router: make sendToRoute omit payment lifecycle

Instead of having SendToRoute pull routes from the payment session in
the payment lifecycle, we utilize the new methods on the paymentShard to
launch and collect the result for this single route.

This also let us remove the check for noRouteError, as we will always
have the result from the tried attempt returned. A result of this is
that we can finally remove lastError from the payment lifecycle (see
next commits).

routing/router.go

@@ -1734,9 +1734,6 @@ func (r *ChannelRouter) preparePayment(payment *LightningPayment) (
 func (r *ChannelRouter) SendToRoute(hash lntypes.Hash, route *route.Route) (
 	lntypes.Preimage, error) {
 
-	// Create a payment session for just this route.
-	paySession := r.cfg.SessionSource.NewPaymentSessionForRoute(route)
-
 	// Calculate amount paid to receiver.
 	amt := route.ReceiverAmt()
 
@@ -1760,34 +1757,57 @@ 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.
-	//
-	// 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(
-		amt, route.TotalFees(), hash, 0, paySession,
-	)
+	// Launch a shard along the given route.
+	sh := &shardHandler{
+		router:      r,
+		paymentHash: hash,
+	}
+
+	var shardError error
+	attempt, outcome, err := sh.launchShard(route)
 	if err != nil {
-		// SendToRoute should return a structured error. In case the
-		// provided route fails, payment lifecycle will return a
-		// noRouteError with the structured error embedded.
-		if noRouteError, ok := err.(errNoRoute); ok {
-			if noRouteError.lastError == nil {
-				return lntypes.Preimage{},
-					errors.New("failure message missing")
-			}
-
-			return lntypes.Preimage{}, noRouteError.lastError
-		}
-
 		return lntypes.Preimage{}, err
 	}
 
-	return preimage, nil
+	switch {
+	// Failed to launch shard.
+	case outcome.err != nil:
+		shardError = outcome.err
+
+	// Shard successfully launched, wait for the result to be available.
+	default:
+		result, err := sh.collectResult(attempt)
+		if err != nil {
+			return lntypes.Preimage{}, err
+		}
+
+		// We got a successful result.
+		if result.err == nil {
+			return result.preimage, nil
+		}
+
+		// The shard failed, break switch to handle it.
+		shardError = result.err
+	}
+
+	// Since for SendToRoute we won't retry in case the shard fails, we'll
+	// mark the payment failed with the control tower immediately. Process
+	// the error to check if it maps into a terminal error code, if not use
+	// a generic NO_ROUTE error.
+	reason := r.processSendError(
+		attempt.AttemptID, &attempt.Route, shardError,
+	)
+	if reason == nil {
+		r := channeldb.FailureReasonNoRoute
+		reason = &r
+	}
+
+	err = r.cfg.Control.Fail(hash, *reason)
+	if err != nil {
+		return lntypes.Preimage{}, err
+	}
+
+	return lntypes.Preimage{}, shardError
 }
 
 // sendPayment attempts to send a payment to the passed payment hash. This