Xunit logger ignores exceptions from output helper (#592)

Author: analogrelay

src/Microsoft.Extensions.Logging.Testing/XunitLoggerProvider.cs

@@ -56,12 +56,12 @@ public void Log<TState>(
             }
             var firstLinePrefix = $"| {_category} {logLevel}: ";
             var lines = formatter(state, exception).Split('\n');
-            _output.WriteLine(firstLinePrefix + lines.First().TrimEnd(NewLineChars));
+            WriteLine(firstLinePrefix + lines.First().TrimEnd(NewLineChars));
 
             var additionalLinePrefix = "|" + new string(' ', firstLinePrefix.Length - 1);
             foreach (var line in lines.Skip(1))
             {
-                _output.WriteLine(additionalLinePrefix + line.TrimEnd(NewLineChars));
+                WriteLine(additionalLinePrefix + line.TrimEnd(NewLineChars));
             }
         }
 
@@ -71,6 +71,21 @@ public bool IsEnabled(LogLevel logLevel)
         public IDisposable BeginScope<TState>(TState state)
             => new NullScope();
 
+        private void WriteLine(string message)
+        {
+            try
+            {
+                _output.WriteLine(message);
+            }
+            catch (Exception)
+            {
+                // We could fail because we're on a background thread and our captured ITestOutputHelper is
+                // busted (if the test "completed" before the background thread fired).
+                // So, ignore this. There isn't really anything we can do but hope the
+                // caller has additional loggers registered
+            }
+        }
+
         private class NullScope : IDisposable
         {
             public void Dispose()

test/Microsoft.Extensions.Logging.Testing.Tests/XunitLoggerProviderTest.cs

@@ -60,19 +60,43 @@ public void LoggerProviderPrependsPrefixToEachLine()
             Assert.Equal(expectedOutput, testTestOutputHelper.Output);
         }
 
+        [Fact]
+        public void LoggerProviderDoesNotThrowIfOutputHelperThrows()
+        {
+            var testTestOutputHelper = new TestTestOutputHelper();
+            var loggerFactory = new LoggerFactory();
+            loggerFactory.AddXunit(testTestOutputHelper);
+            testTestOutputHelper.Throw = true;
+
+            var logger = loggerFactory.CreateLogger("TestCategory");
+            logger.LogInformation("This is a" + Environment.NewLine + "multi-line" + Environment.NewLine + "message");
+
+            Assert.Equal(0, testTestOutputHelper.Output.Length);
+        }
+
         private class TestTestOutputHelper : ITestOutputHelper
         {
             private StringBuilder _output = new StringBuilder();
 
+            public bool Throw { get; set; }
+
             public string Output => _output.ToString();
 
             public void WriteLine(string message)
             {
+                if (Throw)
+                {
+                    throw new Exception("Boom!");
+                }
                 _output.AppendLine(message);
             }
 
             public void WriteLine(string format, params object[] args)
             {
+                if (Throw)
+                {
+                    throw new Exception("Boom!");
+                }
                 _output.AppendLine(string.Format(format, args));
             }
         }