Add compression magic number detection to System.Formats.Tar with helpful error messages

src/libraries/System.Formats.Tar/src/Resources/Strings.resx

@@ -211,4 +211,10 @@
   <data name="ExtHeaderInvalidRecords" xml:space="preserve">
     <value>The extended header contains invalid records.</value>
   </data>
+  <data name="TarInvalidArchiveFormat" xml:space="preserve">
+    <value>The file is not a valid TAR archive format.</value>
+  </data>
+  <data name="TarCompressionArchiveDetected" xml:space="preserve">
+    <value>The file appears to be a {0} archive. TAR format expected.</value>
+  </data>
 </root>

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHeader.Read.cs

@@ -370,7 +370,20 @@ private async Task ProcessDataBlockAsync(Stream archiveStream, bool copyData, Ca
             {
                 return null;
             }
-            int checksum = (int)TarHelpers.ParseOctal<uint>(spanChecksum);
+
+            int checksum;
+            try
+            {
+                checksum = (int)TarHelpers.ParseOctal<uint>(spanChecksum);
+            }
+            catch (InvalidDataException)
+            {
+                // This is likely not a TAR file
+                // Check for compression magic numbers to provide better error message
+                ThrowInvalidFormatWithBetterMessage(buffer);
+                throw; // Ensure control flow never reaches code using 'checksum'
+            }
+
             // Zero checksum means the whole header is empty
             if (checksum == 0)
             {
@@ -789,5 +802,86 @@ private static bool TryGetNextExtendedAttribute(
             buffer = buffer.Slice(newlinePos + 1);
             return true;
         }
+
+
+        /// Analyzes the buffer for known file format magic numbers and throws an InvalidDataException
+        /// with a specific error message. This provides better user experience by identifying the
+        /// actual file type when users pass non-TAR files to the TAR reader.
+        /// <exception cref="InvalidDataException">Always thrown with either a format-specific or generic message</exception>
+        [DoesNotReturn]
+        private static void ThrowInvalidFormatWithBetterMessage(ReadOnlySpan<byte> buffer)
+        {
+            if (buffer.Length < 2)
+            {
+                throw new InvalidDataException(SR.TarInvalidArchiveFormat);
+            }
+
+            byte firstByte = buffer[0];
+            switch (firstByte)
+            {
+                case 0x28: // Zstandard
+                    if (buffer.Length >= 4 &&
+                        buffer[1] == 0xB5 && buffer[2] == 0x2F && buffer[3] == 0xFD)
+                    {
+                        throw new InvalidDataException(SR.Format(SR.TarCompressionArchiveDetected, "Zstandard"));
+                    }
+                    break;
+
+                case 0x37: // 7-Zip
+                    if (buffer.Length >= 6 &&
+                        buffer[1] == 0x7A && buffer[2] == 0xBC &&
+                        buffer[3] == 0xAF && buffer[4] == 0x27 && buffer[5] == 0x1C)
+                    {
+                        throw new InvalidDataException(SR.Format(SR.TarCompressionArchiveDetected, "7-Zip"));
+                    }
+                    break;
+
+                case 0x50: // ZIP files start with "PK"
+                    if (buffer.Length >= 2 && buffer[1] == 0x4B)
+                    {
+                        throw new InvalidDataException(SR.Format(SR.TarCompressionArchiveDetected, "ZIP"));
+                    }
+                    break;
+
+                case 0x1F: // GZIP
+                    if (buffer.Length >= 2 && buffer[1] == 0x8B)
+                    {
+                        throw new InvalidDataException(SR.Format(SR.TarCompressionArchiveDetected, "GZIP"));
+                    }
+                    break;
+
+                case 0x42: // BZIP2 - "BZh"
+                    if (buffer.Length >= 3 && buffer[1] == 0x5A && buffer[2] == 0x68)
+                    {
+                        throw new InvalidDataException(SR.Format(SR.TarCompressionArchiveDetected, "BZIP2"));
+                    }
+                    break;
+
+                case 0xFD: // XZ
+                    if (buffer.Length >= 6 &&
+                        buffer[1] == 0x37 && buffer[2] == 0x7A &&
+                        buffer[3] == 0x58 && buffer[4] == 0x5A && buffer[5] == 0x00)
+                    {
+                        throw new InvalidDataException(SR.Format(SR.TarCompressionArchiveDetected, "XZ"));
+                    }
+                    break;
+
+                case 0x78: // ZLIB (deflate compression)
+                    if (buffer.Length >= 2)
+                    {
+                        byte secondByte = buffer[1];
+                        if (secondByte == 0x01 || secondByte == 0x5E || secondByte == 0x9C ||
+                            secondByte == 0xDA || secondByte == 0x20 || secondByte == 0x7D ||
+                            secondByte == 0xBB || secondByte == 0xF9)
+                        {
+                            throw new InvalidDataException(SR.Format(SR.TarCompressionArchiveDetected, "ZLIB"));
+                        }
+                    }
+                    break;
+            }
+
+            // If we can't identify the specific format, provide a generic message
+            throw new InvalidDataException(SR.TarInvalidArchiveFormat);
+        }
     }
 }