Making pathological files in ZIP archives

A File that uses a Data Descriptor can easily trick this module if care is taken in the construction of the data that goes into the File. First, write some data to a File sink:

sink = zipsink("pathological.zip")
zf = open(sink, "file.txt"; compression=:store)
data = "Hello, Julia!"

write(zf, data)

Second, write the Data Descriptor header to the File (in little-endian format):

write(zf, htol(ZipStreams.SIG_DATA_DESCRIPTOR))

Third, write the CSC-32 checksum of the original data to the File (again, in little-endian format):

write(zf, htol(ZipStreams.crc32(data)))

Finally, write the compressed and uncompressed sizes of the data to the File (you guessed it: in little-endian format). This is made easier by using the compression=:store keyword argument when opening the File for writing. Because the default File format is Zip64 (keyword argument zip64=true), you need to write these as UInt64 integers (if you set zip64=false, write these as UInt32 instead):

write(zf, hotl(sizeof(data) % UInt64)) # compressed size
write(zf, hotl(sizeof(data) % UInt64)) # uncompressed size

Now write whatever you want after that:

write(zf, "Goodbye, Julia!")
close(zf)
close(sink)

When you read the Archive back, you'll find that the stream reader will read up to the fake Data Descriptor that you wrote and ignore the additional data that you wrote afterward:

zipsource("pathological.zip") do source
    for zf in source
        println(read(zf, String))
    end
end

# output

Hello, Julia!