WindowsMultiProcessFileAppender.cs

// 
// Copyright (c) 2004-2021 Jaroslaw Kowalski <jaak@jkowalski.net>, Kim Christensen, Julian Verdurmen
// 
// All rights reserved.
// 
// Redistribution and use in source and binary forms, with or without 
// modification, are permitted provided that the following conditions 
// are met:
// 
// * Redistributions of source code must retain the above copyright notice, 
//   this list of conditions and the following disclaimer. 
// 
// * Redistributions in binary form must reproduce the above copyright notice,
//   this list of conditions and the following disclaimer in the documentation
//   and/or other materials provided with the distribution. 
// 
// * Neither the name of Jaroslaw Kowalski nor the names of its 
//   contributors may be used to endorse or promote products derived from this
//   software without specific prior written permission. 
// 
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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// THE POSSIBILITY OF SUCH DAMAGE.
// 

#if !MONO && !NETSTANDARD

namespace NLog.Internal.FileAppenders
{
    using System;
    using System.IO;
    using System.Security;
    using System.Threading;

    using NLog.Common;

    /// <summary>
    /// Provides a multi process-safe atomic file append while
    /// keeping the files open.
    /// </summary>
    [SecuritySafeCritical]
    internal sealed class WindowsMultiProcessFileAppender : BaseMutexFileAppender
    {
        public static readonly IFileAppenderFactory TheFactory = new Factory();

        private FileStream _fileStream;

        /// <summary>
        /// Initializes a new instance of the <see cref="WindowsMultiProcessFileAppender" /> class.
        /// </summary>
        /// <param name="fileName">Name of the file.</param>
        /// <param name="parameters">The parameters.</param>
        public WindowsMultiProcessFileAppender(string fileName, ICreateFileParameters parameters) : base(fileName, parameters)
        {
            try
            {
                CreateAppendOnlyFile(fileName);
            }
            catch
            {
                CloseFileSafe(ref _fileStream, fileName);
                throw;
            }
        }

        /// <summary>
        /// Creates or opens a file in a special mode, so that writes are automatically
        /// as atomic writes at the file end.
        /// See also "UnixMultiProcessFileAppender" which does a similar job on *nix platforms.
        /// </summary>
        /// <param name="fileName">File to create or open</param>
        private void CreateAppendOnlyFile(string fileName)
        {
            string dir = Path.GetDirectoryName(fileName);
            if (!Directory.Exists(dir))
            {
                if (!CreateFileParameters.CreateDirs)
                {
                    throw new DirectoryNotFoundException(dir);
                }
                Directory.CreateDirectory(dir);
            }

            var fileShare = FileShare.ReadWrite;
            if (CreateFileParameters.EnableFileDelete)
                fileShare |= FileShare.Delete;

            try
            {
                bool fileExists = File.Exists(fileName);

                // https://blogs.msdn.microsoft.com/oldnewthing/20151127-00/?p=92211/
                // https://msdn.microsoft.com/en-us/library/ff548289.aspx
                // If only the FILE_APPEND_DATA and SYNCHRONIZE flags are set, the caller can write only to the end of the file, 
                // and any offset information about writes to the file is ignored.
                // However, the file will automatically be extended as necessary for this type of write operation.
                _fileStream = new FileStream(
                    fileName,
                    FileMode.Append,
                    System.Security.AccessControl.FileSystemRights.AppendData | System.Security.AccessControl.FileSystemRights.Synchronize, // <- Atomic append
                    fileShare,
                    1,  // No internal buffer, write directly from user-buffer
                    FileOptions.None);

                long filePosition = _fileStream.Position;
                if (fileExists || filePosition > 0)
                {
                    CreationTimeUtc = File.GetCreationTimeUtc(FileName);
                    if (CreationTimeUtc < DateTime.UtcNow - TimeSpan.FromSeconds(2) && filePosition == 0)
                    {
                        // File wasn't created "almost now". 
                        // This could mean creation time has tunneled through from another file (see comment below).
                        Thread.Sleep(50);
                        // Having waited for a short amount of time usually means the file creation process has continued
                        // code execution just enough to the above point where it has fixed up the creation time.
                        CreationTimeUtc = File.GetCreationTimeUtc(FileName);
                    }
                }
                else
                {
                    // We actually created the file and eventually concurrent processes 
                    // may have opened the same file in between.
                    // Only the one process creating the file should adjust the file creation time 
                    // to avoid being thwarted by Windows' Tunneling capabilities (https://support.microsoft.com/en-us/kb/172190).
                    // Unfortunately we can't use the native SetFileTime() to prevent opening the file 2nd time.
                    // This would require another desiredAccess flag which would disable the atomic append feature.
                    // See also UpdateCreationTime()
                    CreationTimeUtc = DateTime.UtcNow;
                    File.SetCreationTimeUtc(FileName, CreationTimeUtc);
                }
            }
            catch
            {
                CloseFileSafe(ref _fileStream, fileName);
                throw;
            }
        }

        /// <inheritdoc/>
        public override void Write(byte[] bytes, int offset, int count)
        {
            _fileStream?.Write(bytes, offset, count);
        }

        /// <inheritdoc/>
        public override void Close()
        {
            CloseFileSafe(ref _fileStream, FileName);
        }

        /// <inheritdoc/>
        public override void Flush()
        {
            // do nothing, the file is written directly
        }

        /// <inheritdoc/>
        public override DateTime? GetFileCreationTimeUtc()
        {
            return CreationTimeUtc; // File is kept open, so creation time is static
        }

        /// <inheritdoc/>
        public override long? GetFileLength()
        {
            return _fileStream?.Length;
        }

        /// <summary>
        /// Factory class.
        /// </summary>
        private sealed class Factory : IFileAppenderFactory
        {
            /// <inheritdoc/>
            BaseFileAppender IFileAppenderFactory.Open(string fileName, ICreateFileParameters parameters)
            {
                return new WindowsMultiProcessFileAppender(fileName, parameters);
            }
        }
    }
}

#endif