From 060f22e58b26af431e5c7ab2ee2ae78a2ccfff31 Mon Sep 17 00:00:00 2001
From: "Jeremy D. Miller" <jeremydmiller@yahoo.com>
Date: Fri, 5 Jun 2026 13:13:55 -0500
Subject: [PATCH] Fix InvalidOperationException in EventPage.CalculateCeiling
 on empty page

When error handling is configured to skip events and every event in a full
batch is skipped, the EventPage is empty (Count == 0) while
Count + skippedEvents == batchSize. CalculateCeiling then called Last() on the
empty page and threw "Sequence contains no elements", crashing projection
rebuilds. Guard the Last() access with Count > 0 and fall back to the high
water mark so the daemon keeps making progress.

Fixes https://github.com/JasperFx/marten/issues/4663

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 src/EventTests/Projections/EventPageTests.cs | 48 ++++++++++++++++++++
 src/JasperFx.Events/Projections/EventPage.cs |  7 ++-
 2 files changed, 54 insertions(+), 1 deletion(-)
 create mode 100644 src/EventTests/Projections/EventPageTests.cs

diff --git a/src/EventTests/Projections/EventPageTests.cs b/src/EventTests/Projections/EventPageTests.cs
new file mode 100644
index 00000000..a0311078
--- /dev/null
+++ b/src/EventTests/Projections/EventPageTests.cs
@@ -0,0 +1,48 @@
+using JasperFx.Events;
+using JasperFx.Events.Projections;
+using Shouldly;
+
+namespace EventTests.Projections;
+
+public class EventPageTests
+{
+    [Fact]
+    public void calculate_ceiling_when_page_is_full_uses_last_sequence()
+    {
+        var page = new EventPage(0)
+        {
+            new Event<AEvent>(new AEvent()) { Sequence = 4 },
+            new Event<AEvent>(new AEvent()) { Sequence = 5 }
+        };
+
+        page.CalculateCeiling(2, 1000);
+
+        page.Ceiling.ShouldBe(5);
+    }
+
+    [Fact]
+    public void calculate_ceiling_when_page_is_not_full_uses_high_water_mark()
+    {
+        var page = new EventPage(0)
+        {
+            new Event<AEvent>(new AEvent()) { Sequence = 4 }
+        };
+
+        page.CalculateCeiling(10, 1000);
+
+        page.Ceiling.ShouldBe(1000);
+    }
+
+    [Fact]
+    public void calculate_ceiling_when_full_batch_was_entirely_skipped_does_not_throw()
+    {
+        // Reproduces https://github.com/JasperFx/marten/issues/4663 -- every event in a
+        // full batch was skipped, so the page is empty. CalculateCeiling must not call
+        // Last() on the empty page; it should fall back to the high water mark.
+        var page = new EventPage(0);
+
+        Should.NotThrow(() => page.CalculateCeiling(10, 1000, skippedEvents: 10));
+
+        page.Ceiling.ShouldBe(1000);
+    }
+}
diff --git a/src/JasperFx.Events/Projections/EventPage.cs b/src/JasperFx.Events/Projections/EventPage.cs
index a5d2629d..0d461047 100644
--- a/src/JasperFx.Events/Projections/EventPage.cs
+++ b/src/JasperFx.Events/Projections/EventPage.cs
@@ -14,7 +14,12 @@ public EventPage(long floor)
 
     public void CalculateCeiling(int batchSize, long highWaterMark, int skippedEvents = 0)
     {
-        Ceiling = (Count + skippedEvents) == batchSize
+        // Count == 0 happens when every event in a full batch was skipped (e.g. error
+        // handling is configured to skip serialization/application/unknown event errors).
+        // There is no last event to read a sequence from, so fall back to the high water
+        // mark and let the daemon make progress rather than throwing on an empty page. See
+        // https://github.com/JasperFx/marten/issues/4663
+        Ceiling = (Count + skippedEvents) == batchSize && Count > 0
             ? this.Last().Sequence
             : highWaterMark;
     }