From ce13d92371f3913746a0ff8cbbf204e1779cc2aa Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Sat, 3 Oct 2026 14:06:18 +0200
Subject: [PATCH] CachedUntil - Only reuse cached value within its own period,
 and no overflow for the last period

---
 .../Wrappers/CachedLayoutRendererWrapper.cs   | 17 ++++----
 .../LayoutRenderers/Wrappers/CachedTests.cs   | 40 +++++++++++++++++++
 2 files changed, 48 insertions(+), 9 deletions(-)

diff --git a/src/NLog/LayoutRenderers/Wrappers/CachedLayoutRendererWrapper.cs b/src/NLog/LayoutRenderers/Wrappers/CachedLayoutRendererWrapper.cs
index 7e2cdd1..d6cb07e 100644
--- a/src/NLog/LayoutRenderers/Wrappers/CachedLayoutRendererWrapper.cs
+++ b/src/NLog/LayoutRenderers/Wrappers/CachedLayoutRendererWrapper.cs
@@ -88,6 +88,7 @@ namespace NLog.LayoutRenderers.Wrappers
         private string? _renderedCacheKey;
         private DateTime _cachedValueExpires;
         private TimeSpan? _cachedValueTimeout;
+        private DateTime _cachedValuePeriodStart;
         private DateTime _cachedValuePeriodEnd;
         private CachedUntilOption _cachedUntil;
 
@@ -186,7 +187,7 @@ namespace NLog.LayoutRenderers.Wrappers
                             if (_cachedValueTimeout.HasValue)
                                 _cachedValueExpires = logEvent.TimeStamp + _cachedValueTimeout.Value;
                             if (_cachedUntil != CachedUntilOption.None)
-                                _cachedValuePeriodEnd = CalculatePeriodEnd(_cachedUntil, logEvent.TimeStamp);
+                                UpdateCachedValuePeriod(logEvent.TimeStamp);
                         }
                     }
                 }
@@ -207,20 +208,18 @@ namespace NLog.LayoutRenderers.Wrappers
             if (_cachedValueTimeout.HasValue && logEvent.TimeStamp > _cachedValueExpires)
                 return null;
 
-            if (_cachedUntil != CachedUntilOption.None && logEvent.TimeStamp >= _cachedValuePeriodEnd)
+            if (_cachedUntil != CachedUntilOption.None && (logEvent.TimeStamp < _cachedValuePeriodStart || logEvent.TimeStamp > _cachedValuePeriodEnd))
                 return null;
 
             return _cachedValue;
         }
 
-        private static DateTime CalculatePeriodEnd(CachedUntilOption cachedUntil, DateTime timeStamp)
+        private void UpdateCachedValuePeriod(DateTime timeStamp)
         {
-            switch (cachedUntil)
-            {
-                case CachedUntilOption.DayChange: return new DateTime(timeStamp.Year, timeStamp.Month, timeStamp.Day, 0, 0, 0, timeStamp.Kind).AddDays(1);
-                case CachedUntilOption.HourChange: return new DateTime(timeStamp.Year, timeStamp.Month, timeStamp.Day, timeStamp.Hour, 0, 0, timeStamp.Kind).AddHours(1);
-                default: return DateTime.MaxValue;
-            }
+            var periodTicks = _cachedUntil == CachedUntilOption.DayChange ? TimeSpan.TicksPerDay : TimeSpan.TicksPerHour;
+            _cachedValuePeriodStart = new DateTime(timeStamp.Ticks - timeStamp.Ticks % periodTicks, timeStamp.Kind);
+            // Inclusive end, so the last period ends at DateTime.MaxValue without overflow
+            _cachedValuePeriodEnd = _cachedValuePeriodStart.AddTicks(periodTicks - 1);
         }
 
         string? IStringValueRenderer.GetFormattedString(LogEventInfo logEvent) => Cached ? RenderInner(logEvent) : null;
diff --git a/tests/NLog.UnitTests/LayoutRenderers/Wrappers/CachedTests.cs b/tests/NLog.UnitTests/LayoutRenderers/Wrappers/CachedTests.cs
index 752668c..66a7661 100644
--- a/tests/NLog.UnitTests/LayoutRenderers/Wrappers/CachedTests.cs
+++ b/tests/NLog.UnitTests/LayoutRenderers/Wrappers/CachedTests.cs
@@ -152,5 +152,45 @@ namespace NLog.UnitTests.LayoutRenderers.Wrappers
             var s3 = l.Render(new LogEventInfo() { TimeStamp = timeStamp.AddHours(6.5) });
             Assert.NotEqual(s2, s3);
         }
+
+        [Fact]
+        public void CachedUntilHourChangeEarlierTimeStampTest()
+        {
+            SimpleLayout l = "${guid:cachedUntil=HourChange}";
+            var timeStamp = new DateTime(2021, 2, 27, 17, 30, 0);
+            var s1 = l.Render(new LogEventInfo() { TimeStamp = timeStamp });
+            var s2 = l.Render(new LogEventInfo() { TimeStamp = timeStamp.AddMinutes(30) });
+            Assert.NotEqual(s1, s2);
+            var s3 = l.Render(new LogEventInfo() { TimeStamp = timeStamp.AddMinutes(15) });
+            Assert.NotEqual(s2, s3);
+            var s4 = l.Render(new LogEventInfo() { TimeStamp = timeStamp.AddMinutes(-30) });
+            Assert.Equal(s3, s4);
+        }
+
+        [Fact]
+        public void CachedUntilDayChangeEarlierTimeStampTest()
+        {
+            SimpleLayout l = "${guid:cachedUntil=DayChange}";
+            var timeStamp = new DateTime(2021, 2, 27, 17, 30, 0);
+            var s1 = l.Render(new LogEventInfo() { TimeStamp = timeStamp });
+            var s2 = l.Render(new LogEventInfo() { TimeStamp = timeStamp.AddHours(7) });
+            Assert.NotEqual(s1, s2);
+            var s3 = l.Render(new LogEventInfo() { TimeStamp = timeStamp.AddHours(6) });
+            Assert.NotEqual(s2, s3);
+            var s4 = l.Render(new LogEventInfo() { TimeStamp = timeStamp.AddHours(-17.5) });
+            Assert.Equal(s3, s4);
+        }
+
+        [Theory]
+        [InlineData("HourChange")]
+        [InlineData("DayChange")]
+        public void CachedUntilLastPeriodTest(string cachedUntil)
+        {
+            SimpleLayout l = "${guid:cachedUntil=" + cachedUntil + "}";
+            var s1 = l.Render(new LogEventInfo() { TimeStamp = DateTime.MaxValue.AddMinutes(-30) });
+            var s2 = l.Render(new LogEventInfo() { TimeStamp = DateTime.MaxValue });
+            Assert.NotEmpty(s1);
+            Assert.Equal(s1, s2);
+        }
     }
 }