Support ILoggingFailureListener

src/Serilog.Sinks.File/FileLoggerConfigurationExtensions.cs

@@ -549,17 +549,26 @@ static LoggerConfiguration ConfigureFile(
         }
         catch (Exception ex)
         {
-            SelfLog.WriteLine("Unable to open file sink for {0}: {1}", path, ex);
+            // No logging failure listener can be configured here; in future we might allow for a static
+            // default listener, but in the meantime this improves `SelfLog` usefulness and consistency.
+            SelfLog.FailureListener.OnLoggingFailed(
+                typeof(FileLoggerConfigurationExtensions),
+                LoggingFailureKind.Final,
+                $"unable to open file sink for {path}",
+                events: null,
+                ex);
 
             if (propagateExceptions)
                 throw;
 
-            return addSink(new NullSink(), LevelAlias.Maximum, null);
+            return addSink(new FailedSink(), restrictedToMinimumLevel, levelSwitch);
         }
 
         if (flushToDiskInterval.HasValue)
         {
 #pragma warning disable 618
+            // `LoggerSinkConfiguration.Wrap()` is not used here because the target sink is expected
+            // to support `ILogEventSink`.
             sink = new PeriodicFlushToDiskSink(sink, flushToDiskInterval.Value);
 #pragma warning restore 618
         }

src/Serilog.Sinks.File/Serilog.Sinks.File.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <Description>Write Serilog events to text files in plain or JSON format.</Description>
-    <VersionPrefix>6.0.1</VersionPrefix>
+    <VersionPrefix>7.0.0</VersionPrefix>
     <Authors>Serilog Contributors</Authors>
     <!-- .NET Framework version targeting is frozen at these two TFMs. -->
     <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT'">net471;net462</TargetFrameworks>
@@ -26,7 +26,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Serilog" Version="4.0.0" />
+    <PackageReference Include="Serilog" Version="4.2.0" />
     <PackageReference Include="Nullable" Version="1.3.1" PrivateAssets="All" />
   </ItemGroup>
 

src/Serilog.Sinks.File/Sinks/File/FailedSink.cs

@@ -0,0 +1,34 @@
+// Copyright © Serilog Contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using Serilog.Core;
+using Serilog.Debugging;
+using Serilog.Events;
+
+namespace Serilog.Sinks.File;
+
+sealed class FailedSink : ILogEventSink, ISetLoggingFailureListener
+{
+    ILoggingFailureListener _failureListener = SelfLog.FailureListener;
+
+    public void Emit(LogEvent logEvent)
+    {
+        _failureListener.OnLoggingFailed(this, LoggingFailureKind.Final, "the sink could not be initialized", [logEvent], exception: null);
+    }
+
+    public void SetFailureListener(ILoggingFailureListener failureListener)
+    {
+        _failureListener = failureListener ?? throw new ArgumentNullException(nameof(failureListener));
+    }
+}

src/Serilog.Sinks.File/Sinks/File/FileSink.cs

@@ -13,6 +13,8 @@
 // limitations under the License.
 
 using System.Text;
+using Serilog.Core;
+using Serilog.Debugging;
 using Serilog.Events;
 using Serilog.Formatting;
 
@@ -21,7 +23,7 @@ namespace Serilog.Sinks.File;
 /// <summary>
 /// Write log events to a disk file.
 /// </summary>
-public sealed class FileSink : IFileSink, IDisposable
+public sealed class FileSink : IFileSink, IDisposable, ISetLoggingFailureListener
 {
     readonly TextWriter _output;
     readonly FileStream _underlyingStream;
@@ -31,6 +33,8 @@ public sealed class FileSink : IFileSink, IDisposable
     readonly object _syncRoot = new();
     readonly WriteCountingStream? _countingStreamWrapper;
 
+    ILoggingFailureListener _failureListener = SelfLog.FailureListener;
+
     /// <summary>Construct a <see cref="FileSink"/>.</summary>
     /// <param name="path">Path to the file.</param>
     /// <param name="textFormatter">Formatter used to convert log events to text.</param>
@@ -98,7 +102,7 @@ internal FileSink(
             }
             catch
             {
-                outputStream?.Dispose();
+                outputStream.Dispose();
                 throw;
             }
         }
@@ -132,7 +136,16 @@ bool IFileSink.EmitOrOverflow(LogEvent logEvent)
     /// <exception cref="ArgumentNullException">When <paramref name="logEvent"/> is <code>null</code></exception>
     public void Emit(LogEvent logEvent)
     {
-        ((IFileSink) this).EmitOrOverflow(logEvent);
+        if (!((IFileSink)this).EmitOrOverflow(logEvent))
+        {
+            // Support fallback chains without the overhead of throwing an exception.
+            _failureListener.OnLoggingFailed(
+                this,
+                LoggingFailureKind.Permanent,
+                "the log file size limit has been reached and no rolling behavior was specified",
+                [logEvent],
+                exception: null);
+        }
     }
 
     /// <inheritdoc />
@@ -153,4 +166,9 @@ public void FlushToDisk()
             _underlyingStream.Flush(true);
         }
     }
+
+    void ISetLoggingFailureListener.SetFailureListener(ILoggingFailureListener failureListener)
+    {
+        _failureListener = failureListener ?? throw new ArgumentNullException(nameof(failureListener));
+    }
 }

src/Serilog.Sinks.File/Sinks/File/PeriodicFlushToDiskSink.cs

@@ -22,12 +22,14 @@ namespace Serilog.Sinks.File;
 /// A sink wrapper that periodically flushes the wrapped sink to disk.
 /// </summary>
 [Obsolete("This type will be removed from the public API in a future version; use `WriteTo.File(flushToDiskInterval:)` instead.")]
-public class PeriodicFlushToDiskSink : ILogEventSink, IDisposable
+public sealed class PeriodicFlushToDiskSink : ILogEventSink, IDisposable, ISetLoggingFailureListener
 {
     readonly ILogEventSink _sink;
     readonly Timer _timer;
     int _flushRequired;
 
+    ILoggingFailureListener _failureListener = SelfLog.FailureListener;
+
     /// <summary>
     /// Construct a <see cref="PeriodicFlushToDiskSink"/> that wraps
     /// <paramref name="sink"/> and flushes it at the specified <paramref name="flushInterval"/>.
@@ -46,7 +48,17 @@ public PeriodicFlushToDiskSink(ILogEventSink sink, TimeSpan flushInterval)
         else
         {
             _timer = new Timer(_ => { }, null, Timeout.InfiniteTimeSpan, Timeout.InfiniteTimeSpan);
-            SelfLog.WriteLine("{0} configured to flush {1}, but {2} not implemented", typeof(PeriodicFlushToDiskSink), sink, nameof(IFlushableFileSink));
+
+            // May be an opportunity to improve the failure listener API for these cases - the failure
+            // is important, but not exactly `Final`.
+            SelfLog.FailureListener.OnLoggingFailed(
+                // Class must be sealed in order for this to be safe - `this` may be partially constructed
+                // otherwise.
+                this,
+                LoggingFailureKind.Final,
+                $"configured to flush {sink}, but {nameof(IFlushableFileSink)} not implemented",
+                events: null,
+                exception: null);
         }
     }
 
@@ -77,7 +89,21 @@ void FlushToDisk(IFlushableFileSink flushable)
         }
         catch (Exception ex)
         {
-            SelfLog.WriteLine("{0} could not flush the underlying sink to disk: {1}", typeof(PeriodicFlushToDiskSink), ex);
+            _failureListener.OnLoggingFailed(
+                this,
+                LoggingFailureKind.Temporary,
+                "could not flush the underlying file to disk",
+                events: null,
+                ex);
+        }
+    }
+
+    void ISetLoggingFailureListener.SetFailureListener(ILoggingFailureListener failureListener)
+    {
+        _failureListener = failureListener ?? throw new ArgumentNullException(nameof(failureListener));
+        if (_sink is ISetLoggingFailureListener setLoggingFailureListener)
+        {
+            setLoggingFailureListener.SetFailureListener(failureListener);
         }
     }
 }

src/Serilog.Sinks.File/Sinks/File/RollingFileSink.cs

@@ -20,7 +20,7 @@
 
 namespace Serilog.Sinks.File;
 
-sealed class RollingFileSink : ILogEventSink, IFlushableFileSink, IDisposable
+sealed class RollingFileSink : ILogEventSink, IFlushableFileSink, IDisposable, ISetLoggingFailureListener
 {
     readonly PathRoller _roller;
     readonly ITextFormatter _textFormatter;
@@ -33,6 +33,8 @@ sealed class RollingFileSink : ILogEventSink, IFlushableFileSink, IDisposable
     readonly bool _rollOnFileSizeLimit;
     readonly FileLifecycleHooks? _hooks;
 
+    ILoggingFailureListener _failureListener = SelfLog.FailureListener;
+
     readonly object _syncRoot = new();
     bool _isDisposed;
     DateTime? _nextCheckpoint;
@@ -72,6 +74,7 @@ public void Emit(LogEvent logEvent)
     {
         if (logEvent == null) throw new ArgumentNullException(nameof(logEvent));
 
+        bool failed;
         lock (_syncRoot)
         {
             if (_isDisposed) throw new ObjectDisposedException("The log file has been disposed.");
@@ -84,12 +87,18 @@ public void Emit(LogEvent logEvent)
                 AlignCurrentFileTo(now, nextSequence: true);
             }
 
-            /* TODO: We REALLY should add this to avoid stuff become missing undetected.
-            if (_currentFile == null)
-            {
-                SelfLog.WriteLine("Log event {0} was lost since it was not possible to open the file or create a new one.", logEvent.RenderMessage());
-            }
-            */
+            failed = _currentFile == null;
+        }
+
+        if (failed)
+        {
+            // Support fallback chains without the overhead of throwing an exception.
+            _failureListener.OnLoggingFailed(
+                this,
+                LoggingFailureKind.Permanent,
+                "the target file could not be opened or created",
+                [logEvent],
+                exception: null);
         }
     }
 
@@ -170,14 +179,22 @@ void OpenFile(DateTime now, int? minSequence = null)
                         new FileSink(path, _textFormatter, _fileSizeLimitBytes, _encoding, _buffered, _hooks);
 
                     _currentFileSequence = sequence;
+
+                    if (_currentFile is ISetLoggingFailureListener setLoggingFailureListener)
+                    {
+                        setLoggingFailureListener.SetFailureListener(_failureListener);
+                    }
                 }
                 catch (IOException ex)
                 {
                     if (IOErrors.IsLockedFile(ex))
                     {
-                        SelfLog.WriteLine(
-                            "File target {0} was locked, attempting to open next in sequence (attempt {1})", path,
-                            attempt + 1);
+                        _failureListener.OnLoggingFailed(
+                            this,
+                            LoggingFailureKind.Temporary,
+                            $"file target {path} was locked, attempting to open next in sequence (attempt {attempt + 1})",
+                            events: null,
+                            exception: null);
                         sequence = (sequence ?? 0) + 1;
                         continue;
                     }
@@ -216,7 +233,7 @@ void ApplyRetentionPolicy(string currentFilePath, DateTime now)
         // ReSharper disable once ConvertClosureToMethodGroup
         var potentialMatches = Directory.GetFiles(_roller.LogFileDirectory, _roller.DirectorySearchPattern)
             .Select(f => Path.GetFileName(f))
-            .Union(new[] { currentFileName });
+            .Union([currentFileName]);
 
         var newestFirst = _roller
             .SelectMatches(potentialMatches)
@@ -239,7 +256,12 @@ void ApplyRetentionPolicy(string currentFilePath, DateTime now)
             }
             catch (Exception ex)
             {
-                SelfLog.WriteLine("Error {0} while processing obsolete log file {1}", ex, fullPath);
+                _failureListener.OnLoggingFailed(
+                    this,
+                    LoggingFailureKind.Temporary,
+                    $"error while processing obsolete log file {fullPath}",
+                    events: null,
+                    ex);
             }
         }
     }
@@ -286,4 +308,9 @@ public void FlushToDisk()
             _currentFile?.FlushToDisk();
         }
     }
+
+    public void SetFailureListener(ILoggingFailureListener failureListener)
+    {
+        _failureListener = failureListener ?? throw new ArgumentNullException(nameof(failureListener));
+    }
 }

src/Serilog.Sinks.File/Sinks/File/SharedFileSink.AtomicAppend.cs

@@ -16,6 +16,8 @@
 
 using System.Security.AccessControl;
 using System.Text;
+using Serilog.Core;
+using Serilog.Debugging;
 using Serilog.Events;
 using Serilog.Formatting;
 
@@ -25,7 +27,7 @@ namespace Serilog.Sinks.File;
 /// Write log events to a disk file.
 /// </summary>
 [Obsolete("This type will be removed from the public API in a future version; use `WriteTo.File(shared: true)` instead.")]
-public sealed class SharedFileSink : IFileSink, IDisposable
+public sealed class SharedFileSink : IFileSink, IDisposable, ISetLoggingFailureListener
 {
     readonly MemoryStream _writeBuffer;
     readonly string _path;
@@ -34,6 +36,8 @@ public sealed class SharedFileSink : IFileSink, IDisposable
     readonly long? _fileSizeLimitBytes;
     readonly object _syncRoot = new();
 
+    ILoggingFailureListener _failureListener = SelfLog.FailureListener;
+
     // The stream is reopened with a larger buffer if atomic writes beyond the current buffer size are needed.
     FileStream _fileOutput;
     int _fileStreamBufferLength = DefaultFileStreamBufferLength;
@@ -59,7 +63,7 @@ public sealed class SharedFileSink : IFileSink, IDisposable
     public SharedFileSink(string path, ITextFormatter textFormatter, long? fileSizeLimitBytes, Encoding? encoding = null)
     {
         if (fileSizeLimitBytes.HasValue && fileSizeLimitBytes < 1)
-            throw new ArgumentException("Invalid value provided; file size limit must be at least 1 byte, or null");
+            throw new ArgumentException("Invalid value provided; file size limit must be at least 1 byte, or null.");
 
         _path = path ?? throw new ArgumentNullException(nameof(path));
         _textFormatter = textFormatter ?? throw new ArgumentNullException(nameof(textFormatter));
@@ -149,7 +153,16 @@ bool IFileSink.EmitOrOverflow(LogEvent logEvent)
     /// <exception cref="ArgumentNullException">When <paramref name="logEvent"/> is <code>null</code></exception>
     public void Emit(LogEvent logEvent)
     {
-        ((IFileSink)this).EmitOrOverflow(logEvent);
+        if (!((IFileSink)this).EmitOrOverflow(logEvent))
+        {
+            // Support fallback chains without the overhead of throwing an exception.
+            _failureListener.OnLoggingFailed(
+                this,
+                LoggingFailureKind.Permanent,
+                "the log file size limit has been reached and no rolling behavior was specified",
+                [logEvent],
+                exception: null);
+        }
     }
 
     /// <inheritdoc />
@@ -170,6 +183,11 @@ public void FlushToDisk()
             _fileOutput.Flush(true);
         }
     }
+
+    void ISetLoggingFailureListener.SetFailureListener(ILoggingFailureListener failureListener)
+    {
+        _failureListener = failureListener ?? throw new ArgumentNullException(nameof(failureListener));
+    }
 }
 
 #endif