Support ILoggingFailureListener and fallback sinks

src/Serilog.Sinks.Async/Serilog.Sinks.Async.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <Description>Asynchronous sink wrapper for Serilog.</Description>
-    <VersionPrefix>2.0.1</VersionPrefix>
+    <VersionPrefix>2.1.0</VersionPrefix>
     <Authors>Jezz Santos;Serilog Contributors</Authors>
     <!-- .NET Framework version targeting is frozen at these two TFMs. -->
     <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT'">net471;net462</TargetFrameworks>
@@ -29,7 +29,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Serilog" Version="4.0.0" />
+    <PackageReference Include="Serilog" Version="4.1.0-*" />
     <PackageReference Include="Nullable" Version="1.3.1" PrivateAssets="all" />
   </ItemGroup>
 

src/Serilog.Sinks.Async/Sinks/Async/BackgroundWorkerSink.cs

@@ -22,13 +22,16 @@
 
 namespace Serilog.Sinks.Async;
 
-sealed class BackgroundWorkerSink : ILogEventSink, IAsyncLogEventSinkInspector, IDisposable
+sealed class BackgroundWorkerSink : ILogEventSink, IAsyncLogEventSinkInspector, IDisposable, ISetLoggingFailureListener
 {
     readonly ILogEventSink _wrappedSink;
     readonly bool _blockWhenFull;
     readonly BlockingCollection<LogEvent> _queue;
     readonly Task _worker;
     readonly IAsyncLogEventSinkMonitor? _monitor;
+
+    // By contract, set only during initialization, so updates are not synchronized.
+    ILoggingFailureListener _failureListener = SelfLog.FailureListener;
 
     long _droppedMessages;
 
@@ -46,7 +49,10 @@ public BackgroundWorkerSink(ILogEventSink wrappedSink, int bufferCapacity, bool
     public void Emit(LogEvent logEvent)
     {
         if (_queue.IsAddingCompleted)
+        {
+            _failureListener.OnLoggingFailed(this, LoggingFailureKind.Final, "the sink has been disposed", [logEvent], null);
             return;
+        }
 
         try
         {
@@ -59,14 +65,15 @@ public void Emit(LogEvent logEvent)
                 if (!_queue.TryAdd(logEvent))
                 {
                     Interlocked.Increment(ref _droppedMessages);
-                    SelfLog.WriteLine("{0} unable to enqueue, capacity {1}", typeof(BackgroundWorkerSink), _queue.BoundedCapacity);
+                    _failureListener.OnLoggingFailed(this, LoggingFailureKind.Permanent, $"unable to enqueue, capacity {_queue.BoundedCapacity}", [logEvent], null);
                 }
             }
         }
-        catch (InvalidOperationException)
+        catch (InvalidOperationException ex)
         {
             // Thrown in the event of a race condition when we try to add another event after
             // CompleteAdding has been called
+            _failureListener.OnLoggingFailed(this, LoggingFailureKind.Final, "the sink has been disposed", [logEvent], ex);
         }
     }
 
@@ -95,13 +102,13 @@ void Pump()
                 }
                 catch (Exception ex)
                 {
-                    SelfLog.WriteLine("{0} failed to emit event to wrapped sink: {1}", typeof(BackgroundWorkerSink), ex);
+                    _failureListener.OnLoggingFailed(this, LoggingFailureKind.Permanent, "failed to emit event to wrapped sink", [next], ex);
                 }
             }
         }
         catch (Exception fatal)
         {
-            SelfLog.WriteLine("{0} fatal error in worker thread: {1}", typeof(BackgroundWorkerSink), fatal);
+            _failureListener.OnLoggingFailed(this, LoggingFailureKind.Final, "fatal error in worker thread", null, fatal);
         }
     }
 
@@ -110,4 +117,9 @@ void Pump()
     int IAsyncLogEventSinkInspector.Count => _queue.Count;
 
     long IAsyncLogEventSinkInspector.DroppedMessagesCount => _droppedMessages;
+
+    public void SetFailureListener(ILoggingFailureListener failureListener)
+    {
+        _failureListener = failureListener ?? throw new ArgumentNullException(nameof(failureListener));
+    }
 }

test/Serilog.Sinks.Async.Tests/BackgroundWorkerSinkTests.cs

@@ -1,4 +1,6 @@
-using Serilog.Sinks.Async.Tests.Support;
+using System;
+using Serilog.Events;
+using Serilog.Sinks.Async.Tests.Support;
 using Xunit;
 
 namespace Serilog.Sinks.Async.Tests;
@@ -50,4 +52,19 @@ public void CtorAndDisposeInformMonitor()
 
         Assert.Null(monitor.Inspector);
     }
+
+    [Fact]
+    public void SupportsLoggingFailureListener()
+    {
+        var failureListener = new CollectingFailureListener();
+        var sink = new BackgroundWorkerSink(new NotImplementedSink(), 1, false, null);
+        sink.SetFailureListener(failureListener);
+        var evt = new LogEvent(DateTimeOffset.Now, LogEventLevel.Information, null, MessageTemplate.Empty, []);
+        sink.Emit(evt);
+        sink.Dispose();
+        var collected = Assert.Single(failureListener.Events);
+        Assert.Same(evt, collected);
+        var exception = Assert.Single(failureListener.Exceptions);
+        Assert.IsType<NotImplementedException>(exception);
+    }
 }

test/Serilog.Sinks.Async.Tests/Serilog.Sinks.Async.Tests.csproj

@@ -6,6 +6,7 @@
     <AssemblyOriginatorKeyFile>../../assets/Serilog.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>
     <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
+    <LangVersion>12</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>

test/Serilog.Sinks.Async.Tests/Support/CollectingFailureListener.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Serilog.Core;
+using Serilog.Events;
+
+namespace Serilog.Sinks.Async.Tests.Support;
+
+class CollectingFailureListener: ILoggingFailureListener
+{
+    readonly object _sync = new();
+    readonly List<LogEvent> _events = [];
+    readonly List<Exception> _exceptions = [];
+
+    public IReadOnlyList<LogEvent> Events
+    {
+        get
+        {
+            lock (_sync)
+                return _events.ToList();
+        }
+    }
+    public IReadOnlyList<Exception> Exceptions
+    {
+        get
+        {
+            lock (_sync)
+                return _exceptions.ToList();
+        }
+    }
+
+    public void OnLoggingFailed(object sender, LoggingFailureKind kind, string message, IReadOnlyCollection<LogEvent> events,
+        Exception exception)
+    {
+        lock (_sync)
+        {
+            if (exception != null)
+                _exceptions.Add(exception);
+
+            foreach (var logEvent in events ?? [])
+            {
+                _events.Add(logEvent);
+            }
+        }
+    }
+}

test/Serilog.Sinks.Async.Tests/Support/NotImplementedSink.cs

@@ -0,0 +1,13 @@
+using System;
+using Serilog.Core;
+using Serilog.Events;
+
+namespace Serilog.Sinks.Async.Tests.Support;
+
+class NotImplementedSink: ILogEventSink
+{
+    public void Emit(LogEvent logEvent)
+    {
+        throw new NotImplementedException();
+    }
+}