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# implement client and server side logic to transfer an
# arbitrary file from server to client over a socket; 
# uses a simple control-info protocol, rather than 
# separate sockets for control and data (as in ftp), and 
# must loop to transfer the entire file by blocks;  see
# ftplib examples for a higher-level transport scheme;
# serverthread also compares the size returned by a 
# send to len(bytes) just in case of a network glitch;
# could instead load the whole file at once with read
# and loop until the entire byte count has been sent;
# note: there is no reason to use makefile() and send
# filenames in a terminated line here, except that an
# explicit line simplifies filenames longer than blksz;
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