fix hang at end of PUT to proxied p2p http remote

sendExactly will now be sure to evaluate the whole lazy ByteString.

In this case, the lazy ByteString was exactly the right lenth.
But, it seems that L.take caused it to not actually be fully evaluated.

In servePut, this manifested as gather never being fully evaluated,
which caused the hang.

Very, very subtle, and horrible bug. Clearly the use of lazy ByteString
(or really just laziness) is at fault, and it would be very worth moving
to conduit or whatever to avoid this.

P2P/IO.hs

@@ -335,12 +335,16 @@ debugMessage conn prefix m = do
 -- Must avoid sending too many bytes as it would confuse the other end.
 -- This is easily dealt with by truncating it.
 --
+-- However, the whole ByteString will be evaluated here, even if
+-- the end of it does not get sent.
+--
 -- If too few bytes are sent, the only option is to give up on this
 -- connection. False is returned to indicate this problem.
 sendExactly :: Len -> L.ByteString -> Handle -> MeterUpdate -> IO Bool
 sendExactly (Len n) b h p = do
-	sent <- meteredWrite' p (B.hPut h) (L.take (fromIntegral n) b)
-	return (fromBytesProcessed sent == n)
+	let (x, y) = L.splitAt (fromIntegral n) b
+	sent <- meteredWrite' p (B.hPut h) x
+	L.length y `seq` return (fromBytesProcessed sent == n)
 
 receiveExactly :: Len -> Handle -> MeterUpdate -> IO L.ByteString
 receiveExactly (Len n) h p = hGetMetered h (Just n) p