Add TarEntry conversion constructors

src/libraries/System.Formats.Tar/ref/System.Formats.Tar.cs

@@ -8,12 +8,14 @@ namespace System.Formats.Tar
 {
     public sealed partial class GnuTarEntry : System.Formats.Tar.PosixTarEntry
     {
+        public GnuTarEntry(System.Formats.Tar.TarEntry other) { }
         public GnuTarEntry(System.Formats.Tar.TarEntryType entryType, string entryName) { }
         public System.DateTimeOffset AccessTime { get { throw null; } set { } }
         public System.DateTimeOffset ChangeTime { get { throw null; } set { } }
     }
     public sealed partial class PaxTarEntry : System.Formats.Tar.PosixTarEntry
     {
+        public PaxTarEntry(System.Formats.Tar.TarEntry other) { }
         public PaxTarEntry(System.Formats.Tar.TarEntryType entryType, string entryName) { }
         public PaxTarEntry(System.Formats.Tar.TarEntryType entryType, string entryName, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string>> extendedAttributes) { }
         public System.Collections.Generic.IReadOnlyDictionary<string, string> ExtendedAttributes { get { throw null; } }
@@ -32,6 +34,7 @@ internal TarEntry() { }
         public int Checksum { get { throw null; } }
         public System.IO.Stream? DataStream { get { throw null; } set { } }
         public System.Formats.Tar.TarEntryType EntryType { get { throw null; } }
+        public System.Formats.Tar.TarEntryFormat Format { get { throw null; } }
         public int Gid { get { throw null; } set { } }
         public long Length { get { throw null; } }
         public string LinkName { get { throw null; } set { } }
@@ -98,26 +101,28 @@ public enum TarFileMode
     public sealed partial class TarReader : System.IDisposable
     {
         public TarReader(System.IO.Stream archiveStream, bool leaveOpen = false) { }
-        public System.Formats.Tar.TarEntryFormat Format { get { throw null; } }
         public System.Collections.Generic.IReadOnlyDictionary<string, string>? GlobalExtendedAttributes { get { throw null; } }
         public void Dispose() { }
         public System.Formats.Tar.TarEntry? GetNextEntry(bool copyData = false) { throw null; }
     }
     public sealed partial class TarWriter : System.IDisposable
     {
+        public TarWriter(System.IO.Stream archiveStream) { }
         public TarWriter(System.IO.Stream archiveStream, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string>>? globalExtendedAttributes = null, bool leaveOpen = false) { }
-        public TarWriter(System.IO.Stream archiveStream, System.Formats.Tar.TarEntryFormat archiveFormat, bool leaveOpen = false) { }
+        public TarWriter(System.IO.Stream archiveStream, System.Formats.Tar.TarEntryFormat archiveFormat = System.Formats.Tar.TarEntryFormat.Pax, bool leaveOpen = false) { }
         public System.Formats.Tar.TarEntryFormat Format { get { throw null; } }
         public void Dispose() { }
         public void WriteEntry(System.Formats.Tar.TarEntry entry) { }
         public void WriteEntry(string fileName, string? entryName) { }
     }
     public sealed partial class UstarTarEntry : System.Formats.Tar.PosixTarEntry
     {
+        public UstarTarEntry(System.Formats.Tar.TarEntry other) { }
         public UstarTarEntry(System.Formats.Tar.TarEntryType entryType, string entryName) { }
     }
     public sealed partial class V7TarEntry : System.Formats.Tar.TarEntry
     {
+        public V7TarEntry(System.Formats.Tar.TarEntry other) { }
         public V7TarEntry(System.Formats.Tar.TarEntryType entryType, string entryName) { }
     }
 }

src/libraries/System.Formats.Tar/src/System/Formats/Tar/GnuTarEntry.cs

@@ -9,9 +9,9 @@ namespace System.Formats.Tar
     /// <remarks>Even though the <see cref="TarEntryFormat.Gnu"/> format is not POSIX compatible, it implements and supports the Unix-specific fields that were defined in the POSIX IEEE P1003.1 standard from 1988: <c>devmajor</c>, <c>devminor</c>, <c>gname</c> and <c>uname</c>.</remarks>
     public sealed class GnuTarEntry : PosixTarEntry
     {
-        // Constructor used when reading an existing archive.
+        // Constructor called when reading a TarEntry from a TarReader.
         internal GnuTarEntry(TarHeader header, TarReader readerOfOrigin)
-            : base(header, readerOfOrigin)
+            : base(header, readerOfOrigin, TarEntryFormat.Gnu)
         {
         }
 
@@ -31,6 +31,57 @@ internal GnuTarEntry(TarHeader header, TarReader readerOfOrigin)
         public GnuTarEntry(TarEntryType entryType, string entryName)
             : base(entryType, entryName, TarEntryFormat.Gnu)
         {
+            _header._aTime = _header._mTime; // mtime was set in base constructor
+            _header._cTime = _header._mTime;
+        }
+
+        /// <summary>
+        /// Initializes a new <see cref="GnuTarEntry"/> instance by converting the specified <paramref name="other"/> entry into the GNU format.
+        /// </summary>
+        public GnuTarEntry(TarEntry other)
+            : base(other, TarEntryFormat.Gnu)
+        {
+            if (other is GnuTarEntry gnuOther)
+            {
+                _header._aTime = gnuOther.AccessTime;
+                _header._cTime = gnuOther.ChangeTime;
+                _header._gnuUnusedBytes = other._header._gnuUnusedBytes;
+            }
+            else
+            {
+                bool changedATime = false;
+                bool changedCTime = false;
+
+                if (other is PaxTarEntry paxOther)
+                {
+                    changedATime = TarHelpers.TryGetDateTimeOffsetFromTimestampString(paxOther._header._extendedAttributes, TarHeader.PaxEaATime, out DateTimeOffset aTime);
+                    if (changedATime)
+                    {
+                        _header._aTime = aTime;
+                    }
+
+                    changedCTime = TarHelpers.TryGetDateTimeOffsetFromTimestampString(paxOther._header._extendedAttributes, TarHeader.PaxEaCTime, out DateTimeOffset cTime);
+                    if (changedCTime)
+                    {
+                        _header._cTime = cTime;
+                    }
+                }
+
+                // Either 'other' was V7 or Ustar (those formats do not have atime or ctime),
+                // or 'other' was PAX and at least one of the timestamps was not found in the extended attributes
+                if (!changedATime || !changedCTime)
+                {
+                    DateTimeOffset now = DateTimeOffset.UtcNow;
+                    if (!changedATime)
+                    {
+                        _header._aTime = now;
+                    }
+                    if (!changedCTime)
+                    {
+                        _header._cTime = now;
+                    }
+                }
+            }
         }
 
         /// <summary>

src/libraries/System.Formats.Tar/src/System/Formats/Tar/PaxTarEntry.cs

@@ -14,12 +14,10 @@ public sealed class PaxTarEntry : PosixTarEntry
     {
         private ReadOnlyDictionary<string, string>? _readOnlyExtendedAttributes;
 
-        // Constructor used when reading an existing archive.
+        // Constructor called when reading a TarEntry from a TarReader.
         internal PaxTarEntry(TarHeader header, TarReader readerOfOrigin)
-            : base(header, readerOfOrigin)
+            : base(header, readerOfOrigin, TarEntryFormat.Pax)
         {
-            _header._extendedAttributes ??= new Dictionary<string, string>();
-            _readOnlyExtendedAttributes = null;
         }
 
         /// <summary>
@@ -52,6 +50,11 @@ internal PaxTarEntry(TarHeader header, TarReader readerOfOrigin)
         public PaxTarEntry(TarEntryType entryType, string entryName)
             : base(entryType, entryName, TarEntryFormat.Pax)
         {
+            _header._prefix = string.Empty;
+            _header._extendedAttributes = new Dictionary<string, string>();
+
+            Debug.Assert(_header._mTime != default);
+            AddNewAccessAndChangeTimestampsIfNotExist(useMTime: true);
         }
 
         /// <summary>
@@ -87,7 +90,40 @@ public PaxTarEntry(TarEntryType entryType, string entryName, IEnumerable<KeyValu
             : base(entryType, entryName, TarEntryFormat.Pax)
         {
             ArgumentNullException.ThrowIfNull(extendedAttributes);
-            _header.ReplaceNormalAttributesWithExtended(extendedAttributes);
+
+            _header._prefix = string.Empty;
+            _header._extendedAttributes = new Dictionary<string, string>(extendedAttributes);
+
+            Debug.Assert(_header._mTime != default);
+            AddNewAccessAndChangeTimestampsIfNotExist(useMTime: true);
+        }
+
+        /// <summary>
+        /// Initializes a new <see cref="PaxTarEntry"/> instance by converting the specified <paramref name="other"/> entry into the PAX format.
+        /// </summary>
+        public PaxTarEntry(TarEntry other)
+            : base(other, TarEntryFormat.Pax)
+        {
+            if (other._header._format is TarEntryFormat.Ustar or TarEntryFormat.Pax)
+            {
+                _header._prefix = other._header._prefix;
+            }
+
+            if (other is PaxTarEntry paxOther)
+            {
+                _header._extendedAttributes = new Dictionary<string, string>(paxOther.ExtendedAttributes);
+            }
+            else
+            {
+                _header._extendedAttributes = new Dictionary<string, string>();
+                if (other is GnuTarEntry gnuOther)
+                {
+                    _header._extendedAttributes[TarHeader.PaxEaATime] = TarHelpers.GetTimestampStringFromDateTimeOffset(gnuOther.AccessTime);
+                    _header._extendedAttributes[TarHeader.PaxEaCTime] = TarHelpers.GetTimestampStringFromDateTimeOffset(gnuOther.ChangeTime);
+                }
+            }
+
+            AddNewAccessAndChangeTimestampsIfNotExist(useMTime: false);
         }
 
         /// <summary>
@@ -112,12 +148,38 @@ public IReadOnlyDictionary<string, string> ExtendedAttributes
         {
             get
             {
-                Debug.Assert(_header._extendedAttributes != null);
+                _header._extendedAttributes ??= new Dictionary<string, string>();
                 return _readOnlyExtendedAttributes ??= _header._extendedAttributes.AsReadOnly();
             }
         }
 
         // Determines if the current instance's entry type supports setting a data stream.
         internal override bool IsDataStreamSetterSupported() => EntryType == TarEntryType.RegularFile;
+
+        // Checks if the extended attributes dictionary contains 'atime' and 'ctime'.
+        // If any of them is not found, it is added with the value of either the current entry's 'mtime',
+        // or 'DateTimeOffset.UtcNow', depending on the value of 'useMTime'.
+        private void AddNewAccessAndChangeTimestampsIfNotExist(bool useMTime)
+        {
+            Debug.Assert(!useMTime || (useMTime && _header._mTime != default));
+            Debug.Assert(_header._extendedAttributes != null);
+            bool containsATime = _header._extendedAttributes.ContainsKey(TarHeader.PaxEaATime);
+            bool containsCTime = _header._extendedAttributes.ContainsKey(TarHeader.PaxEaCTime);
+
+            if (!containsATime || !containsCTime)
+            {
+                string secondsFromEpochString = TarHelpers.GetTimestampStringFromDateTimeOffset(useMTime ? _header._mTime : DateTimeOffset.UtcNow);
+
+                if (!containsATime)
+                {
+                    _header._extendedAttributes[TarHeader.PaxEaATime] = secondsFromEpochString;
+                }
+
+                if (!containsCTime)
+                {
+                    _header._extendedAttributes[TarHeader.PaxEaCTime] = secondsFromEpochString;
+                }
+            }
+        }
     }
 }

src/libraries/System.Formats.Tar/src/System/Formats/Tar/PosixTarEntry.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+
 namespace System.Formats.Tar
 {
     /// <summary>
@@ -10,16 +12,37 @@ namespace System.Formats.Tar
     /// Even though the <see cref="TarEntryFormat.Gnu"/> format is not POSIX compatible, it implements and supports the Unix-specific fields that were defined in that POSIX standard.</remarks>
     public abstract partial class PosixTarEntry : TarEntry
     {
-        // Constructor used when reading an existing archive.
-        internal PosixTarEntry(TarHeader header, TarReader readerOfOrigin)
-            : base(header, readerOfOrigin)
+        // Constructor called when reading a TarEntry from a TarReader.
+        internal PosixTarEntry(TarHeader header, TarReader readerOfOrigin, TarEntryFormat format)
+            : base(header, readerOfOrigin, format)
         {
         }
 
-        // Constructor called when creating a new 'TarEntry*' instance that can be passed to a TarWriter.
+        // Constructor called when the user creates a TarEntry instance from scratch.
         internal PosixTarEntry(TarEntryType entryType, string entryName, TarEntryFormat format)
             : base(entryType, entryName, format)
         {
+            _header._uName = string.Empty;
+            _header._gName = string.Empty;
+            _header._devMajor = 0;
+            _header._devMinor = 0;
+        }
+
+        // Constructor called when converting an entry to the selected format.
+        internal PosixTarEntry(TarEntry other, TarEntryFormat format)
+            : base(other, format)
+        {
+            if (other is PosixTarEntry)
+            {
+                Debug.Assert(other._header._uName != null);
+                Debug.Assert(other._header._gName != null);
+                _header._uName = other._header._uName;
+                _header._gName = other._header._gName;
+                _header._devMajor = other._header._devMajor;
+                _header._devMinor = other._header._devMinor;
+            }
+            _header._uName ??= string.Empty;
+            _header._gName ??= string.Empty;
         }
 
         /// <summary>

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarEntry.cs

@@ -1,7 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
 using Microsoft.Win32.SafeHandles;
@@ -15,42 +14,72 @@ namespace System.Formats.Tar
     public abstract partial class TarEntry
     {
         internal TarHeader _header;
+
         // Used to access the data section of this entry in an unseekable file
         private TarReader? _readerOfOrigin;
 
-        // Constructor used when reading an existing archive.
-        internal TarEntry(TarHeader header, TarReader readerOfOrigin)
+        // Constructor called when reading a TarEntry from a TarReader.
+        internal TarEntry(TarHeader header, TarReader readerOfOrigin, TarEntryFormat format)
         {
+            // This constructor is called after reading a header from the archive,
+            // and we should've already detected the format of the header.
+            Debug.Assert(header._format == format);
             _header = header;
             _readerOfOrigin = readerOfOrigin;
         }
 
-        // Constructor called when creating a new 'TarEntry*' instance that can be passed to a TarWriter.
+        // Constructor called when the user creates a TarEntry instance from scratch.
         internal TarEntry(TarEntryType entryType, string entryName, TarEntryFormat format)
         {
             ArgumentException.ThrowIfNullOrEmpty(entryName);
-
-            // Throws if format is unknown or out of range
-            TarHelpers.VerifyEntryTypeIsSupported(entryType, format, forWriting: false);
-
-            _readerOfOrigin = null;
+            TarHelpers.ThrowIfEntryTypeNotSupported(entryType, format);
 
             _header = default;
+            _header._format = format;
 
-            _header._extendedAttributes = new Dictionary<string, string>();
-
+            // Default values for fields shared by all supported formats
             _header._name = entryName;
-            _header._linkName = string.Empty;
-            _header._typeFlag = entryType;
             _header._mode = (int)TarHelpers.DefaultMode;
+            _header._mTime = DateTimeOffset.UtcNow;
+            _header._typeFlag = entryType;
+            _header._linkName = string.Empty;
+        }
 
-            _header._gName = string.Empty;
-            _header._uName = string.Empty;
+        // Constructor called when converting an entry to the selected format.
+        internal TarEntry(TarEntry other, TarEntryFormat format)
+        {
+            TarEntryType compatibleEntryType;
+            if (other.Format is TarEntryFormat.V7 && other.EntryType is TarEntryType.V7RegularFile && format is TarEntryFormat.Ustar or TarEntryFormat.Pax or TarEntryFormat.Gnu)
+            {
+                compatibleEntryType = TarEntryType.RegularFile;
+            }
+            else if (other.Format is TarEntryFormat.Ustar or TarEntryFormat.Pax or TarEntryFormat.Gnu && other.EntryType is TarEntryType.RegularFile && format is TarEntryFormat.V7)
+            {
+                compatibleEntryType = TarEntryType.V7RegularFile;
+            }
+            else
+            {
+                compatibleEntryType = other.EntryType;
+            }
+
+            TarHelpers.ThrowIfEntryTypeNotSupported(compatibleEntryType, format);
 
-            DateTimeOffset now = DateTimeOffset.Now;
-            _header._mTime = now;
-            _header._aTime = now;
-            _header._cTime = now;
+            _readerOfOrigin = other._readerOfOrigin;
+
+            _header = default;
+            _header._format = format;
+
+            _header._name = other._header._name;
+            _header._mode = other._header._mode;
+            _header._uid = other._header._uid;
+            _header._gid = other._header._gid;
+            _header._size = other._header._size;
+            _header._mTime = other._header._mTime;
+            _header._checksum = 0;
+            _header._typeFlag = compatibleEntryType;
+            _header._linkName = other._header._linkName;
+
+            _header._dataStream = other._header._dataStream;
         }
 
         /// <summary>
@@ -63,6 +92,11 @@ internal TarEntry(TarEntryType entryType, string entryName, TarEntryFormat forma
         /// </summary>
         public TarEntryType EntryType => _header._typeFlag;
 
+        /// <summary>
+        /// The format of the entry.
+        /// </summary>
+        public TarEntryFormat Format => _header._format;
+
         /// <summary>
         /// The ID of the group that owns the file represented by this entry.
         /// </summary>