction.Attribute{{
+			Type:  transaction.OracleResponseT,
+			Value: &transaction.OracleResponse{ID: id},
+		}}
+		// sanity check
+		_, ok := mp.TryGetValue(tx.Hash())
+		require.False(t, ok)
+		return tx
+	}
+
+	tx1 := newTx(10, 1)
+	require.NoError(t, mp.Add(tx1, fs))
+
+	// smaller network fee
+	tx2 := newTx(5, 1)
+	err := mp.Add(tx2, fs)
+	require.True(t, errors.Is(err, ErrOracleResponse))
+
+	// ok if old tx is removed
+	mp.Remove(tx1.Hash(), fs)
+	require.NoError(t, mp.Add(tx2, fs))
+
+	// higher network fee
+	tx3 := newTx(6, 1)
+	require.NoError(t, mp.Add(tx3, fs))
+	_, ok := mp.TryGetValue(tx2.Hash())
+	require.False(t, ok)
+	_, ok = mp.TryGetValue(tx3.Hash())
+	require.True(t, ok)
+
+	// another oracle response ID
+	tx4 := newTx(4, 2)
+	require.NoError(t, mp.Add(tx4, fs))
+
+	mp.RemoveStale(func(tx *transaction.Transaction) bool {
+		return tx.Hash() != tx4.Hash()
+	}, fs)
+
+	// check that oracle id was removed.
+	tx5 := newTx(3, 2)
+	require.NoError(t, mp.Add(tx5, fs))
+}
+
 func TestMempoolAddRemoveConflicts(t *testing.T) {
 	capacity := 6
 	mp := New(capacity)