src/log4net/Appender/RollingFileAppender.cs

#region Apache License
//
// Licensed to the Apache Software Foundation (ASF) under one or more 
// contributor license agreements. See the NOTICE file distributed with
// this work for additional information regarding copyright ownership. 
// The ASF licenses this file to you 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.
//
#endregion

using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;

using log4net.Util;
using log4net.Core;
using System.Threading;
using System.ComponentModel;
using System.Linq;

namespace log4net.Appender;

#if CONFIRM_WIN32_FILE_SHAREMODES
// The following sounds good, and I thought it was the case, but after
// further testing on Windows I have not been able to confirm it.

/// On the Windows platform if another process has a write lock on the file 
/// that is to be deleted, but allows shared read access to the file then the
/// file can be moved, but cannot be deleted. If the other process also allows 
/// shared delete access to the file then the file will be deleted once that 
/// process closes the file. If it is necessary to open the log file or any
/// of the backup files outside of this appender for either read or 
/// write access please ensure that read and delete share modes are enabled.
#endif

/// <summary>
/// Appender that rolls log files based on size or date or both.
/// </summary>
/// <remarks>
/// <para>
/// RollingFileAppender can roll log files based on size or date or both
/// depending on the setting of the <see cref="RollingStyle"/> property.
/// When set to <see cref="RollingMode.Size"/> the log file will be rolled
/// once its size exceeds the <see cref="MaximumFileSize"/>.
/// When set to <see cref="RollingMode.Date"/> the log file will be rolled
/// once the date boundary specified in the <see cref="DatePattern"/> property
/// is crossed.
/// When set to <see cref="RollingMode.Composite"/> the log file will be
/// rolled once the date boundary specified in the <see cref="DatePattern"/> property
/// is crossed, but within a date boundary the file will also be rolled
/// once its size exceeds the <see cref="MaximumFileSize"/>.
/// When set to <see cref="RollingMode.Once"/> the log file will be rolled when
/// the appender is configured. This effectively means that the log file can be
/// rolled once per program execution.
/// </para>
/// <para>
/// The following additional features have been added:
/// <list type="bullet">
/// <item>Attach date pattern for current log file <see cref="StaticLogFileName"/></item>
/// <item>Backup number increments for newer files <see cref="CountDirection"/></item>
/// <item>Infinite number of backups by file size <see cref="MaxSizeRollBackups"/></item>
/// </list>
/// </para>
/// 
/// <note>
/// <para>
/// For large or infinite numbers of backup files a <see cref="CountDirection"/> 
/// greater than zero is highly recommended, otherwise all the backup files need
/// to be renamed each time a new backup is created.
/// </para>
/// <para>
/// When Date/Time based rolling is used setting <see cref="StaticLogFileName"/> 
/// to <see langword="true"/> will reduce the number of file renamings to a few or none.
/// </para>
/// </note>
/// 
/// <note type="caution">
/// <para>
/// Changing <see cref="StaticLogFileName"/> or <see cref="CountDirection"/> without clearing
/// the log file directory of backup files will cause unexpected and unwanted side effects.  
/// </para>
/// </note>
/// 
/// <para>
/// If Date/Time based rolling is enabled this appender will attempt to roll existing files
/// in the directory without a Date/Time tag based on the last write date of the base log file.
/// The appender only rolls the log file when a message is logged. If Date/Time based rolling 
/// is enabled then the appender will not roll the log file at the Date/Time boundary but
/// at the point when the next message is logged after the boundary has been crossed.
/// </para>
/// 
/// <para>
/// The <see cref="RollingFileAppender"/> extends the <see cref="FileAppender"/> and
/// has the same behavior when opening the log file.
/// The appender will first try to open the file for writing when <see cref="ActivateOptions"/>
/// is called. This will typically be during configuration.
/// If the file cannot be opened for writing the appender will attempt
/// to open the file again each time a message is logged to the appender.
/// If the file cannot be opened for writing when a message is logged then
/// the message will be discarded by this appender.
/// </para>
/// <para>
/// When rolling a backup file necessitates deleting an older backup file the
/// file to be deleted is moved to a temporary name before being deleted.
/// </para>
/// 
/// <note type="caution">
/// <para>
/// A maximum number of backup files when rolling on date/time boundaries is not supported.
/// </para>
/// </note>
/// </remarks>
/// <author>Nicko Cadell</author>
/// <author>Gert Driesen</author>
/// <author>Aspi Havewala</author>
/// <author>Douglas de la Torre</author>
/// <author>Edward Smit</author>
// ReSharper disable GrammarMistakeInComment
public partial class RollingFileAppender : FileAppender
{
  /// <summary>
  /// Style of rolling to use
  /// </summary>
  public enum RollingMode
  {
    /// <summary>
    /// Roll files once per program execution
    /// </summary>
    /// <remarks>
    /// <para>
    /// Roll files once per program execution.
    /// Well really once each time this appender is configured.
    /// </para>
    /// <para>
    /// Setting this option also sets <c>AppendToFile</c> to <see langword="false"/> on the
    /// <see cref="RollingFileAppender"/>, otherwise this appender would just be a normal file appender.
    /// </para>
    /// </remarks>
    Once = 0,

    /// <summary>
    /// Roll files based only on the size of the file
    /// </summary>
    Size = 1,

    /// <summary>
    /// Roll files based only on the date
    /// </summary>
    Date = 2,

    /// <summary>
    /// Roll files based on both the size and date of the file
    /// </summary>
    Composite = 3
  }

  /// <summary>
  /// The code assumes that the following 'time' constants are in a increasing sequence.
  /// </summary>
  /// <remarks>
  /// </remarks>
  [EditorBrowsable(EditorBrowsableState.Never)]
  public enum RollPoint
  {
    /// <summary>
    /// Roll the log not based on the date
    /// </summary>
    InvalidRollPoint = -1,

    /// <summary>
    /// Roll the log for each minute
    /// </summary>
    TopOfMinute = 0,

    /// <summary>
    /// Roll the log for each hour
    /// </summary>
    TopOfHour = 1,

    /// <summary>
    /// Roll the log twice a day (midday and midnight)
    /// </summary>
    HalfDay = 2,

    /// <summary>
    /// Roll the log each day (midnight)
    /// </summary>
    TopOfDay = 3,

    /// <summary>
    /// Roll the log each week
    /// </summary>
    TopOfWeek = 4,

    /// <summary>
    /// Roll the log each month
    /// </summary>
    TopOfMonth = 5
  }

  /// <summary>
  /// Initializes a new instance of the <see cref="RollingFileAppender" /> class.
  /// </summary>
  public RollingFileAppender()
  { }

  /// <summary>
  /// Cleans up all resources used by this appender.
  /// </summary>
  ~RollingFileAppender() => Interlocked.Exchange(ref _mutexForRolling, null)?.Dispose();

  /// <summary>
  /// Gets or sets the strategy for determining the current date and time. The default
  /// implementation is to use LocalDateTime which internally calls through to DateTime.Now. 
  /// DateTime.UtcNow may be used on frameworks newer than .NET 1.0 by specifying
  /// <see cref="UniversalDateTime"/>.
  /// </summary>
  /// <value>
  /// An implementation of the <see cref="IDateTime"/> interface which returns the current date and time.
  /// </value>
  /// <remarks>
  /// <para>
  /// Gets or sets the <see cref="IDateTime"/> used to return the current date and time.
  /// </para>
  /// <para>
  /// There are two built strategies for determining the current date and time, 
  /// <see cref="LocalDateTime"/>
  /// and <see cref="UniversalDateTime"/>.
  /// </para>
  /// <para>
  /// The default strategy is <see cref="LocalDateTime"/>.
  /// </para>
  /// </remarks>
  public IDateTime DateTimeStrategy { get; set; } = new LocalDateTime();

  /// <summary>
  /// Gets or sets the date pattern to be used for generating file names
  /// when rolling over on date.
  /// </summary>
  /// <value>
  /// The date pattern to be used for generating file names when rolling 
  /// over on date.
  /// </value>
  /// <remarks>
  /// <para>
  /// Takes a string in the same format as expected by 
  /// <see cref="DateFormatter.SimpleDateFormatter" />.
  /// May be set to null to disable date formatting.
  /// </para>
  /// <para>
  /// This property determines the rollover schedule when rolling over
  /// on date.
  /// </para>
  /// </remarks>
  public string? DatePattern { get; set; } = ".yyyy-MM-dd";

  /// <summary>
  /// Gets or sets the maximum number of backup files that are kept before
  /// the oldest is erased.
  /// </summary>
  /// <value>
  /// The maximum number of backup files that are kept before the oldest is
  /// erased.
  /// </value>
  /// <remarks>
  /// <para>
  /// If set to zero, then there will be no backup files and the log file 
  /// will be truncated when it reaches <see cref="MaxFileSize"/>.  
  /// </para>
  /// <para>
  /// If a negative number is supplied then no deletions will be made.  Note 
  /// that this could result in very slow performance as a large number of 
  /// files are rolled over unless <see cref="CountDirection"/> is used.
  /// </para>
  /// <para>
  /// The maximum applies to <b>each</b> time based group of files and 
  /// <b>not</b> the total.
  /// </para>
  /// </remarks>
  public int MaxSizeRollBackups { get; set; }

  /// <summary>
  /// Gets or sets the maximum size that the output file is allowed to reach
  /// before being rolled over to backup files.
  /// </summary>
  /// <value>
  /// The maximum size in bytes that the output file is allowed to reach before being 
  /// rolled over to backup files.
  /// </value>
  /// <remarks>
  /// <para>
  /// This property is equivalent to <see cref="MaximumFileSize"/> except
  /// that it is required for differentiating the setter taking a
  /// <see cref="long"/> argument from the setter taking a <see cref="string"/> 
  /// argument.
  /// </para>
  /// <para>
  /// The default maximum file size is 10MB (10*1024*1024).
  /// </para>
  /// </remarks>
  public long MaxFileSize { get; set; } = 10 * 1024 * 1024;

  /// <summary>
  /// Gets or sets the maximum size that the output file is allowed to reach
  /// before being rolled over to backup files.
  /// </summary>
  /// <value>
  /// The maximum size that the output file is allowed to reach before being 
  /// rolled over to backup files.
  /// </value>
  /// <remarks>
  /// <para>
  /// This property allows you to specify the maximum size with the
  /// suffixes "KB", "MB" or "GB" so that the size is interpreted being 
  /// expressed respectively in kilobytes, megabytes or gigabytes. 
  /// </para>
  /// <para>
  /// For example, the value "10KB" will be interpreted as 10240 bytes.
  /// </para>
  /// <para>
  /// The default maximum file size is 10MB.
  /// </para>
  /// <para>
  /// If you have the option to set the maximum file size programmatically
  /// consider using the <see cref="MaxFileSize"/> property instead as this
  /// allows you to set the size in bytes as a <see cref="long"/>.
  /// </para>
  /// </remarks>
  public string MaximumFileSize
  {
    get => MaxFileSize.ToString(NumberFormatInfo.InvariantInfo);
    set => MaxFileSize = OptionConverter.ToFileSize(value, MaxFileSize + 1);
  }

  /// <summary>
  /// Gets or sets the rolling file count direction. 
  /// </summary>
  /// <value>
  /// The rolling file count direction.
  /// </value>
  /// <remarks>
  /// <para>
  /// Indicates if the current file is the lowest numbered file or the
  /// highest numbered file.
  /// </para>
  /// <para>
  /// By default, newer files have lower numbers (<see cref="CountDirection" /> &lt; 0),
  /// i.e. log.1 is most recent, log.5 is the 5th backup, etc...
  /// </para>
  /// <para>
  /// <see cref="CountDirection" /> &gt;= 0 does the opposite i.e.
  /// log.1 is the first backup made, log.5 is the 5th backup made, etc.
  /// For infinite backups use <see cref="CountDirection" /> &gt;= 0 to reduce 
  /// rollover costs.
  /// </para>
  /// <para>The default file count direction is -1.</para>
  /// </remarks>
  public int CountDirection { get; set; } = -1;

  /// <summary>
  /// Gets or sets the rolling style.
  /// </summary>
  /// <value>The rolling style.</value>
  /// <remarks>
  /// <para>
  /// The default rolling style is <see cref="RollingMode.Composite" />.
  /// </para>
  /// <para>
  /// When set to <see cref="RollingMode.Once"/> this appender's
  /// <see cref="FileAppender.AppendToFile"/> property is set to <see langword="false"/>, otherwise
  /// the appender would append to a single file rather than rolling
  /// the file each time it is opened.
  /// </para>
  /// </remarks>
  public RollingMode RollingStyle
  {
    get => _rollingStyle;
    set
    {
      _rollingStyle = value;
      switch (_rollingStyle)
      {
        case RollingMode.Once:
          _rollDate = false;
          _rollSize = false;

          AppendToFile = false;
          break;

        case RollingMode.Size:
          _rollDate = false;
          _rollSize = true;
          break;

        case RollingMode.Date:
          _rollDate = true;
          _rollSize = false;
          break;

        case RollingMode.Composite:
          _rollDate = true;
          _rollSize = true;
          break;
      }
    }
  }

  /// <summary>
  /// Gets or sets a value indicating whether to preserve the file name extension when rolling.
  /// </summary>
  /// <value>
  /// <see langword="true"/> if the file name extension should be preserved.
  /// </value>
  /// <remarks>
  /// <para>
  /// By default, file.log is rolled to file.log.yyyy-MM-dd or file.log.curSizeRollBackup.
  /// However, under Windows the new file name will lose any program associations as the
  /// extension is changed. Optionally file.log can be renamed to file.yyyy-MM-dd.log or
  /// file.curSizeRollBackup.log to maintain any program associations.
  /// </para>
  /// </remarks>
  public bool PreserveLogFileNameExtension { get; set; }

  /// <summary>
  /// Gets or sets a value indicating whether to always log to
  /// the same file.
  /// </summary>
  /// <value>
  /// <see langword="true"/> if always should be logged to the same file, otherwise <see langword="false"/>.
  /// </value>
  /// <remarks>
  /// <para>
  /// By default, file.log is always the current file.  Optionally
  /// file.log.yyyy-mm-dd for current formatted datePattern can by the currently
  /// logging file (or file.log.curSizeRollBackup or even
  /// file.log.yyyy-mm-dd.curSizeRollBackup).
  /// </para>
  /// <para>
  /// This will make time based rollovers with a large number of backups 
  /// much faster as the appender it won't have to rename all the backups!
  /// </para>
  /// </remarks>
  public bool StaticLogFileName { get; set; } = true;

  /// <summary>
  /// The fully qualified type of the RollingFileAppender class.
  /// </summary>
  /// <remarks>
  /// Used by the internal logger to record the Type of the
  /// log message.
  /// </remarks>
  private static readonly Type _declaringType = typeof(RollingFileAppender);

  /// <summary>
  /// Sets the quiet writer being used.
  /// </summary>
  /// <remarks>
  /// This method can be overridden by subclasses.
  /// </remarks>
  /// <param name="writer">the writer to set</param>
  protected override void SetQWForFiles(TextWriter writer) 
    => QuietWriter = new CountingQuietTextWriter(writer, ErrorHandler);

  /// <summary>
  /// Write out a logging event.
  /// </summary>
  /// <param name="loggingEvent">the event to write to file.</param>
  /// <remarks>
  /// <para>
  /// Handles append time behavior for RollingFileAppender.  This checks
  /// if a roll over either by date (checked first) or time (checked second)
  /// is need and then appends to the file last.
  /// </para>
  /// </remarks>
  protected override void Append(LoggingEvent loggingEvent)
  {
    AdjustFileBeforeAppend();
    base.Append(loggingEvent);
  }

  /// <summary>
  /// Write out an array of logging events.
  /// </summary>
  /// <param name="loggingEvents">the events to write to file.</param>
  /// <remarks>
  /// <para>
  /// Handles append time behavior for RollingFileAppender.  This checks
  /// if a roll over either by date (checked first) or time (checked second)
  /// is need and then appends to the file last.
  /// </para>
  /// </remarks>
  protected override void Append(LoggingEvent[] loggingEvents)
  {
    AdjustFileBeforeAppend();
    base.Append(loggingEvents);
  }

  /// <summary>
  /// Performs any required rolling before outputting the next event
  /// </summary>
  /// <remarks>
  /// <para>
  /// Handles append time behavior for RollingFileAppender.  This checks
  /// if a roll over either by date (checked first) or time (checked second)
  /// is need and then appends to the file last.
  /// </para>
  /// </remarks>
  protected virtual void AdjustFileBeforeAppend()
  {
    // reuse the file appenders locking model to lock the rolling
    try
    {
      // if rolling should be locked, acquire the lock
      _mutexForRolling?.WaitOne();
      if (_rollDate)
      {
        DateTime n = DateTimeStrategy.Now;
        if (n >= _nextCheck)
        {
          _now = n;
          _nextCheck = NextCheckDate(_now, _rollPoint);

          RollOverTime(true);
        }
      }

      if (_rollSize && (File is not null) && ((CountingQuietTextWriter)QuietWriter!).Count >= MaxFileSize)
      {
        RollOverSize();
      }
    }
    finally
    {
      // if rolling should be locked, release the lock
      _mutexForRolling?.ReleaseMutex();
    }
  }

  /// <summary>
  /// Creates and opens the file for logging.  If <see cref="StaticLogFileName"/>
  /// is false then the fully qualified name is determined and used.
  /// </summary>
  /// <param name="fileName">the name of the file to open</param>
  /// <param name="append">true to append to existing file</param>
  /// <remarks>
  /// <para>This method will ensure that the directory structure
  /// for the <paramref name="fileName"/> specified exists.</para>
  /// </remarks>
  protected override void OpenFile(string fileName, bool append)
  {
    lock (LockObj)
    {
      fileName = GetNextOutputFileName(fileName);

      // Calculate the current size of the file
      long currentCount = 0;
      if (append)
      {
        using (SecurityContext?.Impersonate(this))
        {
          if (System.IO.File.Exists(fileName))
          {
            currentCount = new FileInfo(fileName).Length;
          }
        }
      }
      else
      {
        if (LogLog.IsErrorEnabled)
        {
          // Internal check that the file is not being overwritten
          // If not Appending to an existing file we should have rolled the file out of the
          // way. Therefore, we should not be over-writing an existing file.
          // The only exception is if we are not allowed to roll the existing file away.
          if (MaxSizeRollBackups != 0 && FileExists(fileName))
          {
            LogLog.Error(_declaringType, $"RollingFileAppender: INTERNAL ERROR. Append is False but OutputFile [{fileName}] already exists.");
          }
        }
      }

      if (!StaticLogFileName)
      {
        _scheduledFilename = fileName;
      }

      // Open the file (call the base class to do it)
      base.OpenFile(fileName, append);

      // Set the file size onto the counting writer
      ((CountingQuietTextWriter)QuietWriter!).Count = currentCount;
    }
  }

  /// <summary>
  /// Get the current output file name
  /// </summary>
  /// <param name="fileName">the base file name</param>
  /// <returns>the output file name</returns>
  /// <remarks>
  /// The output file name is based on the base fileName specified.
  /// If <see cref="StaticLogFileName"/> is set then the output 
  /// file name is the same as the base file passed in. Otherwise
  /// the output file depends on the date pattern, on the count
  /// direction or both.
  /// </remarks>
  protected string GetNextOutputFileName(string fileName)
  {
    if (!StaticLogFileName)
    {
      fileName = fileName.EnsureNotNull().Trim();

      if (_rollDate)
      {
        fileName = CombinePath(fileName, _now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo));
      }

      if (CountDirection >= 0)
      {
        fileName = CombinePath(fileName, $".{CurrentSizeRollBackups}");
      }
    }

    return fileName;
  }

  /// <summary>
  /// Determines curSizeRollBackups (only within the current roll point)
  /// </summary>
  private void DetermineCurSizeRollBackups()
  {
    CurrentSizeRollBackups = 0;

    string? fullPath;
    string? fileName;

    using (SecurityContext?.Impersonate(this))
    {
      fullPath = Path.GetFullPath(_baseFileName!);
      fileName = Path.GetFileName(fullPath);
    }

    List<string> arrayFiles = GetExistingFiles(fullPath);
    InitializeRollBackups(fileName, arrayFiles);

    LogLog.Debug(_declaringType, $"curSizeRollBackups starts at [{CurrentSizeRollBackups}]");
  }

  /// <summary>
  /// Generates a wildcard pattern that can be used to find all files
  /// that are similar to the base file name.
  /// </summary>
  private string GetWildcardPatternForFile(string baseFileName)
  {
    if (PreserveLogFileNameExtension)
    {
      return $"{Path.GetFileNameWithoutExtension(baseFileName)}*{Path.GetExtension(baseFileName)}";
    }
    return $"{baseFileName}*";
  }

  /// <summary>
  /// Builds a list of filenames for all files matching the base filename plus a file pattern.
  /// </summary>
  [System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA1002:Do not expose generic lists")]
  protected List<string> GetExistingFiles(string baseFilePath)
  {
    List<string> result = [];

    string? directory;
    using (SecurityContext?.Impersonate(this))
    {
      string fullPath = Path.GetFullPath(baseFilePath);

      directory = Path.GetDirectoryName(fullPath);
      if (Directory.Exists(directory))
      {
        string baseFileName = Path.GetFileName(fullPath);

        string[] files = Directory.GetFiles(directory, GetWildcardPatternForFile(baseFileName));
        result.AddRange(files
          .Select(Path.GetFileName)
          .Where(curFileName => curFileName.StartsWith(Path.GetFileNameWithoutExtension(baseFileName))));
      }
    }
    LogLog.Debug(_declaringType, "Searched for existing files in [" + directory + "]");
    return result;
  }

  /// <summary>
  /// Initiates a roll-over if needed for crossing a date boundary since the last run.
  /// </summary>
  private void RollOverIfDateBoundaryCrossing()
  {
    if (StaticLogFileName && _rollDate)
    {
      if (_baseFileName is not null && FileExists(_baseFileName))
      {
        DateTime last;
        using (SecurityContext?.Impersonate(this))
        {
          last = DateTimeStrategy is UniversalDateTime
            ? System.IO.File.GetLastWriteTimeUtc(_baseFileName)
            : System.IO.File.GetLastWriteTime(_baseFileName);
        }

        LogLog.Debug(_declaringType, $"[{last.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo)}] vs. [{_now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo)}]");

        if (!StringComparer.Ordinal.Equals(last.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo), 
                                           _now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo)))
        {
          _scheduledFilename = CombinePath(_baseFileName, last.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo));
          LogLog.Debug(_declaringType, $"Initial roll over to [{_scheduledFilename}]");
          RollOverTime(false);
          LogLog.Debug(_declaringType, $"curSizeRollBackups after rollOver at [{CurrentSizeRollBackups}]");
        }
      }
    }
  }

  /// <summary>
  /// Initializes based on existing conditions at time of <see cref="ActivateOptions"/>.
  /// </summary>
  /// <remarks>
  /// <para>
  /// Initializes based on existing conditions at time of <see cref="ActivateOptions"/>.
  /// The following is done
  /// <list type="bullet">
  ///  <item>determine curSizeRollBackups (only within the current roll point)</item>
  ///  <item>initiates a roll-over if needed for crossing a date boundary since the last run.</item>
  ///  </list>
  ///  </para>
  /// </remarks>
  protected void ExistingInit()
  {
    DetermineCurSizeRollBackups();
    RollOverIfDateBoundaryCrossing();

    // If file exists, and we are not appending then roll it out of the way
    if (AppendToFile)
    {
      return;
    }

    bool fileExists;
    string fileName = GetNextOutputFileName(_baseFileName!);

    using (SecurityContext?.Impersonate(this))
    {
      fileExists = System.IO.File.Exists(fileName);
    }

    if (!fileExists)
    {
      return;
    }

    if (MaxSizeRollBackups == 0)
    {
      LogLog.Debug(_declaringType, $"Output file [{fileName}] already exists. MaxSizeRollBackups is 0; cannot roll. Overwriting existing file.");
    }
    else
    {
      LogLog.Debug(_declaringType, $"Output file [{fileName}] already exists. Not appending to file. Rolling existing file out of the way.");
      RollOverRenameFiles(fileName);
    }
  }

  /// <summary>
  /// Does the work of bumping the 'current' file counter higher
  /// to the highest count when an incremental file name is seen.
  /// The highest count is either the first file (when count direction
  /// is greater than 0) or the last file (when count direction less than 0).
  /// In either case, we want to know the highest count that is present.
  /// </summary>
  /// <param name="baseFile"></param>
  /// <param name="curFileName"></param>
  private void InitializeFromOneFile(string baseFile, string curFileName)
  {
    curFileName = curFileName.ToLowerInvariant();
    baseFile = baseFile.ToLowerInvariant();
    string dir = string.IsNullOrEmpty(baseFile) ? "" : Path.GetDirectoryName(baseFile);
    string baseFileWithoutExtension = Path.Combine(dir ?? "", Path.GetFileNameWithoutExtension(baseFile));
    
    if (curFileName.StartsWith(baseFileWithoutExtension) == false)
    {
      return; // This is not a log file, so ignore
    }

    if (curFileName.Equals(baseFile, StringComparison.Ordinal))
    {
      return; // Base log file is not an incremented logfile (.1 or .2, etc.)
    }

    // Only look for files in the current roll point
    if (_rollDate && !StaticLogFileName)
    {
      string date = DateTimeStrategy.Now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo).ToLowerInvariant();
      string prefix = (PreserveLogFileNameExtension
        ? Path.GetFileNameWithoutExtension(baseFile) + date
        : baseFile + date).ToLowerInvariant();
      string suffix = PreserveLogFileNameExtension
        ? Path.GetExtension(baseFile).ToLowerInvariant()
        : "";
      string curFileNameWithoutDir = Path.GetFileName(curFileName);
      if (!curFileNameWithoutDir.StartsWith(prefix) || !curFileNameWithoutDir.EndsWith(suffix))
      {
        LogLog.Debug(_declaringType, $"Ignoring file [{curFileName}] because it is from a different date period");
        return;
      }
    }

    try
    {
      // Bump the counter up to the highest count seen so far
      int backup = GetBackupIndex(curFileName);

      // caution: we might get a false positive when certain
      // date patterns such as yyyyMMdd are used...those are
      // valid number but aren't the kind of back up index
      // we're looking for
      if (backup > CurrentSizeRollBackups)
      {
        if (0 == MaxSizeRollBackups)
        {
          // Stay at zero when zero backups are desired
        }
        else if (-1 == MaxSizeRollBackups)
        {
          // Infinite backups, so go as high as the highest value
          CurrentSizeRollBackups = backup;
        }
        else
        {
          // Backups limited to a finite number
          if (CountDirection >= 0)
          {
            // Go with the highest file when counting up
            CurrentSizeRollBackups = backup;
          }
          else
          {
            // Clip to the limit when counting down
            if (backup <= MaxSizeRollBackups)
            {
              CurrentSizeRollBackups = backup;
            }
          }
        }
        LogLog.Debug(_declaringType, $"File name [{curFileName}] moves current count to [{CurrentSizeRollBackups}]");
      }
    }
    catch (FormatException)
    {
      //this happens when file.log -> file.log.yyyy-MM-dd which is normal
      //when staticLogFileName == false
      LogLog.Debug(_declaringType, $"Encountered a backup file not ending in .x [{curFileName}]");
    }
  }

  /// <summary>
  /// Attempts to extract a number from the end of the file name that indicates
  /// the number of the times the file has been rolled over.
  /// </summary>
  /// <remarks>
  /// Certain date pattern extensions like yyyyMMdd will be parsed as valid backup indexes.
  /// </remarks>
  private int GetBackupIndex(string curFileName)
  {
    int result = -1;
    string fileName = curFileName;

    if (PreserveLogFileNameExtension)
    {
      fileName = Path.GetFileNameWithoutExtension(fileName);
    }

    int index = fileName.LastIndexOf(".", StringComparison.Ordinal);
    if (index > 0)
    {
      // if the "yyyy-MM-dd" component of file.log.yyyy-MM-dd is passed to TryParse
      // it will gracefully fail and return backUpIndex will be 0
      _ = SystemInfo.TryParse(fileName.Substring(index + 1), out result);
    }

    return result;
  }

  /// <summary>
  /// Takes a list of files and a base file name, and looks for 'incremented' versions of the base file.
  /// Bumps the max count up to the highest count seen.
  /// </summary>
  [EditorBrowsable(EditorBrowsableState.Never)]
  public void InitializeRollBackups(string baseFile, IList<string> arrayFiles)
  {
    string baseFileLower = baseFile.EnsureNotNull().ToLowerInvariant();

    foreach (string curFileName in arrayFiles.EnsureNotNull())
    {
      InitializeFromOneFile(baseFileLower, curFileName.ToLowerInvariant());
    }
  }

  /// <summary>
  /// Calculates the RollPoint for the datePattern supplied.
  /// </summary>
  /// <param name="datePattern">the date pattern to calculate the check period for</param>
  /// <returns>The RollPoint that is most accurate for the date pattern supplied</returns>
  /// <remarks>
  /// Essentially the date pattern is examined to determine what the
  /// most suitable roll point is. The roll point chosen is the roll point
  /// with the smallest period that can be detected using the date pattern
  /// supplied. i.e. if the date pattern only outputs the year, month, day 
  /// and hour then the smallest roll point that can be detected would be
  /// and hourly roll point as minutes could not be detected.
  /// </remarks>
  [EditorBrowsable(EditorBrowsableState.Never)]
  public static RollPoint ComputeCheckPeriod(string datePattern)
  {
    // s_date1970 is 1970-01-01 00:00:00 this is UniversalSortableDateTimePattern 
    // (based on ISO 8601) using universal time. This date is used for reference
    // purposes to calculate the resolution of the date pattern.

    // Get string representation of baseline date
    string r0 = _sDate1970.ToString(datePattern, DateTimeFormatInfo.InvariantInfo);

    // Check each type of rolling mode starting with the smallest increment.
    for (int i = (int)RollPoint.TopOfMinute; i <= (int)RollPoint.TopOfMonth; i++)
    {
      // Get string representation of next pattern
      string r1 = NextCheckDate(_sDate1970, (RollPoint)i).ToString(datePattern, DateTimeFormatInfo.InvariantInfo);

      LogLog.Debug(_declaringType, $"Type = [{i}], r0 = [{r0}], r1 = [{r1}]");

      // Check if the string representations are different
      if (!r0.Equals(r1, StringComparison.Ordinal))
      {
        // Found the highest precision roll point
        return (RollPoint)i;
      }
    }

    return RollPoint.InvalidRollPoint; // Deliberately head for trouble...
  }

  /// <summary>
  /// Initialize the appender based on the options set
  /// </summary>
  /// <remarks>
  /// <para>
  /// This is part of the <see cref="IOptionHandler"/> delayed object
  /// activation scheme. The <see cref="ActivateOptions"/> method must 
  /// be called on this object after the configuration properties have
  /// been set. Until <see cref="ActivateOptions"/> is called this
  /// object is in an undefined state and must not be used. 
  /// </para>
  /// <para>
  /// If any of the configuration properties are modified then 
  /// <see cref="ActivateOptions"/> must be called again.
  /// </para>
  /// <para>
  /// Sets initial conditions including date/time roll over information, first check,
  /// scheduledFilename, and calls <see cref="ExistingInit"/> to initialize
  /// the current number of backups.
  /// </para>
  /// </remarks>
  public override void ActivateOptions()
  {
    if (_rollDate && DatePattern is not null)
    {
      _now = DateTimeStrategy.Now;
      _rollPoint = ComputeCheckPeriod(DatePattern);

      if (_rollPoint == RollPoint.InvalidRollPoint)
      {
        throw new ArgumentException($"Invalid RollPoint, unable to parse [{DatePattern}]");
      }

      // next line added as this removes the name check in rollOver
      _nextCheck = NextCheckDate(_now, _rollPoint);
    }
    else
    {
      if (_rollDate)
      {
        ErrorHandler.Error($"Either DatePattern or rollingStyle options are not set for [{Name}].");
      }
    }

    SecurityContext ??= SecurityContextProvider.DefaultProvider.CreateSecurityContext(this);

    using (SecurityContext.Impersonate(this))
    {
      // Must convert the FileAppender's filePath to an absolute path before we
      // call ExistingInit(). This will be done by the base.ActivateOptions() but
      // we need to duplicate that functionality here first.
      base.File = ConvertToFullPath(base.File!.Trim());

      // Store fully qualified base file name
      _baseFileName = base.File;
    }

    // initialize the mutex that is used to lock rolling
    _mutexForRolling = new Mutex(false, _baseFileName
      .Replace("\\", "_")
      .Replace(":", "_")
      .Replace("/", "_") + "_rolling"
    );

    if (_rollDate && File is not null && _scheduledFilename is null)
    {
      _scheduledFilename = CombinePath(File, _now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo));
    }

    ExistingInit();

    base.ActivateOptions();
  }

  /// <summary>
  /// CombinePath
  /// </summary>
  /// <param name="path1"></param>
  /// <param name="path2">.1, .2, .3, etc.</param>
  /// <returns></returns>
  private string CombinePath(string path1, string path2)
  {
    string extension = Path.GetExtension(path1);
    if (PreserveLogFileNameExtension && extension.Length > 0)
    {
      return Path.Combine(Path.GetDirectoryName(path1) ?? string.Empty, Path.GetFileNameWithoutExtension(path1) + path2 + extension);
    }
    return path1 + path2;
  }

  /// <summary>
  /// Rollover the file(s) to date/time tagged file(s).
  /// </summary>
  /// <param name="fileIsOpen">set to true if the file to be rolled is currently open</param>
  /// <remarks>
  /// <para>
  /// Rollover the file(s) to date/time tagged file(s).
  /// Resets curSizeRollBackups. 
  /// If fileIsOpen is set then the new file is opened (through SafeOpenFile).
  /// </para>
  /// </remarks>
  protected void RollOverTime(bool fileIsOpen)
  {
    if (StaticLogFileName)
    {
      // Compute filename, but only if datePattern is specified
      if (DatePattern is null)
      {
        ErrorHandler.Error("Missing DatePattern option in rollOver().");
        return;
      }

      //is the new file name equivalent to the 'current' one
      //something has gone wrong if we hit this -- we should only
      //roll over if the new file will be different from the old
      string dateFormat = _now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo);
      if (string.Equals(_scheduledFilename, CombinePath(File!, dateFormat), StringComparison.Ordinal))
      {
        ErrorHandler.Error($"Compare {_scheduledFilename} : {CombinePath(File!, dateFormat)}");
        return;
      }

      if (fileIsOpen)
      {
        // close current file, and rename it to datedFilename
        CloseFile();
      }

      //we may have to roll over a large number of backups here
      for (int i = 1; i <= CurrentSizeRollBackups; i++)
      {
        string from = CombinePath(File!, "." + i);
        string to = CombinePath(_scheduledFilename!, "." + i);
        RollFile(from, to);
      }

      RollFile(File!, _scheduledFilename!);
    }

    //We've cleared out the old date and are ready for the new
    CurrentSizeRollBackups = 0;

    //new scheduled name
    _scheduledFilename = CombinePath(File!, _now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo));

    if (fileIsOpen)
    {
      // This will also close the file. This is OK since multiple close operations are safe.
      SafeOpenFile(_baseFileName!, false);
    }
  }

  /// <summary>
  /// Renames file <paramref name="fromFile"/> to file <paramref name="toFile"/>.
  /// </summary>
  /// <param name="fromFile">Name of existing file to roll.</param>
  /// <param name="toFile">New name for file.</param>
  /// <remarks>
  /// <para>
  /// Renames file <paramref name="fromFile"/> to file <paramref name="toFile"/>. It
  /// also checks for existence of target file and deletes if it does.
  /// </para>
  /// </remarks>
  protected void RollFile(string fromFile, string toFile)
  {
    if (FileExists(fromFile))
    {
      // Delete the toFile if it exists
      DeleteFile(toFile);

      // We may not have permission to move the file, or the file may be locked
      try
      {
        LogLog.Debug(_declaringType, $"Moving [{fromFile}] -> [{toFile}]");
        using (SecurityContext?.Impersonate(this))
        {
          System.IO.File.Move(fromFile, toFile);
        }
      }
      catch (Exception e) when (!e.IsFatal())
      {
        ErrorHandler.Error($"Exception while rolling file [{fromFile}] -> [{toFile}]", e, ErrorCode.GenericFailure);
      }
    }
    else
    {
      LogLog.Warn(_declaringType, $"Cannot RollFile [{fromFile}] -> [{toFile}]. Source does not exist");
    }
  }

  /// <summary>
  /// Test if a file exists at a specified path
  /// </summary>
  /// <param name="path">the path to the file</param>
  /// <returns>true if the file exists</returns>
  /// <remarks>
  /// <para>
  /// Test if a file exists at a specified path
  /// </para>
  /// </remarks>
  protected bool FileExists(string path)
  {
    using (SecurityContext?.Impersonate(this))
    {
      return System.IO.File.Exists(path);
    }
  }

  /// <summary>
  /// Deletes the specified file if it exists.
  /// </summary>
  /// <param name="fileName">The file to delete.</param>
  /// <remarks>
  /// <para>
  /// Delete a file if it exists.
  /// The file is first moved to a new filename then deleted.
  /// This allows the file to be removed even when it cannot
  /// be deleted, but it still can be moved.
  /// </para>
  /// </remarks>
  protected void DeleteFile(string fileName)
  {
    LogLog.Debug(_declaringType, $"Trying to delete [{fileName}]");

    if (!FileExists(fileName))
    {
      LogLog.Debug(_declaringType, $"[{fileName}] does not exist");
      return;
    }
    // We may not have permission to delete the file, or the file may be locked
    string fileToDelete = fileName;

    // Try to move the file to temp name.
    // If the file is locked we may still be able to move it
    string tempFileName = fileName + "." + Environment.TickCount + ".DeletePending";
    try
    {
      using (SecurityContext?.Impersonate(this))
      {
        System.IO.File.Move(fileName, tempFileName);
      }
      fileToDelete = tempFileName;
    }
    catch (Exception e) when (!e.IsFatal())
    {
      LogLog.Debug(_declaringType, $"Exception while moving file to be deleted [{fileName}] -> [{tempFileName}]", e);
    }

    // Try to delete the file (either the original or the moved file)
    try
    {
      using (SecurityContext?.Impersonate(this))
      {
        System.IO.File.Delete(fileToDelete);
      }
      LogLog.Debug(_declaringType, $"Deleted file [{fileName}]");
    }
    catch (Exception e) when (!e.IsFatal())
    {
      if (fileToDelete == fileName)
      {
        // Unable to move or delete the file
        ErrorHandler.Error($"Exception while deleting file [{fileToDelete}]", e, ErrorCode.GenericFailure);
      }
      else
      {
        // ReSharper disable once GrammarMistakeInComment
        // Moved the file, but the delete failed. File is probably locked.
        // The file should automatically be deleted when the lock is released.
        LogLog.Debug(_declaringType, $"Exception while deleting temp file [{fileToDelete}]", e);
      }
    }
  }

  /// <summary>
  /// Implements file roll base on file size.
  /// </summary>
  /// <remarks>
  /// <para>
  /// If the maximum number of size based backups is reached
  /// (<c>curSizeRollBackups == maxSizeRollBackups</c>) then the oldest
  /// file is deleted -- its index determined by the sign of countDirection.
  /// If <c>countDirection</c> &lt; 0, then files
  /// {<c>File.1</c>, ..., <c>File.curSizeRollBackups -1</c>}
  /// are renamed to {<c>File.2</c>, ...,
  /// <c>File.curSizeRollBackups</c>}. Moreover, <c>File</c> is
  /// renamed <c>File.1</c> and closed.
  /// </para>
  /// <para>
  /// A new file is created to receive further log output.
  /// </para>
  /// <para>
  /// If <c>maxSizeRollBackups</c> is equal to zero, then the
  /// <c>File</c> is truncated with no backup files created.
  /// </para>
  /// <para>
  /// If <c>maxSizeRollBackups</c> &lt; 0, then <c>File</c> is
  /// renamed if needed and no files are deleted.
  /// </para>
  /// </remarks>
  protected void RollOverSize()
  {
    CloseFile(); // keep windows happy.

    LogLog.Debug(_declaringType, $"rolling over count [{((CountingQuietTextWriter)QuietWriter!).Count}]");
    LogLog.Debug(_declaringType, $"maxSizeRollBackups [{MaxSizeRollBackups}]");
    LogLog.Debug(_declaringType, $"curSizeRollBackups [{CurrentSizeRollBackups}]");
    LogLog.Debug(_declaringType, $"countDirection [{CountDirection}]");

    if (File is not null)
    {
      RollOverRenameFiles(File);
    }

    if (!StaticLogFileName && CountDirection >= 0)
    {
      CurrentSizeRollBackups++;
    }

    // This will also close the file. This is OK since multiple close operations are safe.
    SafeOpenFile(_baseFileName!, false);
  }

  /// <summary>
  /// Implements file roll.
  /// </summary>
  /// <param name="baseFileName">the base name to rename</param>
  /// <remarks>
  /// <para>
  /// If the maximum number of size based backups is reached
  /// (<c>curSizeRollBackups == maxSizeRollBackups</c>) then the oldest
  /// file is deleted -- its index determined by the sign of countDirection.
  /// If <c>countDirection</c> &lt; 0, then files
  /// {<c>File.1</c>, ..., <c>File.curSizeRollBackups -1</c>}
  /// are renamed to {<c>File.2</c>, ...,
  /// <c>File.curSizeRollBackups</c>}. 
  /// </para>
  /// <para>
  /// If <c>maxSizeRollBackups</c> is equal to zero, then the
  /// <c>File</c> is truncated with no backup files created.
  /// </para>
  /// <para>
  /// If <c>maxSizeRollBackups</c> &lt; 0, then <c>File</c> is
  /// renamed if needed and no files are deleted.
  /// </para>
  /// <para>
  /// This is called by <see cref="RollOverSize"/> to rename the files.
  /// </para>
  /// </remarks>
  protected virtual void RollOverRenameFiles(string baseFileName)
  {
    baseFileName.EnsureNotNull();
    // If maxBackups <= 0, then there is no file renaming to be done.
    if (MaxSizeRollBackups == 0)
    {
      return;
    }
    if (CountDirection < 0)
    {
      // Delete the oldest file, to keep Windows happy.
      if (CurrentSizeRollBackups == MaxSizeRollBackups)
      {
        DeleteFile(CombinePath(baseFileName, "." + MaxSizeRollBackups));
        CurrentSizeRollBackups--;
      }

      // Map {(maxBackupIndex - 1), ..., 2, 1} to {maxBackupIndex, ..., 3, 2}
      for (int i = CurrentSizeRollBackups; i >= 1; i--)
      {
        RollFile(CombinePath(baseFileName, "." + i), CombinePath(baseFileName, "." + (i + 1)));
      }

      CurrentSizeRollBackups++;

      // Rename fileName to fileName.1
      RollFile(baseFileName, CombinePath(baseFileName, ".1"));
    }
    else
    {
      //countDirection >= 0
      if (CurrentSizeRollBackups >= MaxSizeRollBackups && MaxSizeRollBackups > 0)
      {
        //delete the first and keep counting up.
        int oldestFileIndex = CurrentSizeRollBackups - MaxSizeRollBackups;

        // If static then there is 1 file without a number, therefore 1 less archive
        if (StaticLogFileName)
        {
          oldestFileIndex++;
        }

        // If using a static log file then the base for the numbered sequence is the baseFileName passed in
        // If not using a static log file then the baseFileName will already have a numbered postfix which
        // we must remove, however it may have a date postfix which we must keep!
        string archiveFileBaseName = baseFileName;
        if (!StaticLogFileName)
        {
          if (PreserveLogFileNameExtension)
          {
            string extension = Path.GetExtension(archiveFileBaseName);
            string baseName = Path.Combine(Path.GetDirectoryName(archiveFileBaseName), Path.GetFileNameWithoutExtension(archiveFileBaseName));
            int lastDotIndex = baseName.LastIndexOf(".", StringComparison.Ordinal);
            if (lastDotIndex >= 0)
            {
              archiveFileBaseName = baseName.Substring(0, lastDotIndex) + extension;
            }
          }
          else
          {
            int lastDotIndex = archiveFileBaseName.LastIndexOf(".", StringComparison.Ordinal);
            if (lastDotIndex >= 0)
            {
              archiveFileBaseName = archiveFileBaseName.Substring(0, lastDotIndex);
            }
          }
        }

        // Delete the archive file
        DeleteFile(CombinePath(archiveFileBaseName, "." + oldestFileIndex));
      }

      if (StaticLogFileName)
      {
        CurrentSizeRollBackups++;
        RollFile(baseFileName, CombinePath(baseFileName, "." + CurrentSizeRollBackups));
      }
    }
  }

  /// <summary>
  /// Get the start time of the next window for the current roll point
  /// </summary>
  /// <param name="currentDateTime">the current date</param>
  /// <param name="rollPoint">the type of roll point we are working with</param>
  /// <returns>the start time for the next roll point an interval after the currentDateTime date</returns>
  /// <remarks>
  /// <para>
  /// Returns the date of the next roll point after the currentDateTime date passed to the method.
  /// </para>
  /// <para>
  /// The basic strategy is to subtract the time parts that are less significant
  /// than the roll point from the current time. This should roll the time back to
  /// the start of the time window for the current roll point. Then we add 1 window
  /// worth of time and get the start time of the next window for the roll point.
  /// </para>
  /// </remarks>
  protected static DateTime NextCheckDate(DateTime currentDateTime, RollPoint rollPoint)
  {
    // Local variable to work on (this does not look very efficient)
    DateTime current = currentDateTime;

    // Do slightly different things depending on what the type of roll point we want.
    switch (rollPoint)
    {
      case RollPoint.TopOfMinute:
        current = current.AddMilliseconds(-current.Millisecond);
        current = current.AddSeconds(-current.Second);
        current = current.AddMinutes(1);
        break;

      case RollPoint.TopOfHour:
        current = current.AddMilliseconds(-current.Millisecond);
        current = current.AddSeconds(-current.Second);
        current = current.AddMinutes(-current.Minute);
        current = current.AddHours(1);
        break;

      case RollPoint.HalfDay:
        current = current.AddMilliseconds(-current.Millisecond);
        current = current.AddSeconds(-current.Second);
        current = current.AddMinutes(-current.Minute);

        if (current.Hour < 12)
        {
          current = current.AddHours(12 - current.Hour);
        }
        else
        {
          current = current.AddHours(-current.Hour);
          current = current.AddDays(1);
        }
        break;

      case RollPoint.TopOfDay:
        current = current.AddMilliseconds(-current.Millisecond);
        current = current.AddSeconds(-current.Second);
        current = current.AddMinutes(-current.Minute);
        current = current.AddHours(-current.Hour);
        current = current.AddDays(1);
        break;

      case RollPoint.TopOfWeek:
        current = current.AddMilliseconds(-current.Millisecond);
        current = current.AddSeconds(-current.Second);
        current = current.AddMinutes(-current.Minute);
        current = current.AddHours(-current.Hour);
        current = current.AddDays(7 - (int)current.DayOfWeek);
        break;

      case RollPoint.TopOfMonth:
        current = current.AddMilliseconds(-current.Millisecond);
        current = current.AddSeconds(-current.Second);
        current = current.AddMinutes(-current.Minute);
        current = current.AddHours(-current.Hour);
        current = current.AddDays(1 - current.Day); /* first day of month is 1 not 0 */
        current = current.AddMonths(1);
        break;
    }
    return current;
  }

  /// <summary>
  /// The actual formatted filename that is currently being written to
  /// or will be the file transferred to on roll over
  /// (based on staticLogFileName).
  /// </summary>
  private string? _scheduledFilename;

  /// <summary>
  /// The timestamp when we shall next recompute the filename.
  /// </summary>
  private DateTime _nextCheck = DateTime.MaxValue;

  /// <summary>
  /// Holds date of last roll over
  /// </summary>
  private DateTime _now;

  /// <summary>
  /// The type of rolling done
  /// </summary>
  private RollPoint _rollPoint;

  /// <summary>
  /// How many sized based backups have been made so far
  /// </summary>
  public int CurrentSizeRollBackups { get; set; }

  /// <summary>
  /// The rolling mode used in this appender.
  /// </summary>
  private RollingMode _rollingStyle = RollingMode.Composite;

  /// <summary>
  /// Cache flag set if we are rolling by date.
  /// </summary>
  private bool _rollDate = true;

  /// <summary>
  /// Cache flag set if we are rolling by size.
  /// </summary>
  private bool _rollSize = true;

  /// <summary>
  /// FileName provided in configuration.  Used for rolling properly
  /// </summary>
  private string? _baseFileName;

  /// <summary>
  /// A mutex that is used to lock rolling of files.
  /// </summary>
  private Mutex? _mutexForRolling;

  /// <summary>
  /// The 1st of January 1970 in UTC
  /// </summary>
  private static readonly DateTime _sDate1970 = new(1970, 1, 1);
}