This generally looks good, and sensible. There is only one piece that seems a little dodgy. Sending back out parts of pieces before you have fully received them.

I might be misunderstanding, but until a piece is fully received, you can't hash it and check it is right. This could therefore lead to many clients having partially incorrect pieces, if they keep passing it around.

I've found (and I'm not 100% sure why) that most bittorrent clients do find they get some number of dodgy pieces over time, so this isn't just a theoretical problem.