[logging] Add FileLogSink, CompositeLogSink, and SHARPEMU_LOG_FILE env support (#50)

- FileLogSink: thread-safe file writer with AutoFlush, FileShare.Read for
  concurrent read access (tail -f), automatic parent directory creation,
  full date-time timestamps, IDisposable for graceful shutdown
- CompositeLogSink: fan-out to multiple sinks with per-sink exception
  isolation (one broken sink cannot silence the others), IDisposable
  propagates to children
- SharpEmuLog: ResolveSinkFromEnvironment() reads SHARPEMU_LOG_FILE and
  creates CompositeLogSink(console + file) when set; Sink setter now
  disposes the previous IDisposable sink to prevent file handle leaks;
  Shutdown() flushes and disposes the active sink
- Program.cs: Main wrapped in try/finally to guarantee SharpEmuLog.Shutdown()
  runs on all exit paths (GUI, mitigated child, normal, exception)

Co-authored-by: Hermes Atlas <hermesatlas@example.com>

src/SharpEmu.CLI/Program.cs

@@ -36,6 +36,18 @@ internal static partial class Program
 
     [STAThread]
     private static int Main(string[] args)
+    {
+        try
+        {
+            return Run(args);
+        }
+        finally
+        {
+            SharpEmuLog.Shutdown();
+        }
+    }
+
+    private static int Run(string[] args)
     {
         args = NormalizeInternalArguments(args, out var isMitigatedChild);
         if (args.Length == 0 && !isMitigatedChild)
@@ -66,7 +78,7 @@ internal static partial class Program
 
         ebootPath = Path.GetFullPath(ebootPath);
         Console.Error.WriteLine($"[DEBUG] Full path: {ebootPath}");
-        
+
         if (!File.Exists(ebootPath))
         {
             Log.Error($"EBOOT file was not found: {ebootPath}");

src/SharpEmu.Logging/CompositeLogSink.cs

@@ -0,0 +1,68 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Logging;
+
+/// <summary>
+/// Dispatches each <see cref="LogEntry"/> to every child sink.
+/// Exceptions thrown by a child are swallowed so one failing sink
+/// (e.g. a closed file) cannot silence the others.
+/// </summary>
+public sealed class CompositeLogSink : ISharpEmuLogSink, IDisposable
+{
+    private readonly ISharpEmuLogSink[] _sinks;
+    private bool _disposed;
+
+    public CompositeLogSink(params ISharpEmuLogSink[] sinks)
+    {
+        ArgumentNullException.ThrowIfNull(sinks);
+
+        foreach (var sink in sinks)
+        {
+            ArgumentNullException.ThrowIfNull(sink, nameof(sinks));
+        }
+
+        _sinks = sinks;
+    }
+
+    public IReadOnlyList<ISharpEmuLogSink> Sinks => _sinks;
+
+    public void Write(in LogEntry entry)
+    {
+        foreach (var sink in _sinks)
+        {
+            try
+            {
+                sink.Write(in entry);
+            }
+            catch
+            {
+                // A broken sink must not prevent the remaining sinks from logging.
+            }
+        }
+    }
+
+    public void Dispose()
+    {
+        if (_disposed)
+        {
+            return;
+        }
+
+        _disposed = true;
+
+        foreach (var sink in _sinks)
+        {
+            if (sink is IDisposable disposable)
+            {
+                try
+                {
+                    disposable.Dispose();
+                }
+                catch
+                {
+                }
+            }
+        }
+    }
+}

src/SharpEmu.Logging/FileLogSink.cs

@@ -0,0 +1,115 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.IO;
+using System.Text;
+
+namespace SharpEmu.Logging;
+
+/// <summary>
+/// Writes log entries to a file. Thread-safe via an internal lock.
+/// <see cref="StreamWriter.AutoFlush"/> is enabled so entries survive a crash.
+/// </summary>
+public sealed class FileLogSink : ISharpEmuLogSink, IDisposable
+{
+    private readonly object _sync = new();
+    private readonly StreamWriter _writer;
+    private bool _disposed;
+
+    /// <param name="path">Absolute or relative file path. Parent directories are created if missing.</param>
+    /// <param name="append"><see langword="true"/> to append to an existing file; <see langword="false"/> to overwrite.</param>
+    /// <param name="includeTimestamp">Always recommended for file logs — entries include a full date-time prefix.</param>
+    public FileLogSink(string path, bool append = true, bool includeTimestamp = true)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(path);
+
+        var directory = Path.GetDirectoryName(path);
+        if (!string.IsNullOrEmpty(directory) && !Directory.Exists(directory))
+        {
+            Directory.CreateDirectory(directory);
+        }
+
+        var fileStream = new FileStream(
+            path,
+            append ? FileMode.Append : FileMode.Create,
+            FileAccess.Write,
+            FileShare.Read,
+            bufferSize: 4096,
+            FileOptions.SequentialScan);
+        _writer = new StreamWriter(fileStream, Encoding.UTF8)
+        {
+            AutoFlush = true
+        };
+
+        IncludeTimestamp = includeTimestamp;
+    }
+
+    public bool IncludeTimestamp { get; set; }
+
+    public void Write(in LogEntry entry)
+    {
+        lock (_sync)
+        {
+            if (_disposed)
+            {
+                return;
+            }
+
+            if (IncludeTimestamp)
+            {
+                _writer.Write('[');
+                _writer.Write(entry.Timestamp.ToString("yyyy-MM-dd HH:mm:ss.fff"));
+                _writer.Write(']');
+            }
+
+            _writer.Write('[');
+            _writer.Write(ToLevelLabel(entry.Level));
+            _writer.Write(']');
+            _writer.Write('[');
+            _writer.Write(entry.Category);
+            _writer.Write(']');
+            _writer.Write(' ');
+
+            _writer.Write(entry.SourceFileName);
+            if (entry.SourceLine > 0)
+            {
+                _writer.Write(':');
+                _writer.Write(entry.SourceLine);
+            }
+
+            _writer.Write(' ');
+            _writer.WriteLine(entry.Message);
+
+            if (entry.Exception is not null)
+            {
+                _writer.WriteLine(entry.Exception);
+            }
+        }
+    }
+
+    public void Dispose()
+    {
+        lock (_sync)
+        {
+            if (_disposed)
+            {
+                return;
+            }
+
+            _disposed = true;
+            _writer.Flush();
+            _writer.Dispose();
+        }
+    }
+
+    private static string ToLevelLabel(LogLevel level) => level switch
+    {
+        LogLevel.Trace => "TRACE",
+        LogLevel.Debug => "DEBUG",
+        LogLevel.Info => "INFO",
+        LogLevel.Warning => "WARNING",
+        LogLevel.Error => "ERROR",
+        LogLevel.Critical => "CRITICAL",
+        _ => "LOG",
+    };
+}

src/SharpEmu.Logging/SharpEmuLog.cs

@@ -12,9 +12,7 @@ public static class SharpEmuLog
         new(StringComparer.Ordinal);
     private static readonly object ConfigurationSync = new();
     private static volatile LogLevel _minimumLevel = ResolveMinimumLevelFromEnvironment();
-    private static ISharpEmuLogSink _sink = new ConsoleLogSink(
-        useColors: ResolveColorEnabledFromEnvironment(),
-        includeTimestamp: false);
+    private static ISharpEmuLogSink _sink = ResolveSinkFromEnvironment();
 
     public static LogLevel MinimumLevel
     {
@@ -37,11 +35,26 @@ public static class SharpEmuLog
             ArgumentNullException.ThrowIfNull(value);
             lock (ConfigurationSync)
             {
+                if (ReferenceEquals(_sink, value))
+                {
+                    return;
+                }
+
+                if (_sink is IDisposable disposable)
+                {
+                    try
+                    {
+                        disposable.Dispose();
+                    }
+                    catch
+                    {
+                    }
+                }
+
                 _sink = value;
             }
         }
     }
-
     public static void Configure(LogLevel? minimumLevel = null, ISharpEmuLogSink? sink = null)
     {
         if (minimumLevel.HasValue)
@@ -55,6 +68,22 @@ public static class SharpEmuLog
         }
     }
 
+    /// <summary>
+    /// Disposes the active sink if it implements <see cref="IDisposable"/>.
+    /// Call at shutdown to flush file buffers. Logging after this call
+    /// continues to work for non-disposable sinks (e.g. <see cref="ConsoleLogSink"/>).
+    /// </summary>
+    public static void Shutdown()
+    {
+        lock (ConfigurationSync)
+        {
+            if (_sink is IDisposable disposable)
+            {
+                disposable.Dispose();
+            }
+        }
+    }
+
     public static SharpEmuLogger For(string category)
     {
         ArgumentException.ThrowIfNullOrWhiteSpace(category);
@@ -146,6 +175,30 @@ public static class SharpEmuLog
         return !IsTrueLike(raw);
     }
 
+    private static ISharpEmuLogSink ResolveSinkFromEnvironment()
+    {
+        var consoleSink = new ConsoleLogSink(
+            useColors: ResolveColorEnabledFromEnvironment(),
+            includeTimestamp: false);
+
+        var logFilePath = Environment.GetEnvironmentVariable("SHARPEMU_LOG_FILE");
+        if (!string.IsNullOrWhiteSpace(logFilePath))
+        {
+            try
+            {
+                var fileSink = new FileLogSink(logFilePath, append: true, includeTimestamp: true);
+                return new CompositeLogSink(consoleSink, fileSink);
+            }
+            catch (Exception ex)
+            {
+                // Bootstrapping — the logging system is not yet active, so stderr is the only channel.
+                Console.Error.WriteLine($"[SHARPEMU_LOG] Failed to open log file '{logFilePath}': {ex.Message}");
+            }
+        }
+
+        return consoleSink;
+    }
+
     private static bool IsTrueLike(string? text)
     {
         if (string.IsNullOrWhiteSpace(text))