evan_tech

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09:22 am, 18 Sep 09

found my bug!

Not too interesting, but this has been bugging me for a week.

Been working on a toy program that proxies a TCP connection. It was working fine for some and then would mysteriously corrupt others; I spent a long time looking and it finally hit me, painfully obvious.

I had threads going in each way doing this in a loop:
 forward :: Socket -> Socket -> IO ByteString
 forward src dst = do
   buf <- recv src (16 * 2^10)
   send dst buf
   return buf
Network.Socket.ByteString.send of course returns an integer of how many bytes were actually sent; you need to loop if you want to be sure to send it all. (I had imagined this higher-level API was handling this looping for me, but it makes more sense that it wouldn't. It does however handle EINTR, so when I had glanced at the source in my desperation I did notice a loop, throwing me off the scent.) Sticking a loop around the send fixed everything.