routing/test: add test for stuck payment with in-flight htlcs

Add an additional stuck-payment case, where our payment gets
a terminal error while it has other htlcs in-flight, and a
shard fails with ErrTerminalPayment. This payment also falls in
our class of expected errors, but is not currently handled. The
mock is updated accordingly, using the same ordering as in our
real RegisterAttempt implementation.

routing/mock_test.go

@@ -333,6 +333,10 @@ func (m *mockControlTower) RegisterAttempt(phash lntypes.Hash,
 		return channeldb.ErrPaymentAlreadyFailed
 	}
 
+	if settled || failed {
+		return channeldb.ErrPaymentTerminal
+	}
+
 	// Add attempt to payment.
 	p.attempts = append(p.attempts, channeldb.HTLCAttempt{
 		HTLCAttemptInfo: *a,

routing/payment_lifecycle_test.go

@@ -650,6 +650,56 @@ func TestRouterPaymentStateMachine(t *testing.T) {
 			},
 			paymentErr: channeldb.ErrPaymentAlreadyFailed,
 		},
+		{
+			// A MP payment scenario when our path finding returns
+			// after we've just received a terminal failure, and
+			// we have another shard still in flight.
+			name: "MP shard in flight after terminal",
+
+			steps: []string{
+				routerInitPayment,
+
+				// shard 0
+				routeRelease,
+				routerRegisterAttempt,
+				sendToSwitchSuccess,
+
+				// shard 1
+				routeRelease,
+				routerRegisterAttempt,
+				sendToSwitchSuccess,
+
+				// shard 2
+				routeRelease,
+				routerRegisterAttempt,
+				sendToSwitchSuccess,
+
+				// We find a path for another shard.
+				routeRelease,
+
+				// shard 0 fails with a terminal error.
+				getPaymentResultTerminalFailure,
+				routerFailAttempt,
+				routerFailPayment,
+
+				// We try to register our final shard after
+				// processing a terminal failure.
+				routerRegisterAttempt,
+
+				// Our in-flight shards fail.
+				getPaymentResultTempFailure,
+				getPaymentResultTempFailure,
+				routerFailAttempt,
+				routerFailAttempt,
+
+				// Payment fails.
+				paymentError,
+			},
+			routes: []*route.Route{
+				shard, shard, shard, shard,
+			},
+			paymentErr: channeldb.ErrPaymentTerminal,
+		},
 	}
 
 	for _, test := range tests {