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TarWriter uses HardLink entries for hard-linked files

TarWriter now detects when multiple files are hard-linked to the same inode. It writes a HardLink entry for subsequent files instead of duplicating the file content.

Version introduced

.NET 11 Preview 3

Previous behavior

Previously, when creating a tar archive with TarWriter, files that were hard-linked to the same inode were treated as separate, independent files. The full file content was duplicated in the archive for each hard-linked file.

using System.Formats.Tar;
using System.IO;

string filePath1 = "file1.txt";
string filePath2 = "file2.txt";

// Create two hard-linked files.
File.WriteAllText(filePath1, "Hello, world!");
File.CreateHardLink(filePath2, filePath1);

using (var stream = File.Create("archive.tar"))
using (var writer = new TarWriter(stream, TarEntryFormat.Pax, leaveOpen: false))
{
    writer.WriteEntry(filePath1, "file1.txt");
    writer.WriteEntry(filePath2, "file2.txt");
}

In the resulting archive.tar, both file1.txt and file2.txt had separate file entries, each containing the full file content.

New behavior

Starting in .NET 11, TarWriter detects when multiple files are hard-linked to the same inode and writes a HardLink entry for subsequent files instead of duplicating the file content.

Using the same code as shown in the Previous behavior section, the resulting archive.tar now contains a full file entry for file1.txt, while file2.txt has a HardLink entry pointing to file1.txt.

Type of breaking change

This change is a behavioral change.

Reason for change

This change improves the efficiency of tar archives created by the System.Formats.Tar library. By using <System.Formats.Tar.TarEntryType.HardLink> entries for files that share the same inode, the size of the resulting archive is reduced and the hard-link relationships between files are preserved. This behavior is consistent with GNU tar and other widely used tar implementations.

If your application depends on duplicating file content for hard-linked files, you can restore the previous behavior. Set the HardLinkMode property to TarHardLinkMode.CopyContents in a new TarWriterOptions instance:

using System.Formats.Tar;
using System.IO;

string filePath1 = "file1.txt";
string filePath2 = "file2.txt";

// Create two hard-linked files.
File.WriteAllText(filePath1, "Hello, world!");
File.CreateHardLink(filePath2, filePath1);

var options = new TarWriterOptions
{
    HardLinkMode = TarHardLinkMode.CopyContents
};

using (var stream = File.Create("archive.tar"))
using (var writer = new TarWriter(stream, options, leaveOpen: false))
{
    writer.WriteEntry(filePath1, "file1.txt");
    writer.WriteEntry(filePath2, "file2.txt");
}

Extracting a tar archive that contains HardLink entries to a file system without hard link support throws an IOException. Use the new TarExtractOptions class to specify whether to extract hard links as hard links or copy them as separate files. This enables extraction to file systems without hard link support.

Affected APIs

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