Ref #248. Ref #132. Tar reader support for symlinks for .NET standard 2 and Posix platforms

Extracts linkname from the tar header, and exposes this on IEntry as the LinkTarget (string) property. If an entry is not a symlink, then that property is null.

Uses Mono.Posix.NETStandard nuget to create a symlink. However, this is only applicable to .NET standard 2.0+. So far, unable to find a nuget that works for older versions.

Also, not sure what to do on Windows.

src/SharpCompress/Common/Entry.cs

@@ -15,6 +15,11 @@ namespace SharpCompress.Common
         /// </summary>
         public abstract string Key { get; }
 
+        /// <summary>
+        /// The target of a symlink entry internal to the Archive. Will be null if not a symlink.
+        /// </summary>
+        public abstract string LinkTarget { get; }
+
         /// <summary>
         /// The compressed file size
         /// </summary>

src/SharpCompress/Common/ExtractionMethods.cs

@@ -68,19 +68,40 @@ namespace SharpCompress.Common
                                             ExtractionOptions options,
                                             Action<string, FileMode> openAndWrite)
         {
-            FileMode fm = FileMode.Create;
-            options = options ?? new ExtractionOptions()
-                                 {
-                                     Overwrite = true
-                                 };
-
-            if (!options.Overwrite)
+#if NETSTANDARD2_0
+            if (entry.LinkTarget != null)
             {
-                fm = FileMode.CreateNew;
+                /*
+                var parDir = System.IO.Path.GetDirectoryName(destinationFileName);
+                if (!System.IO.Directory.Exists(parDir))
+                {
+                    System.IO.Directory.CreateDirectory(parDir);
+                }
+                */
+                var link = new Mono.Unix.UnixSymbolicLinkInfo(destinationFileName);
+                if (System.IO.File.Exists(destinationFileName))
+                {
+                    link.Delete(); // equivalent to ln -s -f
+                }
+                link.CreateSymbolicLinkTo(entry.LinkTarget);
             }
+            else
+#endif
+            {
+                FileMode fm = FileMode.Create;
+                options = options ?? new ExtractionOptions()
+                                     {
+                                         Overwrite = true
+                                     };
 
-            openAndWrite(destinationFileName, fm);
-            entry.PreserveExtractionOptions(destinationFileName, options);
+                if (!options.Overwrite)
+                {
+                    fm = FileMode.CreateNew;
+                }
+
+                openAndWrite(destinationFileName, fm);
+                entry.PreserveExtractionOptions(destinationFileName, options);
+            }
         }
 #endif
     }

src/SharpCompress/Common/GZip/GZipEntry.cs

@@ -20,6 +20,8 @@ namespace SharpCompress.Common.GZip
 
         public override string Key => _filePart.FilePartName;
 
+        public override string LinkTarget => null;
+
         public override long CompressedSize => 0;
 
         public override long Size => 0;

src/SharpCompress/Common/IEntry.cs

@@ -10,6 +10,7 @@ namespace SharpCompress.Common
         long Crc { get; }
         DateTime? CreatedTime { get; }
         string Key { get; }
+        string LinkTarget { get; }
         bool IsDirectory { get; }
         bool IsEncrypted { get; }
         bool IsSplitAfter { get; }

src/SharpCompress/Common/Rar/RarEntry.cs

@@ -22,6 +22,8 @@ namespace SharpCompress.Common.Rar
         /// </summary>
         public override string Key => FileHeader.FileName;
 
+        public override string LinkTarget => null;
+
         /// <summary>
         /// The entry last modified time in the archive, if recorded
         /// </summary>

src/SharpCompress/Common/SevenZip/SevenZipEntry.cs

@@ -18,6 +18,8 @@ namespace SharpCompress.Common.SevenZip
 
         public override string Key => FilePart.Header.Name;
 
+        public override string LinkTarget => null;
+
         public override long CompressedSize => 0;
 
         public override long Size => FilePart.Header.Size;

src/SharpCompress/Common/Tar/Headers/TarHeader.cs

@@ -15,6 +15,7 @@ namespace SharpCompress.Common.Tar.Headers
         }
 
         internal string Name { get; set; }
+        internal string LinkName { get; set; }
 
         //internal int Mode { get; set; }
         //internal int UserId { get; set; }
@@ -98,6 +99,12 @@ namespace SharpCompress.Common.Tar.Headers
                 return false;
             }
 
+            // for symlinks, additionally read the linkname
+            if (ReadEntryType(buffer) == EntryType.SymLink)
+            {
+                LinkName = ArchiveEncoding.Decode(buffer, 157, 100).TrimNulls();
+            }
+
             if (ReadEntryType(buffer) == EntryType.LongName)
             {
                 Name = ReadLongName(reader, buffer);

src/SharpCompress/Common/Tar/TarEntry.cs

@@ -23,6 +23,8 @@ namespace SharpCompress.Common.Tar
 
         public override string Key => _filePart.Header.Name;
 
+        public override string LinkTarget => _filePart.Header.LinkName;
+
         public override long CompressedSize => _filePart.Header.Size;
 
         public override long Size => _filePart.Header.Size;

src/SharpCompress/Common/Zip/ZipEntry.cs

@@ -60,6 +60,8 @@ namespace SharpCompress.Common.Zip
 
         public override string Key => _filePart.Header.Name;
 
+        public override string LinkTarget => null;
+
         public override long CompressedSize => _filePart.Header.CompressedSize;
 
         public override long Size => _filePart.Header.UncompressedSize;

src/SharpCompress/SharpCompress.csproj

@@ -34,6 +34,7 @@
     <DefineConstants>$(DefineConstants);NETCORE</DefineConstants>
   </PropertyGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
+    <PackageReference Include="Mono.Posix.NETStandard" Version="1.0.0" />
     <PackageReference Include="System.Text.Encoding.CodePages" Version="4.5.0" />
   </ItemGroup>
 </Project>