Allow the sink to flush for events of a minimum log level

Author: BernhardMarconato

src/Serilog.Sinks.File/FileLoggerConfigurationExtensions.cs

@@ -306,6 +306,7 @@ public static LoggerConfiguration File(
     /// Must be greater than or equal to <see cref="TimeSpan.Zero"/>.
     /// Ignored if <paramref see="rollingInterval"/> is <see cref="RollingInterval.Infinite"/>.
     /// The default is to retain files indefinitely.</param>
+    /// <param name="flushAtMinimumLevel">The minimum level for events to flush the sink. The default is <see cref="LevelAlias.Off"/>.</param>
     /// <returns>Configuration object allowing method chaining.</returns>
     /// <exception cref="ArgumentNullException">When <paramref name="sinkConfiguration"/> is <code>null</code></exception>
     /// <exception cref="ArgumentNullException">When <paramref name="formatter"/> is <code>null</code></exception>
@@ -331,15 +332,16 @@ public static LoggerConfiguration File(
         int? retainedFileCountLimit = DefaultRetainedFileCountLimit,
         Encoding? encoding = null,
         FileLifecycleHooks? hooks = null,
-        TimeSpan? retainedFileTimeLimit = null)
+        TimeSpan? retainedFileTimeLimit = null,
+        LogEventLevel flushAtMinimumLevel = LevelAlias.Off)
     {
         if (sinkConfiguration == null) throw new ArgumentNullException(nameof(sinkConfiguration));
         if (formatter == null) throw new ArgumentNullException(nameof(formatter));
         if (path == null) throw new ArgumentNullException(nameof(path));
 
         return ConfigureFile(sinkConfiguration.Sink, formatter, path, restrictedToMinimumLevel, fileSizeLimitBytes, levelSwitch,
             buffered, false, shared, flushToDiskInterval, encoding, rollingInterval, rollOnFileSizeLimit,
-            retainedFileCountLimit, hooks, retainedFileTimeLimit);
+            retainedFileCountLimit, hooks, retainedFileTimeLimit, flushAtMinimumLevel);
     }
 
     /// <summary>
@@ -494,7 +496,7 @@ public static LoggerConfiguration File(
         if (path == null) throw new ArgumentNullException(nameof(path));
 
         return ConfigureFile(sinkConfiguration.Sink, formatter, path, restrictedToMinimumLevel, null, levelSwitch, false, true,
-            false, null, encoding, RollingInterval.Infinite, false, null, hooks, null);
+            false, null, encoding, RollingInterval.Infinite, false, null, hooks, null, LevelAlias.Off);
     }
 
     static LoggerConfiguration ConfigureFile(
@@ -513,7 +515,8 @@ static LoggerConfiguration ConfigureFile(
         bool rollOnFileSizeLimit,
         int? retainedFileCountLimit,
         FileLifecycleHooks? hooks,
-        TimeSpan? retainedFileTimeLimit)
+        TimeSpan? retainedFileTimeLimit,
+        LogEventLevel flushAtMinimumLevel)
     {
         if (addSink == null) throw new ArgumentNullException(nameof(addSink));
         if (formatter == null) throw new ArgumentNullException(nameof(formatter));
@@ -530,7 +533,7 @@ static LoggerConfiguration ConfigureFile(
         {
             if (rollOnFileSizeLimit || rollingInterval != RollingInterval.Infinite)
             {
-                sink = new RollingFileSink(path, formatter, fileSizeLimitBytes, retainedFileCountLimit, encoding, buffered, shared, rollingInterval, rollOnFileSizeLimit, hooks, retainedFileTimeLimit);
+                sink = new RollingFileSink(path, formatter, fileSizeLimitBytes, retainedFileCountLimit, encoding, buffered, shared, rollingInterval, rollOnFileSizeLimit, hooks, retainedFileTimeLimit, flushAtMinimumLevel);
             }
             else
             {
@@ -542,7 +545,7 @@ static LoggerConfiguration ConfigureFile(
                 }
                 else
                 {
-                    sink = new FileSink(path, formatter, fileSizeLimitBytes, encoding, buffered, hooks);
+                    sink = new FileSink(path, formatter, fileSizeLimitBytes, encoding, buffered, hooks, flushAtMinimumLevel);
                 }
 
             }

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

@@ -32,6 +32,7 @@ public sealed class FileSink : IFileSink, IDisposable, ISetLoggingFailureListene
     readonly bool _buffered;
     readonly object _syncRoot = new();
     readonly WriteCountingStream? _countingStreamWrapper;
+    readonly LogEventLevel _flushAtMinimumLevel;
 
     ILoggingFailureListener _failureListener = SelfLog.FailureListener;
 
@@ -57,7 +58,7 @@ public sealed class FileSink : IFileSink, IDisposable, ISetLoggingFailureListene
     /// <exception cref="ArgumentException">Invalid <paramref name="path"/></exception>
     [Obsolete("This type and constructor will be removed from the public API in a future version; use `WriteTo.File()` instead.")]
     public FileSink(string path, ITextFormatter textFormatter, long? fileSizeLimitBytes, Encoding? encoding = null, bool buffered = false)
-        : this(path, textFormatter, fileSizeLimitBytes, encoding, buffered, null)
+        : this(path, textFormatter, fileSizeLimitBytes, encoding, buffered, null, LevelAlias.Off)
     {
     }
 
@@ -68,13 +69,15 @@ internal FileSink(
         long? fileSizeLimitBytes,
         Encoding? encoding,
         bool buffered,
-        FileLifecycleHooks? hooks)
+        FileLifecycleHooks? hooks,
+        LogEventLevel flushAtMinimumLevel)
     {
         if (path == null) throw new ArgumentNullException(nameof(path));
         if (fileSizeLimitBytes is < 1) throw new ArgumentException("Invalid value provided; file size limit must be at least 1 byte, or null.");
         _textFormatter = textFormatter ?? throw new ArgumentNullException(nameof(textFormatter));
         _fileSizeLimitBytes = fileSizeLimitBytes;
         _buffered = buffered;
+        _flushAtMinimumLevel = flushAtMinimumLevel;
 
         var directory = Path.GetDirectoryName(path);
         if (!string.IsNullOrWhiteSpace(directory) && !Directory.Exists(directory))
@@ -124,6 +127,8 @@ bool IFileSink.EmitOrOverflow(LogEvent logEvent)
             _textFormatter.Format(logEvent, _output);
             if (!_buffered)
                 _output.Flush();
+            else if (logEvent.Level >= _flushAtMinimumLevel)
+                FlushToDisk();
 
             return true;
         }

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

@@ -27,6 +27,7 @@ sealed class RollingFileSink : ILogEventSink, IFlushableFileSink, IDisposable, I
     readonly long? _fileSizeLimitBytes;
     readonly int? _retainedFileCountLimit;
     readonly TimeSpan? _retainedFileTimeLimit;
+    readonly LogEventLevel _flushAtMinimumLevel;
     readonly Encoding? _encoding;
     readonly bool _buffered;
     readonly bool _shared;
@@ -51,7 +52,8 @@ public RollingFileSink(string path,
                           RollingInterval rollingInterval,
                           bool rollOnFileSizeLimit,
                           FileLifecycleHooks? hooks,
-                          TimeSpan? retainedFileTimeLimit)
+                          TimeSpan? retainedFileTimeLimit,
+                          LogEventLevel flushAtMinimumLevel)
     {
         if (path == null) throw new ArgumentNullException(nameof(path));
         if (fileSizeLimitBytes is < 1) throw new ArgumentException("Invalid value provided; file size limit must be at least 1 byte, or null.");
@@ -63,6 +65,7 @@ public RollingFileSink(string path,
         _fileSizeLimitBytes = fileSizeLimitBytes;
         _retainedFileCountLimit = retainedFileCountLimit;
         _retainedFileTimeLimit = retainedFileTimeLimit;
+        _flushAtMinimumLevel = flushAtMinimumLevel;
         _encoding = encoding;
         _buffered = buffered;
         _shared = shared;
@@ -176,7 +179,7 @@ void OpenFile(DateTime now, int? minSequence = null)
                         new SharedFileSink(path, _textFormatter, _fileSizeLimitBytes, _encoding)
                         :
 #pragma warning restore 618
-                        new FileSink(path, _textFormatter, _fileSizeLimitBytes, _encoding, _buffered, _hooks);
+                        new FileSink(path, _textFormatter, _fileSizeLimitBytes, _encoding, _buffered, _hooks, _flushAtMinimumLevel);
 
                     _currentFileSequence = sequence;
 

test/Serilog.Sinks.File.Tests/FileSinkTests.cs

@@ -1,6 +1,7 @@
-using System.IO.Compression;
+using System.IO.Compression;
 using System.Text;
 using Serilog.Core;
+using Serilog.Events;
 using Xunit;
 using Serilog.Formatting.Json;
 using Serilog.Sinks.File.Tests.Support;
@@ -146,7 +147,7 @@ public void OnOpenedLifecycleHookCanWrapUnderlyingStream()
         var path = tmp.AllocateFilename("txt");
         var evt = Some.LogEvent("Hello, world!");
 
-        using (var sink = new FileSink(path, new JsonFormatter(), null, null, false, gzipWrapper))
+        using (var sink = new FileSink(path, new JsonFormatter(), null, null, false, gzipWrapper, LevelAlias.Off))
         {
             sink.Emit(evt);
             sink.Emit(evt);
@@ -178,12 +179,12 @@ public static void OnOpenedLifecycleHookCanWriteFileHeader()
         var headerWriter = new FileHeaderWriter("This is the file header");
 
         var path = tmp.AllocateFilename("txt");
-        using (new FileSink(path, new JsonFormatter(), null, new UTF8Encoding(false), false, headerWriter))
+        using (new FileSink(path, new JsonFormatter(), null, new UTF8Encoding(false), false, headerWriter, LevelAlias.Off))
         {
             // Open and write header
         }
 
-        using (var sink = new FileSink(path, new JsonFormatter(), null, new UTF8Encoding(false), false, headerWriter))
+        using (var sink = new FileSink(path, new JsonFormatter(), null, new UTF8Encoding(false), false, headerWriter, LevelAlias.Off))
         {
             // Length check should prevent duplicate header here
             sink.Emit(Some.LogEvent());
@@ -203,7 +204,7 @@ public static void OnOpenedLifecycleHookCanCaptureFilePath()
         var capturePath = new CaptureFilePathHook();
 
         var path = tmp.AllocateFilename("txt");
-        using (new FileSink(path, new JsonFormatter(), null, new UTF8Encoding(false), false, capturePath))
+        using (new FileSink(path, new JsonFormatter(), null, new UTF8Encoding(false), false, capturePath, LevelAlias.Off))
         {
             // Open and capture the log file path
         }
@@ -223,7 +224,7 @@ public static void OnOpenedLifecycleHookCanEmptyTheFileContents()
             sink.Emit(Some.LogEvent());
         }
 
-        using (var sink = new FileSink(path, new JsonFormatter(), fileSizeLimitBytes: null, encoding: new UTF8Encoding(false), buffered: false, hooks: emptyFileHook))
+        using (var sink = new FileSink(path, new JsonFormatter(), fileSizeLimitBytes: null, encoding: new UTF8Encoding(false), buffered: false, hooks: emptyFileHook, LevelAlias.Off))
         {
             // Hook will clear the contents of the file before emitting the log events
             sink.Emit(Some.LogEvent());
@@ -235,6 +236,83 @@ public static void OnOpenedLifecycleHookCanEmptyTheFileContents()
         Assert.Equal('{', lines[0][0]);
     }
 
+    [Fact]
+    public void WhenFlushAtMinimumLevelIsNotReachedLineIsNotFlushed()
+    {
+        using var tmp = TempFolder.ForCaller();
+        var path = tmp.AllocateFilename("txt");
+        var formatter = new JsonFormatter();
+
+        using (var sink = new FileSink(path, formatter, null, null, true, null, LogEventLevel.Fatal))
+        {
+            sink.Emit(Some.LogEvent(level: LogEventLevel.Information));
+
+            var lines = ReadAllLinesShared(path);
+            Assert.Empty(lines);
+        }
+
+        var savedLines = System.IO.File.ReadAllLines(path);
+        Assert.Single(savedLines);
+    }
+
+    [Fact]
+    public void WhenFlushAtMinimumLevelIsReachedLineIsFlushed()
+    {
+        using var tmp = TempFolder.ForCaller();
+        var path = tmp.AllocateFilename("txt");
+        var formatter = new JsonFormatter();
+
+        using (var sink = new FileSink(path, formatter, null, null, true, null, LogEventLevel.Fatal))
+        {
+            sink.Emit(Some.LogEvent(level: LogEventLevel.Fatal));
+
+            var lines = ReadAllLinesShared(path);
+            Assert.Single(lines);
+        }
+
+        var savedLines = System.IO.File.ReadAllLines(path);
+        Assert.Single(savedLines);
+    }
+
+    [Fact]
+    public void WhenFlushAtMinimumLevelIsOffLineIsNotFlushed()
+    {
+        using var tmp = TempFolder.ForCaller();
+        var path = tmp.AllocateFilename("txt");
+        var formatter = new JsonFormatter();
+
+        using (var sink = new FileSink(path, formatter, null, null, true, null, LevelAlias.Off))
+        {
+            sink.Emit(Some.LogEvent(level: LogEventLevel.Fatal));
+
+            var lines = ReadAllLinesShared(path);
+            Assert.Empty(lines);
+        }
+
+        var savedLines = System.IO.File.ReadAllLines(path);
+        Assert.Single(savedLines);
+    }
+
+    [Fact]
+    public void WhenFlushAtMinimumLevelIsReachedMultipleLinesAreFlushed()
+    {
+        using var tmp = TempFolder.ForCaller();
+        var path = tmp.AllocateFilename("txt");
+        var formatter = new JsonFormatter();
+
+        using (var sink = new FileSink(path, formatter, null, null, true, null, LogEventLevel.Error))
+        {
+            sink.Emit(Some.LogEvent(level: LogEventLevel.Information));
+            sink.Emit(Some.LogEvent(level: LogEventLevel.Fatal));
+
+            var lines = ReadAllLinesShared(path);
+            Assert.Equal(2, lines.Length);
+        }
+
+        var savedLines = System.IO.File.ReadAllLines(path);
+        Assert.Equal(2, savedLines.Length);
+    }
+
     static void WriteTwoEventsAndCheckOutputFileLength(long? maxBytes, Encoding encoding)
     {
         using var tmp = TempFolder.ForCaller();
@@ -260,4 +338,21 @@ static void WriteTwoEventsAndCheckOutputFileLength(long? maxBytes, Encoding enco
         size = new FileInfo(path).Length;
         Assert.Equal(encoding.GetPreamble().Length + eventOuputLength * 2, size);
     }
+
+    private static string[] ReadAllLinesShared(string path)
+    {
+        // ReadAllLines cannot be used here, as it can't read files even if they are opened with FileShare.Read
+        using var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
+        using var reader = new StreamReader(fs);
+
+        string? line;
+        List<string> lines = [];
+
+        while ((line = reader.ReadLine()) != null)
+        {
+            lines.Add(line);
+        }
+
+        return [.. lines];
+    }
 }