routing/payment_lifeycle: remove payment level attempt and circuit

We replace the cached attempt, and instead use the control tower
(database) to fetch any in-flight attempt. This is done as a
preparation for having multiple attempts in flight.

In addition we remove the cached circuit, as it won't be applicable when
multiple shards are in flight.

Instead of tracking the attemp we consult the database on every
iteration, and pick up any existing attempt. This also let us avoid
having to pass in the existing attempts from the payment loop, as we
just fetch them direclty.

routing/router.go

@@ -533,20 +533,13 @@ func (r *ChannelRouter) Start() error {
 			// result for the in-flight attempt is received.
 			paySession := r.cfg.SessionSource.NewPaymentSessionEmpty()
 
-			// TODO(joostjager): For mpp, possibly relaunch multiple
-			// in-flight htlcs here.
-			var attempt *channeldb.HTLCAttemptInfo
-			if len(payment.Attempts) > 0 {
-				attempt = &payment.Attempts[0]
-			}
-
 			// We pass in a zero timeout value, to indicate we
 			// don't need it to timeout. It will stop immediately
 			// after the existing attempt has finished anyway. We
 			// also set a zero fee limit, as no more routes should
 			// be tried.
 			_, _, err := r.sendPayment(
-				attempt, payment.Info.Value, 0,
+				payment.Info.Value, 0,
 				payment.Info.PaymentHash, 0, paySession,
 			)
 			if err != nil {
@@ -1646,7 +1639,7 @@ func (r *ChannelRouter) SendPayment(payment *LightningPayment) ([32]byte,
 	// Since this is the first time this payment is being made, we pass nil
 	// for the existing attempt.
 	return r.sendPayment(
-		nil, payment.Amount, payment.FeeLimit, payment.PaymentHash,
+		payment.Amount, payment.FeeLimit, payment.PaymentHash,
 		payment.PayAttemptTimeout, paySession,
 	)
 }
@@ -1669,9 +1662,8 @@ func (r *ChannelRouter) SendPaymentAsync(payment *LightningPayment) error {
 			spewPayment(payment))
 
 		_, _, err := r.sendPayment(
-			nil, payment.Amount, payment.FeeLimit,
-			payment.PaymentHash, payment.PayAttemptTimeout,
-			paySession,
+			payment.Amount, payment.FeeLimit, payment.PaymentHash,
+			payment.PayAttemptTimeout, paySession,
 		)
 		if err != nil {
 			log.Errorf("Payment with hash %x failed: %v",
@@ -1770,14 +1762,14 @@ func (r *ChannelRouter) SendToRoute(hash lntypes.Hash, route *route.Route) (
 
 	// We set the timeout to a zero value, indicating the payment shouldn't
 	// timeout. It is only a single attempt, so no more attempts will be
-	// done anyway. Since this is the first time this payment is being
-	// made, we pass nil for the existing attempt.
+	// done anyway.
+	//
 	// We pass the route receiver amount as the total payment amount such
 	// that the payment loop will request a route for this amount. As fee
 	// limit we pass the route's total fees, since we already know this is
 	// the route that is going to be used.
 	preimage, _, err := r.sendPayment(
-		nil, amt, route.TotalFees(), hash, 0, paySession,
+		amt, route.TotalFees(), hash, 0, paySession,
 	)
 	if err != nil {
 		// SendToRoute should return a structured error. In case the
@@ -1815,7 +1807,6 @@ func (r *ChannelRouter) SendToRoute(hash lntypes.Hash, route *route.Route) (
 // router will call this method for every payment still in-flight according to
 // the ControlTower.
 func (r *ChannelRouter) sendPayment(
-	existingAttempt *channeldb.HTLCAttemptInfo,
 	totalAmt, feeLimit lnwire.MilliSatoshi, paymentHash lntypes.Hash,
 	timeout time.Duration,
 	paySession PaymentSession) ([32]byte, *route.Route, error) {
@@ -1836,8 +1827,6 @@ func (r *ChannelRouter) sendPayment(
 		paymentHash:   paymentHash,
 		paySession:    paySession,
 		currentHeight: currentHeight,
-		attempt:       existingAttempt,
-		circuit:       nil,
 		lastError:     nil,
 	}