routing+channeldb: make MPPayment into an interface

This commit turns `MPPayment` into an interface inside `routing`. Having
this interface gives us the benefit to write more granular unit tests
inside payment lifecycle. As seen from the modified unit tests, several
hacky ways of testing the `SendPayment` method is now replaced by a mock
over `MPPayment`.

routing/mock_test.go

@@ -491,7 +491,7 @@ func (m *mockControlTowerOld) FailPayment(phash lntypes.Hash,
 }
 
 func (m *mockControlTowerOld) FetchPayment(phash lntypes.Hash) (
-	*channeldb.MPPayment, error) {
+	dbMPPayment, error) {
 
 	m.Lock()
 	defer m.Unlock()
@@ -750,10 +750,8 @@ func (m *mockControlTower) FailPayment(phash lntypes.Hash,
 }
 
 func (m *mockControlTower) FetchPayment(phash lntypes.Hash) (
-	*channeldb.MPPayment, error) {
+	dbMPPayment, error) {
 
-	m.Lock()
-	defer m.Unlock()
 	args := m.Called(phash)
 
 	// Type assertion on nil will fail, so we check and return here.
@@ -761,15 +759,7 @@ func (m *mockControlTower) FetchPayment(phash lntypes.Hash) (
 		return nil, args.Error(1)
 	}
 
-	// Make a copy of the payment here to avoid data race.
-	p := args.Get(0).(*channeldb.MPPayment)
-	payment := &channeldb.MPPayment{
-		Info:          p.Info,
-		FailureReason: p.FailureReason,
-	}
-	payment.HTLCs = make([]channeldb.HTLCAttempt, len(p.HTLCs))
-	copy(payment.HTLCs, p.HTLCs)
-
+	payment := args.Get(0).(*mockMPPayment)
 	return payment, args.Error(1)
 }
 
@@ -794,6 +784,54 @@ func (m *mockControlTower) SubscribeAllPayments() (
 	return args.Get(0).(ControlTowerSubscriber), args.Error(1)
 }
 
+type mockMPPayment struct {
+	mock.Mock
+}
+
+var _ dbMPPayment = (*mockMPPayment)(nil)
+
+func (m *mockMPPayment) GetState() *channeldb.MPPaymentState {
+	args := m.Called()
+	return args.Get(0).(*channeldb.MPPaymentState)
+}
+
+func (m *mockMPPayment) GetStatus() channeldb.PaymentStatus {
+	args := m.Called()
+	return args.Get(0).(channeldb.PaymentStatus)
+}
+
+func (m *mockMPPayment) Terminated() bool {
+	args := m.Called()
+
+	return args.Bool(0)
+}
+
+func (m *mockMPPayment) NeedWaitAttempts() (bool, error) {
+	args := m.Called()
+	return args.Bool(0), args.Error(1)
+}
+
+func (m *mockMPPayment) GetHTLCs() []channeldb.HTLCAttempt {
+	args := m.Called()
+	return args.Get(0).([]channeldb.HTLCAttempt)
+}
+
+func (m *mockMPPayment) InFlightHTLCs() []channeldb.HTLCAttempt {
+	args := m.Called()
+	return args.Get(0).([]channeldb.HTLCAttempt)
+}
+
+func (m *mockMPPayment) GetFailureReason() *channeldb.FailureReason {
+	args := m.Called()
+
+	reason := args.Get(0)
+	if reason == nil {
+		return nil
+	}
+
+	return reason.(*channeldb.FailureReason)
+}
+
 type mockLink struct {
 	htlcswitch.ChannelLink
 	bandwidth         lnwire.MilliSatoshi