multi: query mempool spend when a new input is received

This commit changes how a new input sweep request is handled - now we
will query the mempool and see if it's already been spent. If so, we'll
update its state as we may need to RBF this input.

chainntnfs/mocks.go

@@ -0,0 +1,41 @@
+package chainntnfs
+
+import (
+	"github.com/btcsuite/btcd/wire"
+	"github.com/stretchr/testify/mock"
+)
+
+// MockMempoolWatcher is a mock implementation of the MempoolWatcher interface.
+// This is used by other subsystems to mock the behavior of the mempool
+// watcher.
+type MockMempoolWatcher struct {
+	mock.Mock
+}
+
+// NewMockMempoolWatcher returns a new instance of a mock mempool watcher.
+func NewMockMempoolWatcher() *MockMempoolWatcher {
+	return &MockMempoolWatcher{}
+}
+
+// Compile-time check to ensure MockMempoolWatcher implements MempoolWatcher.
+var _ MempoolWatcher = (*MockMempoolWatcher)(nil)
+
+// SubscribeMempoolSpent implements the MempoolWatcher interface.
+func (m *MockMempoolWatcher) SubscribeMempoolSpent(
+	op wire.OutPoint) (*MempoolSpendEvent, error) {
+
+	args := m.Called(op)
+
+	if args.Get(0) == nil {
+		return nil, args.Error(1)
+	}
+
+	return args.Get(0).(*MempoolSpendEvent), args.Error(1)
+}
+
+// CancelMempoolSpendEvent implements the MempoolWatcher interface.
+func (m *MockMempoolWatcher) CancelMempoolSpendEvent(
+	sub *MempoolSpendEvent) {
+
+	m.Called(sub)
+}