FTP Offset larger than file.

[Bakes]

I am writing a python script that performs an identical function to
the 'tail' unix utility, except that it connects to its files over
FTP, rather than the local hard disk.

I am currently using this python script to generate an increasing
'logfile' of garbage.

> import time
> for i in range(1, 20000):
>	time.sleep(0.2)
>	print i
>	f = open("data1.log","a")
>	f.write('%s: This logfile is being automatically generated to help Bakes test his python ftptail. \n' % i)
>	f.close()

and use this script to actually download it.

>import time
>import os.path
>from ftplib import FTP
>
>#Empty the file
>filename = 'data1.log'
>file = open(filename, 'w')
>file.write('')
>file.close()
>
>def handleDownload(block):
>    file.write(block)
>    print ".",
>
># Create an instance of the FTP object
># Optionally, you could specify username and password:
>ftp=FTP(host, user, pass)
>
>directory = '/temp'
>ftp.cwd(directory)
>
>file = open(filename, 'a')
>
>for i in range(1,20000):
>  size=os.path.getsize('data1.log')
>  ftp.retrbinary('RETR ' + filename, handleDownload, rest=size)
>
>file.close()
>
>print ftp.close()

Now, my problem is that I get a very strange error. What should be
happening is the script gets the size of the local file before
downloading all of the external file after that offset.

The error I get is:
ftplib.error_temp: 451-Restart offset 24576 is too large for file size
22852.
451 Restart offset reset to 0
which tells me that the local file is larger than the external file,
by about a kilobyte. Certainly, the local file is indeed that size, so
my local script is doing the right things. I do wonder what is going
wrong, can anyone enlighten me?