Open an RPM archive and then open the error messages window. You will see cpio related warnings such as:

cpio: warning: skipped 4094 bytes of junk
cpio: warning: skipped 10235 bytes of junk
cpio: warning: skipped 12282 bytes of junk
cpio: warning: skipped 8188 bytes of junk
cpio: warning: skipped 6141 bytes of junk
cpio: warning: skipped 4094 bytes of junk
cpio: warning: skipped 2047 bytes of junk

I think the fault is that the code reads from the GIOChannel of the CPIO file before data are ready to be read. I tried using g_io_channel_get_buffer_condition but it didn't work.