Sending data to socket (after reading) only occurs when the socket connection ends - sockets

Question

I'm trying to write a very basic webserver in Haskell. This is my code:
{-# LANGUAGE OverloadedStrings #-}
import Network (withSocketsDo, listenOn, PortID(..))
import Network.Socket (Socket, accept, close, setSocketOption, SocketOption(..))
import Network.Socket.ByteString (send, sendAll, recv)
import Control.Concurrent.Async (async)
import Control.Monad (forever)
import Data.ByteString.Char8 (unpack)
import Request
main = withSocketsDo $ do
sock <- listenOn $ PortNumber 3000
putStrLn "Listening on port 3000..."
forever $ do
(conn, _) <- accept sock
async $ handleAccept conn
handleAccept :: Socket -> IO ()
handleAccept sock = do
putStrLn $ "Connected!"
rawReq <- recv sock 4096
let req = parseRawRequest $ unpack rawReq -- returns Maybe Request
putStrLn $ show req
handleRequest sock req
handleRequest :: Socket -> Maybe Request -> IO ()
handleRequest sock Nothing = do
putStrLn "Closing..."
handleRequest sock req = do
sendAll sock "In handleRequest!" -- Doesn't appear until server is killed.
This is what I expected to happen:
Start server.
"Listening on port 3000..." is printed on server-side.
Do curl localhost:3000
"Connected!" is printed server-side.
The request is printed server-side.
"In handleRequest!" is printed.
What actually happens:
Start server.
"Listening on port 3000..." is printed on server-side.
Do curl localhost:3000
"Connected!" is printed server-side.
The request is printed server-side.
I wait patiently
I kill the server with CTRL+C
"In handleRequest!" prints client-side.
I suspect this has something to do with possible laziness in recv, although I use the value immediately afterwards (I parse the raw request into a Request type), so theoretically it should be evaluated.
If I put sendAll sock "Yadda yadda at the end of handleAccept, everything works fine. It's when I move this behaviour into a new function, handleRequest, that things go wonky.
Any thoughts? I'm new-ish to Haskell, so I'd appreciate any comments on the issue, or my code generally.
Cheers.
EDIT:
This is super weird! I "fixed" it, but I have no idea why this occurs.
This is the line that only appeared after I killed the server:
handleRequest sock req = do
sendAll sock "In handleRequest!" -- Doesn't appear until server is killed.
If I intentionally close the socket after sending, it works:
handleRequest sock req = do
sendAll sock "In handleRequest!" -- Now appears without killing the server
close sock
So it sends when the connection is closed. This is consistent with previous behaviour, since the connection automatically closes when the server is killed.
Now for the confusing bit. If I replace it with:
handleRequest sock req = do
sendAll sock "In handleRequest!\n" -- Works perfect
This works without closing the connection! It does what I expected, just by adding a newline. Why does this occur?
What on earth? Is it a printing problem with my terminal, not the code? (OSX iTerm2)
EDIT 2:
Was asked to provide the code for my Request module:
import Data.List (isInfixOf)
import Data.List.Split (splitOn)
data RequestType = GET | PUT
deriving Show
data Request =
Request {
reqType :: RequestType,
path :: String,
options :: [(String, String)]
} deriving Show
-- Turn a raw HTTP request into a request
-- object.
parseRawRequest :: String -> Maybe Request
parseRawRequest rawReq =
Request <$> parseRawRequestType rawReq
<*> parseRawRequestPath rawReq
<*> parseRawRequestOps rawReq
-- Turn an (entire) raw HTTP request into just
-- the request type.
parseRawRequestType :: String -> Maybe RequestType
parseRawRequestType rawReq =
case typ of
"GET" -> Just GET
"PUT" -> Just PUT
_ -> Nothing
where typ = (head . words . head . lines) rawReq
-- Turn an (entire) raw HTTP request into just
-- the path.
parseRawRequestPath :: String -> Maybe String
parseRawRequestPath = Just . (!! 1) . words . head . lines
-- Turn an (entire) raw HTTP request into just
-- a lookup table of their options.
parseRawRequestOps :: String -> Maybe [(String, String)]
parseRawRequestOps rawReq = Just [("One", "Two")] -- Test impl

Answer 1

I have one answer and one suggestion.
The suggestion is for you to turn off the naggle algorithm after accept:
setSocketOption conn NoDelay 1
The answer is that your sendAll is sending data but curl is not printing it. You can confirm this with netcat, for example. I commented out your Nothing case so that no matter what I typed in netcat I was sure to get the "In handleRequest!" message back:
server:
% ghc so.hs && ./so
Listening on port 3000...
Connected!
Nothing
client:
% nc localhost 3000
test ; My input, with a newline
In handleRequest! ; Printed out, no newline
Alternatively, you can use curl's -N option to disable buffering.
% curl -N localhost:3000
In handleRequest!