When running multithreaded, sometimes CRC calculations for PNG fail

[grisharav]

When I run in parallel lots of File.Create(...) calls (on different paths) I sometimes get

TagLib.CorruptFileException: CRC check failed for IHDR Chunk (expected: 0xFFFFFFFE, read: 0x5446E2B7 at TagLib.File.Create(IFileAbstraction abstraction, String mimetype, ReadStyle propertiesStyle)

on the first file

I believe this is due to unsafe code in File.cs:

    static ByteVector ComputeCRC (params ByteVector[] datas)
    {
        uint crc = 0xFFFFFFFF;

        if (crc_table == null)
            BuildCRCTable ();

        foreach (var data in datas) {
            foreach (byte b in data) {
                crc = crc_table[(crc ^ b) & 0xFF] ^ (crc >> 8);
            }
        }

            // Invert
        return ByteVector.FromUInt (crc ^ 0xFFFFFFFF);
    }

in the if (crc_table==null) check, multiple threads can start building the crc table and corrupt the results.

[Starwer]

I think you're right. A simple fix for this would be to assign the crc_table onlly at the end of the function BuildCRCTable instead of initializing it at the begining, i.e.

        static void BuildCRCTable ()
        {
            uint polynom = 0xEDB88320;

            //crc_table = new uint[256];
            var table = new uint[256];

            for (int i = 0; i < 256; i++) {

                uint c = (uint)i;
                for (int k = 0; k < 8; k++) {
                    if ((c & 0x00000001) != 0x00)
                        c = polynom ^ (c >> 1);
                    else
                        c = c >> 1;
                }
                //crc_table[i] = c;
                table[i] = c;
            }
            crc_table = table;

        }

[grisharav]

Will probably work, I would simply make crc_table a Lazy<uint[]>

[Starwer]

Good idea! This be even cleaner.