Use ByteString instead of String for stats callback

This avoids the round-trip via String if one wants to pass the
statistics JSON to aeson for further parsing.

Author: Philipp Hausmann

src/Kafka/Callbacks.hs

@@ -5,6 +5,7 @@ module Kafka.Callbacks
 )
 where
 
+import Data.ByteString (ByteString)
 import Kafka.Internal.RdKafka (rdKafkaConfSetErrorCb, rdKafkaConfSetLogCb, rdKafkaConfSetStatsCb)
 import Kafka.Internal.Setup (HasKafkaConf(..), getRdKafkaConf)
 import Kafka.Types (KafkaError(..), KafkaLogLevel(..))
@@ -39,7 +40,7 @@ logCallback callback k =
   let realCb _ = callback . toEnum
   in rdKafkaConfSetLogCb (getRdKafkaConf k) realCb
 
--- | Add a callback for stats.
+-- | Add a callback for stats. The passed ByteString contains an UTF-8 encoded JSON document and can e.g. be parsed using Data.Aeson.decodeStrict. For more information about the content of the JSON document see <https://github.com/edenhill/librdkafka/blob/master/STATISTICS.md>.
 --
 -- ==== __Examples__
 --
@@ -49,7 +50,7 @@ logCallback callback k =
 -- >
 -- > myStatsCallback :: String -> IO ()
 -- > myStatsCallback stats = print $ show stats
-statsCallback :: HasKafkaConf k => (String -> IO ()) -> k -> IO ()
+statsCallback :: HasKafkaConf k => (ByteString -> IO ()) -> k -> IO ()
 statsCallback callback k =
   let realCb _ = callback
   in rdKafkaConfSetStatsCb (getRdKafkaConf k) realCb

src/Kafka/Internal/RdKafka.chs

@@ -3,6 +3,8 @@
 
 module Kafka.Internal.RdKafka where
 
+import Data.ByteString (ByteString)
+import qualified Data.ByteString as BS
 import Data.Text (Text)
 import qualified Data.Text as Text
 import Control.Monad (liftM)
@@ -15,7 +17,7 @@ import Foreign.Storable (Storable(..))
 import Foreign.Ptr (Ptr, FunPtr, castPtr, nullPtr)
 import Foreign.ForeignPtr (FinalizerPtr, addForeignPtrFinalizer, newForeignPtr_, withForeignPtr)
 import Foreign.C.Error (Errno(..), getErrno)
-import Foreign.C.String (CString, newCString, withCAString, peekCAString, peekCAStringLen, peekCString)
+import Foreign.C.String (CString, newCString, withCAString, peekCAString, peekCString)
 import Foreign.C.Types (CFile, CInt(..), CSize, CChar)
 import System.IO (Handle, stdin, stdout, stderr)
 import System.Posix.IO (handleToFd)
@@ -414,7 +416,7 @@ rdKafkaConfSetLogCb conf cb = do
 
 ---- Stats Callback
 type StatsCallback' = Ptr RdKafkaT -> CString -> CSize -> Word8Ptr -> IO ()
-type StatsCallback = Ptr RdKafkaT -> String -> IO ()
+type StatsCallback = Ptr RdKafkaT -> ByteString -> IO ()
 
 foreign import ccall safe "wrapper"
     mkStatsCallback :: StatsCallback' -> IO (FunPtr StatsCallback')
@@ -424,7 +426,7 @@ foreign import ccall safe "rd_kafka.h rd_kafka_conf_set_stats_cb"
 
 rdKafkaConfSetStatsCb :: RdKafkaConfTPtr -> StatsCallback -> IO ()
 rdKafkaConfSetStatsCb conf cb = do
-    cb' <- mkStatsCallback $ \k j jl _ -> peekCAStringLen (j, cIntConv jl) >>= cb k
+    cb' <- mkStatsCallback $ \k j jl _ -> BS.packCStringLen (j, cIntConv jl) >>= cb k
     withForeignPtr conf $ \c -> rdKafkaConfSetStatsCb' c cb'
     return ()