[Bug-cpio] cpio 2.4.2 put NULL pad in the middle of a file header

[Matthew Braithwaite]

cpio 2.4.2, on RedHat Linux 7.3, invoked as:

  cpio --quiet --format=crc -o

gave me the following message:

  cpio: File <long file name> shrunk by 13906 bytes, padding with zeros

On attempting to extract the archive, cpio said:

  cpio: cannot remove current <first half of filename>: Directory not empty
  cpio: warning: skipped 28160 bytes of junk

I opened the archive in Emacs and searched for the <long file name>.
I found:

  <first half of filename>
  <42066 null bytes>
  <second half of filename>

Looking at util.c I can make a guess about why this happened, and if
my guess is correct the problem would also be present in version 2.5.

For most output, the functions copy_files_disk_to_*() call
*_buffered_write() -- which functions of course buffer.  However, when
cpio emits the error message above, it calls write_nuls_to_file(),
which does *not* buffer:

  blocks = num_bytes / 512;
  extra_bytes = num_bytes % 512;
  for (i = 0; i < extra_bytes; ++i)
    {
      if (write (out_des, zeros_512, 512) != 512)
        error (1, errno, "error writing NUL's");
    }
  if (extra_bytes != 0)
    {
      if (write (out_des, zeros_512, extra_bytes) != extra_bytes)
        error (1, errno, "error writing NUL's");
    }