Include a large event body sample when generating oversized event placeholders

src/Serilog.Sinks.Seq/Sinks/Seq/ConstrainedBufferedFormatter.cs

@@ -107,9 +107,8 @@ static bool CheckEventBodySize(string jsonLine, long? eventBodyLimitBytes)
 
         static LogEvent CreateOversizeEventPlaceholder(LogEvent logEvent, string jsonLine, long eventBodyLimitBytes)
         {
-            // If the limit is so constrained as to disallow sending 512 bytes + packaging, that's okay - we'll just drop
-            // the placeholder, too.
-            var sample = jsonLine.Substring(0, Math.Min(jsonLine.Length, 512));
+            var sampleLength = GetOversizeEventSampleLength(eventBodyLimitBytes);
+            var sample = jsonLine.Substring(0, Math.Min(jsonLine.Length, (int)sampleLength));
             return new LogEvent(
                 logEvent.Timestamp,
                 LogEventLevel.Error,
@@ -121,5 +120,22 @@ static LogEvent CreateOversizeEventPlaceholder(LogEvent logEvent, string jsonLin
                     new LogEventProperty("EventBodySample", new ScalarValue(sample)),
                 });
         }
+
+        internal static long GetOversizeEventSampleLength(long eventBodyLimitBytes)
+        {
+            // A quick estimate of how much of the original event payload we should send along with the oversized event
+            // placeholder. If the limit is so constrained as to disallow sending the sample, that's okay - we'll
+            // just drop the placeholder, too.
+
+            // In reality the timestamp and other envelope components won't be anything close to this.
+            const long packagingAllowance = 2048;
+            var byteBudget = eventBodyLimitBytes - packagingAllowance;
+
+            // Allow for multibyte characters and JSON escape sequences.
+            var withEncoding = byteBudget / 2;
+
+            const long minimumSampleSize = 512;
+            return Math.Max(withEncoding, minimumSampleSize);
+        }
     }
 }

test/Serilog.Sinks.Seq.Tests/ConstrainedBufferedFormatterTests.cs

@@ -40,5 +40,19 @@ public void PlaceholdersAreLoggedWhenTheEventSizeLimitIsExceeded()
             Assert.Contains("\"EventBodySample\"", jsonString);
             Assert.Contains("aaaaa", jsonString);
         }
+
+        [Theory]
+        [InlineData(0, 512)]
+        [InlineData(1, 512)]
+        [InlineData(512, 512)]
+        [InlineData(1000, 512)]
+        [InlineData(5000, 1476)]
+        [InlineData(10000, 3976)]
+        [InlineData(130048, 64000)]
+        public void PlaceholderSampleSizeIsComputedFromEventBodyLimitBytes(long eventBodyLimitBytes, long expectedSampleSize)
+        {
+            var actual = ConstrainedBufferedFormatter.GetOversizeEventSampleLength(eventBodyLimitBytes);
+            Assert.Equal(expectedSampleSize, actual);
+        }
     }
 }