-- | This module can implement any query\/response protocol.  It was written
-- with Kademlia implementations in mind.

{-# LANGUAGE CPP                   #-}
{-# LANGUAGE GADTs                 #-}
{-# LANGUAGE LambdaCase            #-}
{-# LANGUAGE PartialTypeSignatures #-}
{-# LANGUAGE RankNTypes            #-}
{-# LANGUAGE ScopedTypeVariables   #-}
module Network.QueryResponse where

#ifdef THREAD_DEBUG
import Control.Concurrent.Lifted.Instrument
#else
import Control.Concurrent
import GHC.Conc           (labelThread)
#endif
import Control.Concurrent.STM
import Control.Exception
import Control.Monad
import qualified Data.ByteString as B
         ;import Data.ByteString (ByteString)
import Data.Function
import qualified Data.IntMap     as IntMap
         ;import Data.IntMap     (IntMap)
import qualified Data.Map        as Map
         ;import Data.Map        (Map)
import Data.Maybe
import Data.Typeable
import Network.Socket
import Network.Socket.ByteString as B
import System.IO.Error
import System.Timeout

-- * Using a query\/response 'Client'.

-- | Fork a thread that handles inbound packets.  The returned action may be used
-- to terminate the thread and clean up any related state.
--
--  Example usage:
--
--  > -- Start client.
--  > quitServer <- forkListener client
--  > -- Send a query q, recieve a response r.
--  > r <- sendQuery client method q
--  > -- Quit client.
--  > quitServer
forkListener :: Client err meth tid addr x ctx -> IO (IO ())
forkListener client = do
    thread_id <- forkIO $ do
        myThreadId >>= flip labelThread "listener"
        fix $ handleMessage client
    return $ do
        closeTransport (clientNet client)
        killThread thread_id

-- | Send a query to a remote peer.  Note that this funciton will always time
-- out if 'forkListener' was never invoked to spawn a thread receive and
-- dispatch the response.
sendQuery ::
    forall err a b tbl x ctx meth tid addr.
        Client err meth tid addr x ctx          -- ^ A query/response implementation.
        -> MethodSerializer addr x ctx meth a b -- ^ Information for marshalling the query.
        -> a                                    -- ^ The outbound query.
        -> addr                                 -- ^ Destination address of query.
        -> IO (Maybe b)                         -- ^ The response, or 'Nothing' if it timed out.
sendQuery (Client net d err pending whoami) meth q addr = do
    mvar <- newEmptyMVar
    tid <- atomically $ do
        tbl <- readTVar pending
        let (tid, tbl') = dispatchRegister (tableMethods d) mvar tbl
        writeTVar pending tbl'
        return tid
    (self,ctx) <- whoami
    sendMessage net addr (wrapQuery meth ctx self addr q)
    mres <- timeout (methodTimeout meth) $ takeMVar mvar
    case mres of
        Just x -> return $ Just $ unwrapResponse meth x
        Nothing -> do
            atomically $ modifyTVar' pending (dispatchCancel (tableMethods d) tid)
            reportTimeout err (method meth) tid addr
            return Nothing

-- * Implementing a query\/response 'Client'.

-- | All inputs required to implement a query\/response client.
data Client err meth tid addr x ctx = forall tbl. Client
    { -- | The 'Transport' used to dispatch and receive packets.
      clientNet :: Transport err addr x
      -- | Methods for handling inbound packets.
    , clientDispatcher :: DispatchMethods tbl err meth tid addr x ctx
      -- | Methods for reporting various conditions.
    , clientErrorReporter :: ErrorReporter addr x meth tid err
      -- | State necessary for routing inbound responses and assigning unique
      -- /tid/ values for outgoing queries.
    , clientPending :: TVar tbl
      -- | An action yielding this client\'s own address along with some
      -- context neccessary for serializing outgoing packets.  It is invoked
      -- once on each outbound and inbound packet.  It is valid for this to
      -- always return the same value.
    , clientContext :: IO (addr,ctx)
    }

-- | An incomming message can be classified into three cases.
data MessageClass err meth tid
    = IsQuery meth tid -- ^ An unsolicited query is handled based on it's /meth/ value.  Any response
                       -- should include the provided /tid/ value.
    | IsResponse tid   -- ^ A response to a outgoing query we associated with a /tid/ value.
    | IsUnknown err    -- ^ None of the above.

-- | Handler for an inbound query of type _x_ from an address of type _addr_.
data MethodHandler err tid addr x ctx = forall a b. MethodHandler
    { -- | Parse the query into a more specific type for this method.
      methodParse :: x -> Either err a
      -- | Serialize the response for transmission, given a context /ctx/ and the origin
      -- and destination addresses.
    , methodSerialize :: ctx -> tid -> addr -> addr -> b -> x
      -- | Fully typed action to perform upon the query.  The remote origin
      -- address of the query is provided to the handler.
    , methodAction :: addr -> a -> IO b
    }

-- | Attempt to invoke a 'MethodHandler' upon a given inbound query.  If the
-- parse is successful, the returned IO action will construct our reply.
-- Otherwise, a parse err is returned.
dispatchQuery :: MethodHandler err tid addr x ctx -- ^ Handler to invoke.
                 -> ctx                           -- ^ Arbitrary context used during serialization.
                 -> tid                           -- ^ The transaction id for this query\/response session.
                 -> addr                          -- ^ Our own address, to which the query was sent.
                 -> x                             -- ^ The query packet.
                 -> addr                          -- ^ The origin address of the query.
                 -> Either err (IO x)
dispatchQuery (MethodHandler unwrapQ wrapR f) ctx tid self x addr =
    fmap (\a -> wrapR ctx tid self addr <$> f addr a) $ unwrapQ x

-- | These four parameters are required to implement an ougoing query.  A
-- peer-to-peer algorithm will define a 'MethodSerializer' for every 'MethodHandler' that
-- might be returned by 'lookupHandler'.
data MethodSerializer addr x ctx meth a b = MethodSerializer
    { -- | Seconds to wait for a response.
      methodTimeout :: Int
      -- | A method identifier used for error reporting.  This needn't be the
      -- same as the /meth/ argument to 'MethodHandler', but it is suggested.
    , method :: meth
      -- | Serialize the outgoing query /a/ into a transmitable packet /x/.
      -- The /addr/ arguments are, respectively, our own origin address and the
      -- destination of the request.  The /ctx/ argument is useful for attaching
      -- auxillary notations on all outgoing packets.
    , wrapQuery :: ctx -> addr -> addr -> a -> x
      -- | Parse an inbound packet /x/ into a response /b/ for this query.
    , unwrapResponse :: x -> b
    }


-- | Three methods are required to implement a datagram based query\/response protocol.
data Transport err addr x = Transport
    { -- | Blocks until an inbound packet is available. Returns 'Nothing' when
      -- no more packets are expected due to a shutdown or close event.
      -- Otherwise, the packet will be parsed as type /x/ and an origin address
      -- /addr/.  Parse failure is indicated by the type 'err'.
      awaitMessage :: IO (Maybe (Either err (x, addr)))
      -- | Send an /x/ packet to the given destination /addr/.
    , sendMessage :: addr -> x -> IO ()
      -- | Shutdown and clean up any state related to this 'Transport'.
    , closeTransport :: IO ()
    }

-- | This function modifies a 'Transport' to use higher-level addresses and
-- packet representations.  It could be used to change UDP 'ByteString's into
-- bencoded syntax trees or to add an encryption layer in which addresses have
-- associated public keys.
layerTransport ::
        (x -> addr -> Either err (x', addr'))
        -- ^ Function that attempts to transform a low-level address/packet
        -- pair into a higher level representation.
        -> (x' -> addr' -> (x, addr))
        -- ^ Function to encode a high-level address/packet into a lower level
        -- representation.
        -> Transport err addr x
        -- ^ The low-level transport to be transformed.
        -> Transport err addr' x'
layerTransport parse encode tr =
    tr { awaitMessage = do
                m <- awaitMessage tr
                return $ fmap (>>= uncurry parse) m
       , sendMessage = \addr' msg' -> do
                let (msg,addr) = encode msg' addr'
                sendMessage tr addr msg
       }


-- | To dipatch responses to our outbound queries, we require three primitives.
-- See the 'transactionMethods' function to create these primitives out of a
-- lookup table and a generator for transaction ids.
--
-- The type variable /d/ is used to represent the current state of the
-- transaction generator and the table of pending transactions.
data TransactionMethods d tid x = TransactionMethods
    {
      -- | Before a query is sent, this function stores an 'MVar' to which the
      -- response will be written too.  The returned _tid_ is a transaction id
      -- that can be used to forget the 'MVar' if the remote peer is not
      -- responding.
      dispatchRegister :: MVar x -> d -> (tid, d)
      -- | This method is invoked when an incomming packet _x_ indicates it is
      -- a response to the transaction with id _tid_.  The returned IO action
      -- is will write the packet to the correct 'MVar' thus completing the
      -- dispatch.
    , dispatchResponse :: tid -> x -> d -> (d, IO ())
      -- | When a timeout interval elapses, this method is called to remove the
      -- transaction from the table.
    , dispatchCancel :: tid -> d -> d
    }

-- | The standard lookup table methods for use as input to 'transactionMethods'
-- in lieu of directly implementing 'TransactionMethods'.
data TableMethods t tid = TableMethods
    { -- | Insert a new _tid_ entry into the transaction table.
      tblInsert :: forall a. tid -> a -> t a -> t a
      -- | Delete transaction _tid_ from the transaction table.
    , tblDelete :: forall a. tid -> t a -> t a
       -- | Lookup the value associated with transaction /tid/.
    , tblLookup :: forall a. tid -> t a -> Maybe a
    }

-- | Methods for using 'Data.IntMap.
intMapMethods :: TableMethods IntMap Int
intMapMethods = TableMethods IntMap.insert IntMap.delete IntMap.lookup

-- | Methods for using 'Data.Map'
mapMethods :: Ord tid => TableMethods (Map tid) tid
mapMethods = TableMethods Map.insert Map.delete Map.lookup

-- | Change the key type for a lookup table implementation.
--
-- This can be used with 'intMapMethods' or 'mapMethods' to restrict lookups to
-- only a part of the generated /tid/ value.  This is useful for /tid/ types
-- that are especially large due their use for other purposes, such as secure
-- nonces for encryption.
contramapT f (TableMethods ins del lookup) =
    TableMethods (\k v t -> ins (f k) v t)
                 (\k t   -> del (f k) t)
                 (\k t   -> lookup (f k) t)

-- | Since 'Int' may be 32 or 64 bits, this function is provided as a
-- convenience to test if an integral type, such as 'Data.Word.Word64', can be
-- safely transformed into an 'Int' for use with 'IntMap'.
--
-- Returns 'True' if the proxied type can be losslessly convered to 'Int' using
-- 'fromIntegral'.
fitsInInt :: forall word. (Bounded word, Integral word) => Proxy word -> Bool
fitsInInt Proxy = (original == casted)
 where
    original = div maxBound 2 :: word
    casted   = fromIntegral (fromIntegral original :: Int) :: word

-- | Construct 'TransactionMethods' methods out of 3 lookup table primitives and a
-- function for generating unique transaction ids.
transactionMethods ::
    TableMethods t tid  -- ^ Table methods to lookup values by /tid/.
    -> (g -> (tid,g))   -- ^ Generate a new unique /tid/ value and update the generator state /g/.
    -> TransactionMethods (g,t (MVar x)) tid x
transactionMethods (TableMethods insert delete lookup) generate = TransactionMethods
    { dispatchCancel = \tid (g,t) -> (g, delete tid t)
    , dispatchRegister = \v (g,t) ->
        let (tid,g') = generate g
            t' = insert tid v t
        in ( tid, (g',t') )
    , dispatchResponse = \tid x (g,t) ->
        case lookup tid t of
            Just v -> let t' = delete tid t
                      in ((g,t'),void $ tryPutMVar v x)
            Nothing -> ((g,t), return ())
    }

-- | A set of methods neccessary for dispatching incomming packets.
data DispatchMethods tbl err meth tid addr x ctx = DispatchMethods
    { -- | Clasify an inbound packet as a query or response.
      classifyInbound :: x -> MessageClass err meth tid
      -- | Lookup the handler for a inbound query.
    , lookupHandler :: meth -> Maybe (MethodHandler err tid addr x ctx)
      -- | Methods for handling incomming responses.
    , tableMethods :: TransactionMethods tbl tid x
    }

-- | These methods indicate what should be done upon various conditions.  Write
-- to a log file, make debug prints, or simply ignore them.
--
--   [ /addr/ ]  Address of remote peer.
--
--   [ /x/ ]     Incomming or outgoing packet.
--
--   [ /meth/ ]  Method id of incomming or outgoing request.
--
--   [ /tid/ ]   Transaction id for outgoing packet.
--
--   [ /err/ ]   Error information, typically a 'String'.
data ErrorReporter addr x meth tid err = ErrorReporter
    { -- | Incomming: failed to parse packet.
      reportParseError :: err -> IO ()
      -- | Incomming: no handler for request.
    , reportMissingHandler :: meth -> addr -> x -> IO ()
      -- | Incomming: unable to identify request.
    , reportUnknown :: addr -> x -> err -> IO ()
      -- | Outgoing: remote peer is not responding.
    , reportTimeout :: meth -> tid -> addr -> IO ()
    }

ignoreErrors :: ErrorReporter addr x meth tid err
ignoreErrors = ErrorReporter
    { reportParseError = \_ -> return ()
    , reportMissingHandler = \_ _ _ -> return ()
    , reportUnknown = \_ _ _ -> return ()
    , reportTimeout = \_ _ _ -> return ()
    }

-- Change the /err/ type for an 'ErrorReporter'.
contramapE f (ErrorReporter pe mh unk tim)
    = ErrorReporter (\e -> pe (f e))
                    mh
                    (\addr x e -> unk addr x (f e))
                    tim

-- | Handle a single inbound packet and then invoke the given continuation.
-- The 'forkListener' function is implemeneted by passing this function to
-- 'fix' in a forked thread that loops until 'awaitMessage' returns 'Nothing'
-- or throws an exception.
handleMessage ::
    Client err meth tid addr x ctx
    -> IO ()
    -> IO ()
handleMessage (Client net d err pending whoami) again = do
  awaitMessage net >>= \case
    Just (Left e)              -> do reportParseError err e
                                     again
    Just (Right (plain, addr)) -> do
        case classifyInbound d plain of
            IsQuery meth tid -> case lookupHandler d meth of
                                Nothing -> reportMissingHandler err meth addr plain
                                Just m  -> do
                                  (self,ctx) <- whoami
                                  either (reportParseError err)
                                         (>>= sendMessage net addr)
                                         (dispatchQuery m ctx tid self plain addr)
            IsResponse tid -> do
                action <- atomically $ do
                    ts0 <- readTVar pending
                    let (ts, action) = dispatchResponse (tableMethods d) tid plain ts0
                    writeTVar pending ts
                    return action
                action
            IsUnknown e -> reportUnknown err addr plain e
        again
    Nothing -> return ()

-- * UDP Datagrams.

-- | Access the address family of a given 'SockAddr'.  This convenient accessor
-- is missing from 'Network.Socket', so I implemented it here.
sockAddrFamily :: SockAddr -> Family
sockAddrFamily (SockAddrInet  _ _    ) = AF_INET
sockAddrFamily (SockAddrInet6 _ _ _ _) = AF_INET6
sockAddrFamily (SockAddrUnix  _      ) = AF_UNIX
sockAddrFamily (SockAddrCan _        ) = AF_CAN

-- | Packets with an empty payload may trigger eof exception.
-- 'udpTransport' uses this function to avoid throwing in that
-- case.
ignoreEOF def e | isEOFError e = pure def
                | otherwise    = throwIO e

-- | Hardcoded maximum packet size for incomming udp packets received via
-- 'udpTransport'.
udpBufferSize :: Int
udpBufferSize = 65536

-- | A 'udpTransport' uses a UDP socket to send and receive 'ByteString's.  The
-- argument is the listen-address for incomming packets.  This is a useful
-- low-level 'Transport' that can be transformed for higher-level protocols
-- using 'layerTransport'.
udpTransport :: SockAddr -> IO (Transport err SockAddr ByteString)
udpTransport bind_address = do
  let family = sockAddrFamily bind_address
  sock <- socket family Datagram defaultProtocol
  when (family == AF_INET6) $ do
    setSocketOption sock IPv6Only 0
  bind sock bind_address
  return Transport
    { awaitMessage = handle (ignoreEOF $ Just $ Right (B.empty, SockAddrInet 0 0)) $ do
        r <- B.recvFrom sock udpBufferSize
        return $ Just $ Right r
    , sendMessage = \addr bs -> void $ B.sendTo sock bs addr
    , closeTransport = close sock
    }