Handle special case, empty file

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

@@ -2,6 +2,7 @@ using System;
 using System.Buffers.Binary;
 using System.IO;
 using System.Text;
+using ZstdSharp.Unsafe;
 
 namespace SharpCompress.Common.Tar.Headers;
 
@@ -134,9 +135,8 @@ internal sealed class TarHeader
             hasLongValue = false;
         } while (hasLongValue);
 
-        var crc = ReadAsciiInt64Base8(buffer, 148, 7);
-
-        if (crc != RecalculateChecksum(buffer))
+        // Check header checksum
+        if (!checkChecksum(buffer))
         {
             return false;
         }
@@ -318,6 +318,42 @@ internal sealed class TarHeader
         (byte)' '
     };
 
+    internal static bool checkChecksum(byte[] buf)
+    {
+        const int eightSpacesChksum = 256;
+        var buffer = new Span<byte>(buf).Slice(0, 512);
+        int posix_sum = eightSpacesChksum;
+        int sun_sum = eightSpacesChksum;
+
+        foreach (byte b in buffer)
+        {
+            posix_sum += b;
+            sun_sum += unchecked((sbyte)b);
+        }
+
+        // Special case, empty file header
+        if (posix_sum == eightSpacesChksum)
+        {
+            return true;
+        }
+
+        // Remove current checksum from calculation
+        foreach (byte b in buffer.Slice(148, 8))
+        {
+            posix_sum -= b;
+            sun_sum -= unchecked((sbyte)b);
+        }
+
+        // Read and compare checksum for header
+        var crc = ReadAsciiInt64Base8(buf, 148, 7);
+        if (crc != posix_sum && crc != sun_sum)
+        {
+            return false;
+        }
+
+        return true;
+    }
+
     internal static int RecalculateChecksum(byte[] buf)
     {
         // Set default value for checksum. That is 8 spaces.

tests/SharpCompress.Test/Tar/TarArchiveTests.cs

@@ -217,7 +217,8 @@ public class TarArchiveTests : ArchiveTests
     {
         var archiveFullPath = Path.Combine(TEST_ARCHIVES_PATH, "Tar.Empty.tar");
         using Stream stream = File.OpenRead(archiveFullPath);
-        Assert.Throws<InvalidOperationException>(() => ArchiveFactory.Open(stream));
+        using var archive = ArchiveFactory.Open(stream);
+        Assert.True(archive.Type == ArchiveType.Tar);
     }
 
     [Theory]