routing: add regression test to catch the panic

routing/payment_lifecycle_test.go

@@ -1809,3 +1809,61 @@ func TestHandleAttemptResultSuccess(t *testing.T) {
 	require.NoError(t, err, "expected no error")
 	require.Equal(t, attempt, attemptResult.attempt)
 }
+
+// TestReloadInflightAttemptsLegacy checks that when handling a legacy HTLC
+// attempt, `collectResult` behaves as expected.
+func TestReloadInflightAttemptsLegacy(t *testing.T) {
+	t.Parallel()
+
+	// Create a test paymentLifecycle with the initial two calls mocked.
+	p, m := newTestPaymentLifecycle(t)
+
+	// Mount the resultCollector to check the full call path.
+	p.resultCollector = p.collectResultAsync
+
+	// Create testing params.
+	paymentAmt := 10_000
+	preimage := lntypes.Preimage{1}
+	attempt := makeSettledAttempt(t, paymentAmt, preimage)
+
+	// Make the attempt.Hash to be nil to mock a legacy payment.
+	attempt.Hash = nil
+
+	// Create a mock result returned from the switch.
+	result := &htlcswitch.PaymentResult{
+		Preimage: preimage,
+	}
+
+	// 1. calls `FetchPayment` and return the payment.
+	m.control.On("FetchPayment", p.identifier).Return(m.payment, nil).Once()
+
+	// 2. calls `InFlightHTLCs` and return the attempt.
+	attempts := []channeldb.HTLCAttempt{*attempt}
+	m.payment.On("InFlightHTLCs").Return(attempts).Once()
+
+	// 3. Mock the htlcswitch to return a the result chan.
+	resultChan := make(chan *htlcswitch.PaymentResult, 1)
+	m.payer.On("GetAttemptResult",
+		attempt.AttemptID, p.identifier, mock.Anything,
+	).Return(resultChan, nil).Once().Run(func(args mock.Arguments) {
+		// Send the preimage to the result chan.
+		resultChan <- result
+	})
+
+	// Now call the method under test.
+	payment, err := p.reloadInflightAttempts()
+	require.NoError(t, err)
+	require.Equal(t, m.payment, payment)
+
+	var r *switchResult
+
+	// Assert the result is returned within testTimeout.
+	waitErr := wait.NoError(func() error {
+		r = <-p.resultCollected
+		return nil
+	}, testTimeout)
+	require.NoError(t, waitErr, "timeout waiting for result")
+
+	// Assert the result is received as expected.
+	require.Equal(t, result, r.result)
+}