routing/payment_lifecycle: return recorded errors

In preparation for MPP we return the terminal errors recorded with the
control tower. The reason is that we cannot return immediately when a
shard fails for MPP, since there might be more shards in flight that we
must wait for. For that reason we instead mark the payment failed in the
control tower, then return this error when we inspect the payment,
seeing it has been failed and there are no shards in flight.

routing/payment_lifecycle.go

@@ -1,7 +1,6 @@
 package routing
 
 import (
-	"fmt"
 	"time"
 
 	"github.com/davecgh/go-spew/spew"
@@ -95,6 +94,12 @@ func (p *paymentLifecycle) resumePayment() ([32]byte, *route.Route, error) {
 			return settle.Settle.Preimage, &settle.Route, nil
 		}
 
+		// If the payment already is failed, and there is no in-flight
+		// HTLC, return immediately.
+		if attempt == nil && payment.FailureReason != nil {
+			return [32]byte{}, nil, *payment.FailureReason
+		}
+
 		// If this payment had no existing payment attempt, we create
 		// and send one now.
 		if attempt == nil {
@@ -113,10 +118,7 @@ func (p *paymentLifecycle) resumePayment() ([32]byte, *route.Route, error) {
 					return [32]byte{}, nil, err
 				}
 
-				errStr := fmt.Sprintf("payment attempt not completed " +
-					"before timeout")
-
-				return [32]byte{}, nil, newErr(ErrPaymentAttemptTimeout, errStr)
+				continue
 
 			// The payment will be resumed from the current state
 			// after restart.
@@ -149,8 +151,7 @@ func (p *paymentLifecycle) resumePayment() ([32]byte, *route.Route, error) {
 					return [32]byte{}, nil, saveErr
 				}
 
-				// Terminal state, return.
-				return [32]byte{}, nil, err
+				continue
 			}
 
 			// With the route in hand, launch a new shard.
@@ -511,7 +512,9 @@ func (p *shardHandler) sendPaymentAttempt(
 }
 
 // handleSendError inspects the given error from the Switch and determines
-// whether we should make another payment attempt.
+// whether we should make another payment attempt, or if it should be
+// considered a terminal error. Terminal errors will be recorded with the
+// control tower.
 func (p *shardHandler) handleSendError(attempt *channeldb.HTLCAttemptInfo,
 	sendErr error) error {
 
@@ -525,19 +528,12 @@ func (p *shardHandler) handleSendError(attempt *channeldb.HTLCAttemptInfo,
 	log.Debugf("Payment %x failed: final_outcome=%v, raw_err=%v",
 		p.paymentHash, *reason, sendErr)
 
-	// Mark the payment failed with no route.
-	//
-	// TODO(halseth): make payment codes for the actual reason we don't
-	// continue path finding.
-	err := p.router.cfg.Control.Fail(
-		p.paymentHash, *reason,
-	)
+	err := p.router.cfg.Control.Fail(p.paymentHash, *reason)
 	if err != nil {
 		return err
 	}
 
-	// Terminal state, return the error we encountered.
-	return sendErr
+	return nil
 }
 
 // failAttempt calls control tower to fail the current payment attempt.