edge-install/DebUnpack.cs

using System.Text;
using Serilog;
namespace EdgeInstaller;

sealed class DebFile : IDisposable
{
    const int HeaderBytes = 60;
    const string FileMagic = "!<arch>\n";

    private MemoryStream _fileStream;

    public List<DebEntry> fileEntries { get; private set; }

    public DebFile(Stream fs)
    {
        Log.Information("Creating new debFile from stream");
        fileEntries = new();
        if (fs is null)
        {
            throw new NullReferenceException();
        }

        fs.Position = 0;

        // read the first 8 bytes and check for the signature
        byte[] magicBuffer = new byte[8];

        fs.Read(magicBuffer);
        Log.Debug("Magic = {magicBuffer}", Encoding.ASCII.GetString(magicBuffer));
        if (Encoding.ASCII.GetString(magicBuffer) != FileMagic)
        {
            throw new ArgumentException(message: "Magic Fail");
        }

        fs.Position = 0;
        _fileStream = new();
        fs.CopyTo(_fileStream);

        fs.Dispose();
        // if we got here then we must have a proper archive.
        // We shall now get the files that are inside.

        // from OUR copy of the stream we seek to after the magic byte
        _fileStream.Position = 8;
        while (true) // beware making an infinite loop
        {
            // Read a header
            byte[] header = new byte[60];
            var read = _fileStream.Read(header);
            // If nothing was read we have hit the end
            if (read < 60) // also handle broken headers
            {
                break;
            }
            // Otherwise process as a header
            var entry = new DebEntry(header, _fileStream.Position);
            fileEntries.Add(entry);

            // Using the obtained size seek forwards to the next potential header
            _fileStream.Seek(entry.sizeBytes, SeekOrigin.Current);
        }
    }

    public bool getFile(DebEntry file, out Stream output)
    {
        // If we are given a file that isn't in this archive we exit nicely
        if (!fileEntries.Contains(file))
        {
            Log.Error("The given file entry is not valid for this DebFile");
            output = Stream.Null;
            return false;
        }

        // Since we have a file that is in the archive
        // grab the metadata we need
        long offset = file.offset;
        long size = file.sizeBytes;
        // stream the file to the output stream
        _fileStream.Seek(offset, SeekOrigin.Begin);
        byte[] fileBuffer = new byte[size];

        var count = _fileStream.Read(fileBuffer);
        if (count != size)
        {
            Log.Error("Didn't read the correct amount of data from stream (expected:{size} got:{count})", size, count);
            output = Stream.Null;
            return false;
        }

        output = new MemoryStream(fileBuffer); // convert the read buffer to a stream
        return true;
    }

    public void Dispose()
    { // make sure we can clean up our own mess
        _fileStream.Dispose();
    }
}

sealed class DebEntry
{
    // Escape sequnces are used in this string read as "`\n"
    const string HeaderMagic = "\x60\n";

    public string name { get; private set; }
    public long mtime { get; private set; }
    public int mode { get; private set; }
    public int gid { get; private set; }
    public int uid { get; private set; }
    public long sizeBytes { get; private set; }
    public long offset { get; private set; } // where does the file start

    public DebEntry(byte[] buffer, long Position)
    {
        Log.Debug("Reading header at position {p}", Position - 60); // since we are being passed a header we take 60 to get the header position
        if (buffer is null)
        {
            throw new NullReferenceException();
        }

        if (buffer.Length < 60)
        {
            throw new ArgumentException("buffer too short");
        }

        name = Encoding.ASCII.GetString(buffer[0..16]).Trim(); // Trim any trailing whitespace
        mtime = long.Parse(Encoding.ASCII.GetString(buffer[16..28]));
        mode = int.Parse(Encoding.ASCII.GetString(buffer[28..34]));
        gid = int.Parse(Encoding.ASCII.GetString(buffer[34..40]));
        uid = int.Parse(Encoding.ASCII.GetString(buffer[40..48]));
        sizeBytes = long.Parse(Encoding.ASCII.GetString(buffer[48..58]));
        string magic = Encoding.ASCII.GetString(buffer[58..^0]);

        offset = Position;

        Log.Debug("Header Magic = {magic}", magic);
        Log.Debug("{@this}", this);
        if (magic != HeaderMagic)
        {
            throw new ArgumentException("magic failed");
        }

    }

}