{-# LANGUAGE RecordWildCards #-}

-- | This module defines data structures and operations pertaining to registered
-- servers using registered calls; for unregistered support, see
-- `Network.GRPC.LowLevel.Server.Unregistered`.
module Network.GRPC.LowLevel.Server where

import           Control.Exception                       (bracket, finally)
import           Control.Monad
import           Data.ByteString                         (ByteString)
import           Foreign.Ptr                             (nullPtr)
import qualified Network.GRPC.Unsafe                     as C
import qualified Network.GRPC.Unsafe.Op                  as C

import           Network.GRPC.LowLevel.Call
import           Network.GRPC.LowLevel.CompletionQueue   (CompletionQueue,
                                                          TimeoutSeconds,
                                                          createCompletionQueue,
                                                          pluck,
                                                          serverRegisterCompletionQueue,
                                                          serverRequestRegisteredCall,
                                                          serverShutdownAndNotify,
                                                          shutdownCompletionQueue)
import           Network.GRPC.LowLevel.GRPC
import           Network.GRPC.LowLevel.Op

-- | Wraps various gRPC state needed to run a server.
data Server = Server
  { internalServer    :: C.Server
  , serverCQ          :: CompletionQueue
  , registeredMethods :: [RegisteredMethod]
  , serverConfig      :: ServerConfig
  }

-- | Configuration needed to start a server.
data ServerConfig = ServerConfig
  { host              :: Host
    -- ^ Name of the host the server is running on. Not sure how this is
    -- used. Setting to "localhost" works fine in tests.
  , port              :: Port
    -- ^ Port on which to listen for requests.
  , methodsToRegister :: [(MethodName, GRPCMethodType)]
    -- ^ List of (method name, method type) tuples specifying all methods to
    -- register. You can also handle other unregistered methods with
    -- `serverHandleNormalCall`.
  }
  deriving (Show, Eq)

serverEndpoint :: ServerConfig -> Endpoint
serverEndpoint ServerConfig{..} = endpoint host port

startServer :: GRPC -> ServerConfig -> IO Server
startServer grpc conf@ServerConfig{..} = do
  let e = serverEndpoint conf
  server <- C.grpcServerCreate nullPtr C.reserved
  actualPort <- C.grpcServerAddInsecureHttp2Port server (unEndpoint e)
  when (actualPort /= unPort port) $
    error $ "Unable to bind port: " ++ show port
  cq <- createCompletionQueue grpc
  serverRegisterCompletionQueue server cq
  methods <- forM methodsToRegister $ \(name, mtype) ->
               serverRegisterMethod server name e mtype
  C.grpcServerStart server
  return $ Server server cq methods conf

stopServer :: Server -> IO ()
-- TODO: Do method handles need to be freed?
stopServer (Server server cq _ _) = do
  grpcDebug "stopServer: calling shutdownNotify."
  shutdownNotify
  grpcDebug "stopServer: cancelling all calls."
  C.grpcServerCancelAllCalls server
  grpcDebug "stopServer: call grpc_server_destroy."
  C.grpcServerDestroy server
  grpcDebug "stopServer: shutting down CQ."
  shutdownCQ

  where shutdownCQ = do
          shutdownResult <- shutdownCompletionQueue cq
          case shutdownResult of
            Left _ -> do putStrLn "Warning: completion queue didn't shut down."
                         putStrLn "Trying to stop server anyway."
            Right _ -> return ()
        shutdownNotify = do
          let shutdownTag = C.tag 0
          serverShutdownAndNotify server cq shutdownTag
          shutdownEvent <- pluck cq shutdownTag 30
          case shutdownEvent of
            -- This case occurs when we pluck but the queue is already in the
            -- 'shuttingDown' state, implying we already tried to shut down.
            (Left GRPCIOShutdown) -> error "Called stopServer twice!"
            (Left _) -> error "Failed to stop server."
            (Right _) -> return ()

-- Uses 'bracket' to safely start and stop a server, even if exceptions occur.
withServer :: GRPC -> ServerConfig -> (Server -> IO a) -> IO a
withServer grpc cfg f = bracket (startServer grpc cfg) stopServer f

-- | Register a method on a server. The 'RegisteredMethod' type can then be used
-- to wait for a request to arrive. Note: gRPC claims this must be called before
-- the server is started, so we do it during startup according to the
-- 'ServerConfig'.
serverRegisterMethod :: C.Server
                     -> MethodName
                     -- ^ method name, e.g. "/foo"
                     -> Endpoint
                     -- ^ Endpoint name name, e.g. "localhost:9999". I have no
                     -- idea why this is needed since we have to provide these
                     -- parameters to start a server in the first place. It
                     -- doesn't seem to have any effect, even if it's filled
                     -- with nonsense.
                     -> GRPCMethodType
                     -- ^ Type of method this will be. In the future, this will
                     -- be used to switch to the correct handling logic.
                     -- Currently, the only valid choice is 'Normal'.
                     -> IO RegisteredMethod
serverRegisterMethod internalServer meth e Normal = do
  handle <- C.grpcServerRegisterMethod internalServer
              (unMethodName meth) (unEndpoint e)
  grpcDebug $ "registered method to handle " ++ show handle
  return $ RegisteredMethod Normal meth e handle
serverRegisterMethod _ _ _ _ = error "Streaming methods not implemented yet."

-- | Create a 'Call' with which to wait for the invocation of a registered
-- method.
serverCreateRegisteredCall :: Server -> RegisteredMethod -> TimeoutSeconds
                              -> MetadataMap
                              -> IO (Either GRPCIOError ServerRegCall)
serverCreateRegisteredCall Server{..} rm timeLimit initMeta =
  serverRequestRegisteredCall internalServer serverCQ timeLimit rm initMeta

withServerRegisteredCall :: Server -> RegisteredMethod -> TimeoutSeconds
                            -> MetadataMap
                            -> (ServerRegCall
                                -> IO (Either GRPCIOError a))
                            -> IO (Either GRPCIOError a)
withServerRegisteredCall server regmethod timeout initMeta f = do
  createResult <- serverCreateRegisteredCall server regmethod timeout initMeta
  case createResult of
    Left x -> return $ Left x
    Right call -> f call `finally` logDestroy call
      where logDestroy c = grpcDebug "withServerRegisteredCall: destroying."
                           >> destroyServerRegCall c

-- | Sequence of 'Op's needed to receive a normal (non-streaming) call.
serverOpsGetNormalCall :: MetadataMap -> [Op]
serverOpsGetNormalCall initMetadata =
  [OpSendInitialMetadata initMetadata,
   OpRecvMessage]

-- | Sequence of 'Op's needed to respond to a normal (non-streaming) call.
serverOpsSendNormalResponse :: ByteString
                               -> MetadataMap
                               -> C.StatusCode
                               -> StatusDetails
                               -> [Op]
serverOpsSendNormalResponse body metadata code details =
  [OpRecvCloseOnServer,
   OpSendMessage body,
   OpSendStatusFromServer metadata code details]

serverOpsSendNormalRegisteredResponse :: ByteString
                                         -> MetadataMap
                                         -- ^ initial metadata
                                         -> MetadataMap
                                         -- ^ trailing metadata
                                         -> C.StatusCode
                                         -> StatusDetails
                                         -> [Op]
serverOpsSendNormalRegisteredResponse
  body initMetadata trailingMeta code details =
  [OpSendInitialMetadata initMetadata,
   OpRecvCloseOnServer,
   OpSendMessage body,
   OpSendStatusFromServer trailingMeta code details]

-- TODO: we will want to replace this with some more general concept that also
-- works with streaming calls in the future.
-- | Wait for and then handle a normal (non-streaming) call.
serverHandleNormalRegisteredCall :: Server
                          -> RegisteredMethod
                          -> TimeoutSeconds
                          -> MetadataMap
                          -- ^ Initial server metadata
                          -> (ByteString -> MetadataMap
                              -> IO (ByteString,
                                     MetadataMap,
                                     MetadataMap,
                                     StatusDetails))
                             -- ^ Handler function takes a request body and
                             -- metadata and returns a response body and
                             -- metadata.
                          -> IO (Either GRPCIOError ())
serverHandleNormalRegisteredCall s@Server{..} rm timeLimit srvMetadata f = do
  -- TODO: we use this timeLimit twice, so the max time spent is 2*timeLimit.
  -- Should we just hard-code time limits instead? Not sure if client
  -- programmer cares, since this function will likely just be put in a loop
  -- anyway.
  withServerRegisteredCall s rm timeLimit srvMetadata $ \call -> do
    grpcDebug "serverHandleNormalRegisteredCall: starting batch."
    debugServerRegCall call
    payload <- serverRegCallGetPayload call
    case payload of
      --TODO: what should we do with an empty payload? Have the handler take
      -- @Maybe ByteString@? Need to figure out when/why payload would be empty.
      Nothing -> error "serverHandleNormalRegisteredCall: payload empty."
      Just requestBody -> do
        requestMeta <- serverRegCallGetMetadata call
        (respBody, initMeta, trailingMeta, details) <- f requestBody requestMeta
        let status = C.GrpcStatusOk
        let respOps = serverOpsSendNormalRegisteredResponse
                        respBody initMeta trailingMeta status details
        respOpsResults <- runServerRegOps call serverCQ respOps timeLimit
        grpcDebug "serverHandleNormalRegisteredCall: finished response ops."
        case respOpsResults of
          Left x -> return $ Left x
          Right _ -> return $ Right ()