Fix NullReferenceException with structured logging (#2132)

normj · web-flow · commit 4b7587a3adcd · 2025-09-16T11:44:38.000-07:00
diff --git a/.autover/changes/84836f7b-8e69-44a1-87dc-22971f426990.json b/.autover/changes/84836f7b-8e69-44a1-87dc-22971f426990.json
@@ -0,0 +1,18 @@
+{
+  "Projects": [
+    {
+      "Name": "Amazon.Lambda.RuntimeSupport",
+      "Type": "Patch",
+      "ChangelogMessages": [
+        "Fix NullReferenceException when logging a null logging message with structured logging enabled."
+      ]
+    },
+    {
+      "Name": "Amazon.Lambda.Logging.AspNetCore",
+      "Type": "Patch",
+      "ChangelogMessages": [
+        "Fix NullReferenceException when logging a null logging message with structured logging enabled."
+      ]
+    }	
+  ]
+}
diff --git a/Libraries/src/Amazon.Lambda.Logging.AspNetCore/LambdaILogger.cs b/Libraries/src/Amazon.Lambda.Logging.AspNetCore/LambdaILogger.cs
@@ -70,6 +70,11 @@ public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Except
                     }
                 }
 
+                if (messageTemplate == null)
+                {
+                    messageTemplate = formatter.Invoke(state, exception);
+                }
+
                 Amazon.Lambda.Core.LambdaLogger.Log(lambdaLogLevel, exception, messageTemplate, parameters.ToArray());
             }
             else
diff --git a/Libraries/src/Amazon.Lambda.RuntimeSupport/Helpers/ConsoleLoggerWriter.cs b/Libraries/src/Amazon.Lambda.RuntimeSupport/Helpers/ConsoleLoggerWriter.cs
@@ -401,7 +401,7 @@ internal void FormattedWriteLine(string level, Exception exeception, string mess
 
                     var messageState = new MessageState();
 
-                    messageState.MessageTemplate = messageTemplate;
+                    messageState.MessageTemplate = messageTemplate ?? string.Empty;
                     messageState.MessageArguments = args;
                     messageState.TimeStamp = DateTime.UtcNow;
                     messageState.AwsRequestId = CurrentAwsRequestId;
diff --git a/Libraries/src/Amazon.Lambda.RuntimeSupport/Helpers/Logging/AbstractLogMessageFormatter.cs b/Libraries/src/Amazon.Lambda.RuntimeSupport/Helpers/Logging/AbstractLogMessageFormatter.cs
@@ -1,4 +1,4 @@
-﻿#if NET6_0_OR_GREATER
+#if NET6_0_OR_GREATER
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
@@ -285,4 +285,4 @@ public bool UsingPositionalArguments(IReadOnlyList<MessageProperty> messagePrope
         }
     }
 }
-#endif
+#endif
diff --git a/Libraries/src/Amazon.Lambda.RuntimeSupport/Helpers/Logging/JsonLogMessageFormatter.cs b/Libraries/src/Amazon.Lambda.RuntimeSupport/Helpers/Logging/JsonLogMessageFormatter.cs
@@ -1,4 +1,4 @@
-﻿#if NET6_0_OR_GREATER
+#if NET6_0_OR_GREATER
 using System;
 using System.Buffers;
 using System.Collections;
@@ -48,15 +48,15 @@ public override string FormatMessage(MessageState state)
             if (state.MessageArguments?.Length == 0)
             {
                 messageProperties = _emptyMessageProperties;
-                message = state.MessageTemplate;
+                message = state.MessageTemplate ?? string.Empty;
             }
             else
             {
                 // Parse the message template for any message properties like "{count}".
-                messageProperties = ParseProperties(state.MessageTemplate);
+                messageProperties = ParseProperties(state.MessageTemplate ?? string.Empty);
 
                 // Replace any message properties in the message template with the provided argument values.
-                message = ApplyMessageProperties(state.MessageTemplate, messageProperties, state.MessageArguments);
+                message = ApplyMessageProperties(state.MessageTemplate ?? string.Empty, messageProperties, state.MessageArguments);
             }
 
 
@@ -310,4 +310,4 @@ private void FormatJsonValue(Utf8JsonWriter writer, object value, string formatA
         private static string ToInvariantString(object obj) => Convert.ToString(obj, CultureInfo.InvariantCulture);
     }
 }
-#endif
+#endif
diff --git a/Libraries/test/Amazon.Lambda.Logging.AspNetCore.Tests/LoggingTests.cs b/Libraries/test/Amazon.Lambda.Logging.AspNetCore.Tests/LoggingTests.cs
@@ -6,6 +6,7 @@
 using System.IO;
 using System.Linq;
 using System.Reflection;
+using System.Text;
 using Xunit;
 
 namespace Amazon.Lambda.Tests
@@ -581,6 +582,46 @@ public void TestJSONParameterLogging()
 
         }
 
+        [Fact]
+        public void JsonLoggingWithNoOriginalFormat()
+        {
+            Environment.SetEnvironmentVariable("AWS_LAMBDA_LOG_FORMAT", "JSON");
+            try
+            {
+                using (var writer = new StringWriter())
+                {
+                    ConnectLoggingActionToLogger(message => writer.Write(message));
+
+                    var configuration = new ConfigurationBuilder()
+                        .AddJsonFile(GetAppSettingsPath("appsettings.json"))
+                        .Build();
+
+                    var loggerOptions = new LambdaLoggerOptions(configuration);
+                    var loggerFactory = new TestLoggerFactory()
+                        .AddLambdaLogger(loggerOptions);
+
+                    var logger = loggerFactory.CreateLogger("JSONLogging");
+
+                    logger.Log(LogLevel.Error, new EventId(1), new Dictionary<string, object>() { { "Param1", "Value1" } }, null, (state, e) =>
+                    {
+                        var sb = new StringBuilder();
+                        foreach(var kvp in state)
+                        {
+                            sb.AppendFormat("{0}:{1}\n", kvp.Key, kvp.Value);
+                        }
+                        return sb.ToString();
+                    });
+
+                    var text = writer.ToString();
+                    Assert.Contains("Param1:Value1", text);
+                }
+            }
+            finally
+            {
+                Environment.SetEnvironmentVariable("AWS_LAMBDA_LOG_FORMAT", null);
+            }
+        }
+
         private static string GetAppSettingsPath(string fileName)
 		{
 			return Path.Combine(APPSETTINGS_DIR, fileName);
@@ -609,7 +650,7 @@ private static void ConnectLoggingActionToLogger(Action<string> loggingAction)
             loggingWithLevelActionField.SetValue(null, loggingWithLevelAction);
 
             Action<string, Exception, string, object[]> loggingWithExceptionLevelAction = (level, exception, message, parameters) => {
-                var formattedMessage = $"{level}: {message}: parameter count: {parameters?.Length}\n{exception.Message}";
+                var formattedMessage = $"{level}: {message}: parameter count: {parameters?.Length}\n{exception?.Message}";
                 loggingAction(formattedMessage);
             };
 
diff --git a/Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/LogMessageFormatterTests.cs b/Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/LogMessageFormatterTests.cs
@@ -1,4 +1,4 @@
-﻿using Amazon.Lambda.RuntimeSupport.Helpers;
+using Amazon.Lambda.RuntimeSupport.Helpers;
 using Amazon.Lambda.RuntimeSupport.Helpers.Logging;
 using Microsoft.VisualStudio.TestPlatform.CommunicationUtilities;
 using System;
@@ -103,6 +103,41 @@ public void FormatJsonWithStringMessageProperties()
             Assert.Equal("Hello AWS", doc.RootElement.GetProperty("message").GetString());
         }
 
+        [Fact]
+        public void FormatJsonNullMessageTemplate()
+        {
+            var timestamp = DateTime.UtcNow;
+            var formatter = new JsonLogMessageFormatter();
+            var state = new MessageState()
+            {
+                MessageTemplate = null,
+                AwsRequestId = "1234",
+                Level = Helpers.LogLevelLoggerWriter.LogLevel.Warning,
+                TimeStamp = timestamp
+            };
+
+            var json = formatter.FormatMessage(state);
+            var doc = JsonDocument.Parse(json);
+            Assert.Equal(string.Empty, doc.RootElement.GetProperty("message").GetString());
+            Assert.Equal("1234", doc.RootElement.GetProperty("requestId").GetString());
+
+            // Currently the arguments are ignored because they don't exist in the message template but 
+            // having arguments uses a different code path so we need to make sure a NPE doesn't happen.
+            state = new MessageState()
+            {
+                MessageTemplate = null,
+                MessageArguments = new object[] { true },
+                AwsRequestId = "1234",
+                Level = Helpers.LogLevelLoggerWriter.LogLevel.Warning,
+                TimeStamp = timestamp
+            };
+
+            json = formatter.FormatMessage(state);
+            doc = JsonDocument.Parse(json);
+            Assert.Equal(string.Empty, doc.RootElement.GetProperty("message").GetString());
+            Assert.Equal("1234", doc.RootElement.GetProperty("requestId").GetString());
+        }
+
         [Fact]
         public void FormatJsonWithAllPossibleTypes()
         {

Original file line number	Diff line number	Diff line change
`@@ -70,6 +70,11 @@ public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Except`
`70`	`70`	`}`
`71`	`71`	`}`
`72`	`72`
	`73`	`+ if (messageTemplate == null)`
	`74`	`+ {`
	`75`	`+ messageTemplate = formatter.Invoke(state, exception);`
	`76`	`+ }`
	`77`	`+`
`73`	`78`	`Amazon.Lambda.Core.LambdaLogger.Log(lambdaLogLevel, exception, messageTemplate, parameters.ToArray());`
`74`	`79`	`}`
`75`	`80`	`else`
Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-#if NET6_0_OR_GREATER`
	`1`	`+#if NET6_0_OR_GREATER`
`2`	`2`	`using System;`
`3`	`3`	`using System.Collections.Concurrent;`
`4`	`4`	`using System.Collections.Generic;`
`@@ -285,4 +285,4 @@ public bool UsingPositionalArguments(IReadOnlyList<MessageProperty> messagePrope`
`285`	`285`	`}`
`286`	`286`	`}`
`287`	`287`	`}`
`288`		`-#endif`
	`288`	`+#endif`