Synnefo » snf-ganeti

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,

  BangPatterns, TemplateHaskell #-}

{-| Implementation of the RPC client.

-}

{-

Copyright (C) 2012, 2013 Google Inc.

This program is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2 of the License, or

(at your option) any later version.

This program is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

You should have received a copy of the GNU General Public License

along with this program; if not, write to the Free Software

Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

02110-1301, USA.

-}

module Ganeti.Rpc

  ( RpcCall

  , Rpc

  , RpcError(..)

  , ERpcError

  , explainRpcError

  , executeRpcCall

  , executeRpcCalls

  , logRpcErrors

  , rpcCallName

  , rpcCallTimeout

  , rpcCallData

  , rpcCallAcceptOffline

  , rpcResultFill

  , RpcCallInstanceInfo(..)

  , InstanceState(..)

  , InstanceInfo(..)

  , RpcResultInstanceInfo(..)

  , RpcCallAllInstancesInfo(..)

  , RpcResultAllInstancesInfo(..)

  , InstanceConsoleInfoParams(..)

  , InstanceConsoleInfo(..)

  , RpcCallInstanceConsoleInfo(..)

  , RpcResultInstanceConsoleInfo(..)

  , RpcCallInstanceList(..)

  , RpcResultInstanceList(..)

  , HvInfo(..)

  , StorageInfo(..)

  , RpcCallNodeInfo(..)

  , RpcResultNodeInfo(..)

  , RpcCallVersion(..)

  , RpcResultVersion(..)

  , RpcCallStorageList(..)

  , RpcResultStorageList(..)

  , RpcCallTestDelay(..)

  , RpcResultTestDelay(..)

  , RpcCallExportList(..)

  , RpcResultExportList(..)

  , RpcCallJobqueueUpdate(..)

  , RpcCallSetWatcherPause(..)

  , RpcCallSetDrainFlag(..)

  ) where

import Control.Arrow (second)

import qualified Codec.Compression.Zlib as Zlib

import qualified Data.ByteString.Lazy.Char8 as BL

import qualified Data.Map as Map

import Data.Maybe (fromMaybe)

import qualified Text.JSON as J

import Text.JSON.Pretty (pp_value)

import qualified Data.ByteString.Base64.Lazy as Base64

import System.Directory

import Network.Curl hiding (content)

import qualified Ganeti.Path as P

import Ganeti.BasicTypes

import qualified Ganeti.Constants as C

import Ganeti.JSON

import Ganeti.Logging

import Ganeti.Objects

import Ganeti.THH

import Ganeti.Types

import Ganeti.Curl.Multi

import Ganeti.Utils

-- * Base RPC functionality and types

-- | The curl options used for RPC.

curlOpts :: [CurlOption]

curlOpts = [ CurlFollowLocation False

           , CurlSSLVerifyHost 0

           , CurlSSLVerifyPeer True

           , CurlSSLCertType "PEM"

           , CurlSSLKeyType "PEM"

           , CurlConnectTimeout (fromIntegral C.rpcConnectTimeout)

           ]

-- | Data type for RPC error reporting.

data RpcError

  = CurlLayerError String

  | JsonDecodeError String

  | RpcResultError String

  | OfflineNodeError

  deriving (Show, Eq)

-- | Provide explanation to RPC errors.

explainRpcError :: RpcError -> String

explainRpcError (CurlLayerError code) =

    "Curl error:" ++ code

explainRpcError (JsonDecodeError msg) =

    "Error while decoding JSON from HTTP response: " ++ msg

explainRpcError (RpcResultError msg) =

    "Error reponse received from RPC server: " ++ msg

explainRpcError OfflineNodeError =

    "Node is marked offline"

type ERpcError = Either RpcError

-- | A generic class for RPC calls.

class (J.JSON a) => RpcCall a where

  -- | Give the (Python) name of the procedure.

  rpcCallName :: a -> String

  -- | Calculate the timeout value for the call execution.

  rpcCallTimeout :: a -> Int

  -- | Prepare arguments of the call to be send as POST.

  rpcCallData :: Node -> a -> String

  -- | Whether we accept offline nodes when making a call.

  rpcCallAcceptOffline :: a -> Bool

-- | Generic class that ensures matching RPC call with its respective

-- result.

class (RpcCall a, J.JSON b) => Rpc a b  | a -> b, b -> a where

  -- | Create a result based on the received HTTP response.

  rpcResultFill :: a -> J.JSValue -> ERpcError b

-- | Http Request definition.

data HttpClientRequest = HttpClientRequest

  { requestUrl  :: String       -- ^ The actual URL for the node endpoint

  , requestData :: String       -- ^ The arguments for the call

  , requestOpts :: [CurlOption] -- ^ The various curl options

  }

-- | Check if a string represented address is IPv6

isIpV6 :: String -> Bool

isIpV6 = (':' `elem`)

-- | Prepare url for the HTTP request.

prepareUrl :: (RpcCall a) => Node -> a -> String

prepareUrl node call =

  let node_ip = nodePrimaryIp node

      node_address = if isIpV6 node_ip

                     then "[" ++ node_ip ++ "]"

                     else node_ip

      port = C.defaultNodedPort

      path_prefix = "https://" ++ node_address ++ ":" ++ show port

  in path_prefix ++ "/" ++ rpcCallName call

-- | Create HTTP request for a given node provided it is online,

-- otherwise create empty response.

prepareHttpRequest :: (RpcCall a) => [CurlOption] -> Node -> a

                   -> ERpcError HttpClientRequest

prepareHttpRequest opts node call

  | rpcCallAcceptOffline call || not (nodeOffline node) =

      Right HttpClientRequest { requestUrl  = prepareUrl node call

                              , requestData = rpcCallData node call

                              , requestOpts = opts ++ curlOpts

                              }

  | otherwise = Left OfflineNodeError

-- | Parse an HTTP reply.

parseHttpReply :: (Rpc a b) =>

                  a -> ERpcError (CurlCode, String) -> ERpcError b

parseHttpReply _ (Left e) = Left e

parseHttpReply call (Right (CurlOK, body)) = parseHttpResponse call body

parseHttpReply _ (Right (code, err)) =

  Left . CurlLayerError $ "code: " ++ show code ++ ", explanation: " ++ err

-- | Parse a result based on the received HTTP response.

parseHttpResponse :: (Rpc a b) => a -> String -> ERpcError b

parseHttpResponse call res =

  case J.decode res of

    J.Error val -> Left $ JsonDecodeError val

    J.Ok (True, res'') -> rpcResultFill call res''

    J.Ok (False, jerr) -> case jerr of

       J.JSString msg -> Left $ RpcResultError (J.fromJSString msg)

       _ -> Left . JsonDecodeError $ show (pp_value jerr)

-- | Scan the list of results produced by executeRpcCall and log all the RPC

-- errors.

logRpcErrors :: [(a, ERpcError b)] -> IO ()

logRpcErrors allElems =

  let logOneRpcErr (_, Right _) = return ()

      logOneRpcErr (_, Left err) =

        logError $ "Error in the RPC HTTP reply: " ++ show err

  in mapM_ logOneRpcErr allElems

-- | Get options for RPC call

getOptionsForCall :: (Rpc a b) => FilePath -> FilePath -> a -> [CurlOption]

getOptionsForCall cert_path client_cert_path call =

  [ CurlTimeout (fromIntegral $ rpcCallTimeout call)

  , CurlSSLCert client_cert_path

  , CurlSSLKey client_cert_path

  , CurlCAInfo cert_path

  ]

-- | Execute multiple RPC calls in parallel

executeRpcCalls :: (Rpc a b) => [(Node, a)] -> IO [(Node, ERpcError b)]

executeRpcCalls nodeCalls = do

  cert_file <- P.nodedCertFile

  client_cert_file_name <- P.nodedClientCertFile

  client_file_exists <- doesFileExist client_cert_file_name

  -- FIXME: This is needed to ensure upgradability to 2.11

  -- Remove in 2.12.

  let client_cert_file = if client_file_exists

                         then client_cert_file_name

                         else cert_file

      (nodes, calls) = unzip nodeCalls

      opts = map (getOptionsForCall cert_file client_cert_file) calls

      opts_urls = zipWith3 (\n c o ->

                         case prepareHttpRequest o n c of

                           Left v -> Left v

                           Right request ->

                             Right (CurlPostFields [requestData request]:

                                    requestOpts request,

                                    requestUrl request)

                    ) nodes calls opts

  -- split the opts_urls list; we don't want to pass the

  -- failed-already nodes to Curl

  let (lefts, rights, trail) = splitEithers opts_urls

  results <- execMultiCall rights

  results' <- case recombineEithers lefts results trail of

                Bad msg -> error msg

                Ok r -> return r

  -- now parse the replies

  let results'' = zipWith parseHttpReply calls results'

      pairedList = zip nodes results''

  logRpcErrors pairedList

  return pairedList

-- | Execute an RPC call for many nodes in parallel.

executeRpcCall :: (Rpc a b) => [Node] -> a -> IO [(Node, ERpcError b)]

executeRpcCall nodes call = executeRpcCalls . zip nodes $ repeat call

-- | Helper function that is used to read dictionaries of values.

sanitizeDictResults :: [(String, J.Result a)] -> ERpcError [(String, a)]

sanitizeDictResults =

  foldr sanitize1 (Right [])

  where

    sanitize1 _ (Left e) = Left e

    sanitize1 (_, J.Error e) _ = Left $ JsonDecodeError e

    sanitize1 (name, J.Ok v) (Right res) = Right $ (name, v) : res

-- | Helper function to tranform JSON Result to Either RpcError b.

-- Note: For now we really only use it for b s.t. Rpc c b for some c

fromJResultToRes :: J.Result a -> (a -> b) -> ERpcError b

fromJResultToRes (J.Error v) _ = Left $ JsonDecodeError v

fromJResultToRes (J.Ok v) f = Right $ f v

-- | Helper function transforming JSValue to Rpc result type.

fromJSValueToRes :: (J.JSON a) => J.JSValue -> (a -> b) -> ERpcError b

fromJSValueToRes val = fromJResultToRes (J.readJSON val)

-- * RPC calls and results

-- ** Instance info

-- | Returns information about a single instance

$(buildObject "RpcCallInstanceInfo" "rpcCallInstInfo"

  [ simpleField "instance" [t| String |]

  , simpleField "hname" [t| Hypervisor |]

  ])

$(declareILADT "InstanceState"

  [ ("InstanceStateRunning", 0)

  , ("InstanceStateShutdown", 1)

  ])

$(makeJSONInstance ''InstanceState)

instance PyValue InstanceState where

  showValue = show . instanceStateToRaw

$(buildObject "InstanceInfo" "instInfo"

  [ simpleField "memory" [t| Int|]

  , simpleField "state"  [t| InstanceState |]

  , simpleField "vcpus"  [t| Int |]

  , simpleField "time"   [t| Int |]

  ])

-- This is optional here because the result may be empty if instance is

-- not on a node - and this is not considered an error.

$(buildObject "RpcResultInstanceInfo" "rpcResInstInfo"

  [ optionalField $ simpleField "inst_info" [t| InstanceInfo |]])

instance RpcCall RpcCallInstanceInfo where

  rpcCallName _          = "instance_info"

  rpcCallTimeout _       = rpcTimeoutToRaw Urgent

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode

    ( rpcCallInstInfoInstance call

    , rpcCallInstInfoHname call

    )

instance Rpc RpcCallInstanceInfo RpcResultInstanceInfo where

  rpcResultFill _ res =

    case res of

      J.JSObject res' ->

        case J.fromJSObject res' of

          [] -> Right $ RpcResultInstanceInfo Nothing

          _ -> fromJSValueToRes res (RpcResultInstanceInfo . Just)

      _ -> Left $ JsonDecodeError

           ("Expected JSObject, got " ++ show (pp_value res))

-- ** AllInstancesInfo

-- | Returns information about all running instances on the given nodes

$(buildObject "RpcCallAllInstancesInfo" "rpcCallAllInstInfo"

  [ simpleField "hypervisors" [t| [(Hypervisor, HvParams)] |] ])

$(buildObject "RpcResultAllInstancesInfo" "rpcResAllInstInfo"

  [ simpleField "instances" [t| [(String, InstanceInfo)] |] ])

instance RpcCall RpcCallAllInstancesInfo where

  rpcCallName _          = "all_instances_info"

  rpcCallTimeout _       = rpcTimeoutToRaw Urgent

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode (

    map fst $ rpcCallAllInstInfoHypervisors call,

    GenericContainer . Map.fromList $ rpcCallAllInstInfoHypervisors call)

instance Rpc RpcCallAllInstancesInfo RpcResultAllInstancesInfo where

  -- FIXME: Is there a simpler way to do it?

  rpcResultFill _ res =

    case res of

      J.JSObject res' ->

        let res'' = map (second J.readJSON) (J.fromJSObject res')

                        :: [(String, J.Result InstanceInfo)] in

        case sanitizeDictResults res'' of

          Left err -> Left err

          Right insts -> Right $ RpcResultAllInstancesInfo insts

      _ -> Left $ JsonDecodeError

           ("Expected JSObject, got " ++ show (pp_value res))

-- ** InstanceConsoleInfo

-- | Returns information about how to access instances on the given node

$(buildObject "InstanceConsoleInfoParams" "instConsInfoParams"

  [ simpleField "instance"    [t| Instance |]

  , simpleField "node"        [t| Node |]

  , simpleField "group"       [t| NodeGroup |]

  , simpleField "hvParams"    [t| HvParams |]

  , simpleField "beParams"    [t| FilledBeParams |]

  ])

$(buildObject "RpcCallInstanceConsoleInfo" "rpcCallInstConsInfo"

  [ simpleField "instanceInfo" [t| [(String, InstanceConsoleInfoParams)] |] ])

$(buildObject "InstanceConsoleInfo" "instConsInfo"

  [ simpleField "instance"    [t| String |]

  , simpleField "kind"        [t| String |]

  , optionalField $

    simpleField "message"     [t| String |]

  , optionalField $

    simpleField "host"        [t| String |]

  , optionalField $

    simpleField "port"        [t| Int |]

  , optionalField $

    simpleField "user"        [t| String |]

  , optionalField $

    simpleField "command"     [t| [String] |]

  , optionalField $

    simpleField "display"     [t| String |]

  ])

$(buildObject "RpcResultInstanceConsoleInfo" "rpcResInstConsInfo"

  [ simpleField "instancesInfo" [t| [(String, InstanceConsoleInfo)] |] ])

instance RpcCall RpcCallInstanceConsoleInfo where

  rpcCallName _          = "instance_console_info"

  rpcCallTimeout _       = rpcTimeoutToRaw Urgent

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode .

    GenericContainer $ Map.fromList (rpcCallInstConsInfoInstanceInfo call)

instance Rpc RpcCallInstanceConsoleInfo RpcResultInstanceConsoleInfo where

  rpcResultFill _ res =

    case res of

      J.JSObject res' ->

        let res'' = map (second J.readJSON) (J.fromJSObject res')

                        :: [(String, J.Result InstanceConsoleInfo)] in

        case sanitizeDictResults res'' of

          Left err -> Left err

          Right instInfos -> Right $ RpcResultInstanceConsoleInfo instInfos

      _ -> Left $ JsonDecodeError

           ("Expected JSObject, got " ++ show (pp_value res))

-- ** InstanceList

-- | Returns the list of running instances on the given nodes

$(buildObject "RpcCallInstanceList" "rpcCallInstList"

  [ simpleField "hypervisors" [t| [Hypervisor] |] ])

$(buildObject "RpcResultInstanceList" "rpcResInstList"

  [ simpleField "instances" [t| [String] |] ])

instance RpcCall RpcCallInstanceList where

  rpcCallName _          = "instance_list"

  rpcCallTimeout _       = rpcTimeoutToRaw Urgent

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode [rpcCallInstListHypervisors call]

instance Rpc RpcCallInstanceList RpcResultInstanceList where

  rpcResultFill _ res = fromJSValueToRes res RpcResultInstanceList

-- ** NodeInfo

-- | Returns node information

$(buildObject "RpcCallNodeInfo" "rpcCallNodeInfo"

  [ simpleField "storage_units" [t| Map.Map String [StorageUnit] |]

  , simpleField "hypervisors" [t| [ (Hypervisor, HvParams) ] |]

  ])

$(buildObject "StorageInfo" "storageInfo"

  [ simpleField "name" [t| String |]

  , simpleField "type" [t| String |]

  , optionalField $ simpleField "storage_free" [t| Int |]

  , optionalField $ simpleField "storage_size" [t| Int |]

  ])

-- | We only provide common fields as described in hv_base.py.

$(buildObject "HvInfo" "hvInfo"

  [ simpleField "memory_total" [t| Int |]

  , simpleField "memory_free" [t| Int |]

  , simpleField "memory_dom0" [t| Int |]

  , simpleField "cpu_total" [t| Int |]

  , simpleField "cpu_nodes" [t| Int |]

  , simpleField "cpu_sockets" [t| Int |]

  , simpleField "cpu_dom0" [t| Int |]

  ])

$(buildObject "RpcResultNodeInfo" "rpcResNodeInfo"

  [ simpleField "boot_id" [t| String |]

  , simpleField "storage_info" [t| [StorageInfo] |]

  , simpleField "hv_info" [t| [HvInfo] |]

  ])

instance RpcCall RpcCallNodeInfo where

  rpcCallName _          = "node_info"

  rpcCallTimeout _       = rpcTimeoutToRaw Urgent

  rpcCallAcceptOffline _ = False

  rpcCallData n call     = J.encode

    ( fromMaybe (error $ "Programmer error: missing parameter for node named "

                         ++ nodeName n)

          $ Map.lookup (nodeUuid n) (rpcCallNodeInfoStorageUnits call)

    , rpcCallNodeInfoHypervisors call

    )

instance Rpc RpcCallNodeInfo RpcResultNodeInfo where

  rpcResultFill _ res =

    fromJSValueToRes res (\(b, vg, hv) -> RpcResultNodeInfo b vg hv)

-- ** Version

-- | Query node version.

$(buildObject "RpcCallVersion" "rpcCallVersion" [])

-- | Query node reply.

$(buildObject "RpcResultVersion" "rpcResultVersion"

  [ simpleField "version" [t| Int |]

  ])

instance RpcCall RpcCallVersion where

  rpcCallName _          = "version"

  rpcCallTimeout _       = rpcTimeoutToRaw Urgent

  rpcCallAcceptOffline _ = True

  rpcCallData _          = J.encode

instance Rpc RpcCallVersion RpcResultVersion where

  rpcResultFill _ res = fromJSValueToRes res RpcResultVersion

-- ** StorageList

$(buildObject "RpcCallStorageList" "rpcCallStorageList"

  [ simpleField "su_name" [t| StorageType |]

  , simpleField "su_args" [t| [String] |]

  , simpleField "name"    [t| String |]

  , simpleField "fields"  [t| [StorageField] |]

  ])

-- FIXME: The resulting JSValues should have types appropriate for their

-- StorageField value: Used -> Bool, Name -> String etc

$(buildObject "RpcResultStorageList" "rpcResStorageList"

  [ simpleField "storage" [t| [[(StorageField, J.JSValue)]] |] ])

instance RpcCall RpcCallStorageList where

  rpcCallName _          = "storage_list"

  rpcCallTimeout _       = rpcTimeoutToRaw Normal

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode

    ( rpcCallStorageListSuName call

    , rpcCallStorageListSuArgs call

    , rpcCallStorageListName call

    , rpcCallStorageListFields call

    )

instance Rpc RpcCallStorageList RpcResultStorageList where

  rpcResultFill call res =

    let sfields = rpcCallStorageListFields call in

    fromJSValueToRes res (RpcResultStorageList . map (zip sfields))

-- ** TestDelay

-- | Call definition for test delay.

$(buildObject "RpcCallTestDelay" "rpcCallTestDelay"

  [ simpleField "duration" [t| Double |]

  ])

-- | Result definition for test delay.

data RpcResultTestDelay = RpcResultTestDelay

                          deriving Show

-- | Custom JSON instance for null result.

instance J.JSON RpcResultTestDelay where

  showJSON _        = J.JSNull

  readJSON J.JSNull = return RpcResultTestDelay

  readJSON _        = fail "Unable to read RpcResultTestDelay"

instance RpcCall RpcCallTestDelay where

  rpcCallName _          = "test_delay"

  rpcCallTimeout         = ceiling . (+ 5) . rpcCallTestDelayDuration

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode [rpcCallTestDelayDuration call]

instance Rpc RpcCallTestDelay RpcResultTestDelay where

  rpcResultFill _ res = fromJSValueToRes res id

-- ** ExportList

-- | Call definition for export list.

$(buildObject "RpcCallExportList" "rpcCallExportList" [])

-- | Result definition for export list.

$(buildObject "RpcResultExportList" "rpcResExportList"

  [ simpleField "exports" [t| [String] |]

  ])

instance RpcCall RpcCallExportList where

  rpcCallName _          = "export_list"

  rpcCallTimeout _       = rpcTimeoutToRaw Fast

  rpcCallAcceptOffline _ = False

  rpcCallData _          = J.encode

instance Rpc RpcCallExportList RpcResultExportList where

  rpcResultFill _ res = fromJSValueToRes res RpcResultExportList

-- ** Job Queue Replication

-- | Update a job queue file

$(buildObject "RpcCallJobqueueUpdate" "rpcCallJobqueueUpdate"

  [ simpleField "file_name" [t| String |]

  , simpleField "content" [t| String |]

  ])

$(buildObject "RpcResultJobQueueUpdate" "rpcResultJobQueueUpdate" [])

instance RpcCall RpcCallJobqueueUpdate where

  rpcCallName _          = "jobqueue_update"

  rpcCallTimeout _       = rpcTimeoutToRaw Fast

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode

    ( rpcCallJobqueueUpdateFileName call

    , ( C.rpcEncodingZlibBase64

      , BL.unpack . Base64.encode . Zlib.compress . BL.pack

          $ rpcCallJobqueueUpdateContent call

      )

    )

instance Rpc RpcCallJobqueueUpdate RpcResultJobQueueUpdate where

  rpcResultFill _ res =

    case res of

      J.JSNull ->  Right RpcResultJobQueueUpdate

      _ -> Left $ JsonDecodeError

           ("Expected JSNull, got " ++ show (pp_value res))

-- ** Watcher Status Update

-- | Set the watcher status

$(buildObject "RpcCallSetWatcherPause" "rpcCallSetWatcherPause"

  [ simpleField "time" [t| Maybe Double |]

  ])

instance RpcCall RpcCallSetWatcherPause where

  rpcCallName _          = "set_watcher_pause"

  rpcCallTimeout _       = rpcTimeoutToRaw Fast

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode

    [ maybe J.JSNull J.showJSON $ rpcCallSetWatcherPauseTime call ]

$(buildObject "RpcResultSetWatcherPause" "rpcResultSetWatcherPause" [])

instance Rpc RpcCallSetWatcherPause RpcResultSetWatcherPause where

  rpcResultFill _ res =

    case res of

      J.JSNull ->  Right RpcResultSetWatcherPause

      _ -> Left $ JsonDecodeError

           ("Expected JSNull, got " ++ show (pp_value res))

-- ** Queue drain status

-- | Set the queu drain flag

$(buildObject "RpcCallSetDrainFlag" "rpcCallSetDrainFlag"

  [ simpleField "value" [t| Bool |]

  ])

instance RpcCall RpcCallSetDrainFlag where

  rpcCallName _          = "jobqueue_set_drain_flag"

  rpcCallTimeout _       = rpcTimeoutToRaw Fast

  rpcCallAcceptOffline _ = False

  rpcCallData _ call     = J.encode [ rpcCallSetDrainFlagValue call ]

$(buildObject "RpcResultSetDrainFlag" "rpcResultSetDrainFalg" [])

instance Rpc RpcCallSetDrainFlag RpcResultSetDrainFlag where

  rpcResultFill _ res =

    case res of

      J.JSNull ->  Right RpcResultSetDrainFlag

      _ -> Left $ JsonDecodeError

           ("Expected JSNull, got " ++ show (pp_value res))