Fix invoice expiry calculation to accommodate retry windows in RebalanceService

src/Services/RebalanceService.cs

@@ -183,7 +183,7 @@ public async Task<Rebalance> ExecuteAsync(int rebalanceId, CancellationToken ct
         {
             var memo = $"NG rebalance #{rebalance.Id} attempt {rebalance.AttemptNumber}";
             var invoice = await _lightningService.AddInvoiceAsync(node, rebalance.SatsAmount, memo,
-                rebalance.TimeoutSeconds + 60);
+                ComputeInvoiceExpirySeconds(rebalance));
             if (invoice == null || string.IsNullOrEmpty(invoice.PaymentRequest))
             {
                 rebalance.Status = RebalanceStatus.Failed;
@@ -324,6 +324,30 @@ private static double ResolveMaxFeePct(double? userSupplied, decimal defaultPct)
     private static long ComputeFeeLimitMsat(long satsAmount, double maxFeePct)
         => (long)Math.Round(satsAmount * (decimal)maxFeePct * 10m, MidpointRounding.AwayFromZero);
 
+    /// <summary>
+    /// Sizes the self-invoice's expiry to outlive the full retry window — per-attempt
+    /// payment timeout + every backoff gap between retries + a safety buffer. The probe is
+    /// unbounded and an exponential backoff (60s → 120s → 240s) can push later attempts
+    /// well past a short expiry; matching the retry budget guarantees the invoice is still
+    /// honoured by LND when SendPaymentV2 finally fires.
+    /// </summary>
+    private static long ComputeInvoiceExpirySeconds(Rebalance rebalance)
+    {
+        var maxAttempts = Math.Max(1, rebalance.MaxAttempts ?? Constants.REBALANCE_MAX_ATTEMPTS);
+        var initialDelay = Constants.REBALANCE_INITIAL_RETRY_DELAY_SECONDS;
+        var multiplier = Constants.REBALANCE_RETRY_BACKOFF_MULTIPLIER;
+
+        // Mirrors ScheduleRetryIfEligibleAsync: delay between attempt N and N+1 is
+        // initialDelay * multiplier^(N-1). Sum across all retries we might schedule.
+        double totalBackoffSeconds = 0;
+        for (var i = 0; i < maxAttempts - 1; i++)
+        {
+            totalBackoffSeconds += initialDelay * Math.Pow(multiplier, i);
+        }
+
+        return rebalance.TimeoutSeconds + (long)totalBackoffSeconds + 60;
+    }
+
 
     internal static void ApplyTerminalPayment(Rebalance rebalance, Payment payment)
     {

test/NodeGuard.Tests/Services/RebalanceServiceTests.cs

@@ -286,6 +286,63 @@ public async Task ExecuteAsync_ProbeSucceeds_PaymentSucceeds_StatusSucceeded()
         result.PreimageHex.Should().Be("deadbeef");
     }
 
+    [Fact]
+    public async Task ExecuteAsync_InvoiceExpiryCoversFullRetryWindowPlusBuffer()
+    {
+        // Invoice expiry must outlive the worst-case retry timeline so a delayed attempt
+        // still has a live invoice when SendPaymentV2 fires. With defaults that's
+        // TimeoutSeconds + (initial + initial*mult + ... over MaxAttempts-1 retries) + buffer.
+        var node = CreateNode();
+        _nodeRepo.Setup(x => x.GetById(node.Id, It.IsAny<bool>())).ReturnsAsync(node);
+        StubRepoForCapture();
+
+        long capturedExpiry = 0;
+        _lightning.Setup(x => x.AddInvoiceAsync(node, It.IsAny<long>(), It.IsAny<string>(), It.IsAny<long>()))
+            .Callback<Node, long, string, long>((_, _, _, expiry) => capturedExpiry = expiry)
+            .ReturnsAsync(new AddInvoiceResponse { PaymentRequest = "lnbc..." });
+        _lightning.Setup(x => x.ProbeRouteAsync(node, It.IsAny<long>(), It.IsAny<long>(),
+                It.IsAny<ulong?>(), It.IsAny<string?>(), It.IsAny<double>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new ProbeResult.NoRoute("stop"));
+
+        var service = CreateService();
+        // TimeoutSeconds=60, MaxAttempts=3 → backoff = 60 + 120 = 180 → 60 + 180 + buffer.
+        var request = new RebalanceRequest(node.Id, null, null, 100_000, MaxFeePct: 0.05,
+            TimeoutSeconds: 60, MaxAttempts: 3);
+
+        await service.RebalanceAsync(request);
+
+        long expected = 60
+            + Constants.REBALANCE_INITIAL_RETRY_DELAY_SECONDS
+            + (long)(Constants.REBALANCE_INITIAL_RETRY_DELAY_SECONDS * Constants.REBALANCE_RETRY_BACKOFF_MULTIPLIER)
+            + 60;
+        capturedExpiry.Should().Be(expected);
+        capturedExpiry.Should().BeGreaterThan(60 + 60, "old TimeoutSeconds+60 expiry was demonstrably too short");
+    }
+
+    [Fact]
+    public async Task ExecuteAsync_InvoiceExpiry_MaxAttempts1_OmitsBackoffSum()
+    {
+        var node = CreateNode();
+        _nodeRepo.Setup(x => x.GetById(node.Id, It.IsAny<bool>())).ReturnsAsync(node);
+        StubRepoForCapture();
+
+        long capturedExpiry = 0;
+        _lightning.Setup(x => x.AddInvoiceAsync(node, It.IsAny<long>(), It.IsAny<string>(), It.IsAny<long>()))
+            .Callback<Node, long, string, long>((_, _, _, expiry) => capturedExpiry = expiry)
+            .ReturnsAsync(new AddInvoiceResponse { PaymentRequest = "lnbc..." });
+        _lightning.Setup(x => x.ProbeRouteAsync(node, It.IsAny<long>(), It.IsAny<long>(),
+                It.IsAny<ulong?>(), It.IsAny<string?>(), It.IsAny<double>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new ProbeResult.NoRoute("stop"));
+
+        var service = CreateService();
+        var request = new RebalanceRequest(node.Id, null, null, 100_000, MaxFeePct: 0.05,
+            TimeoutSeconds: 45, MaxAttempts: 1);
+
+        await service.RebalanceAsync(request);
+
+        capturedExpiry.Should().Be(45 + 60);
+    }
+
     [Fact]
     public async Task ExecuteAsync_PersistsPaymentHashHexBeforePayment()
     {