Close thunk leak in roundstats.

The function performFetches collects statistics about the requests
performed in each round. However, the list is not evaluated (unless
tracing is on), leading to an accumulation of thunks. Forcing the list
via deepseq prevents this.

Haxl/Core/Monad.hs

@@ -56,6 +56,7 @@ import Control.Exception (AllocationLimitExceeded(..))
 import Control.Monad
 import qualified Control.Exception as Exception
 import Control.Applicative hiding (Const)
+import Control.DeepSeq
 import GHC.Exts (IsString(..))
 #if __GLASGOW_HASKELL__ < 706
 import Prelude hiding (catch)
@@ -526,7 +527,7 @@ performFetches n env reqs = do
   let roundtime = realToFrac (diffUTCTime t1 t0) :: Double
 
   ifReport f 1 $
-    modifyIORef' sref $ \(Stats rounds) ->
+    modifyIORef' sref $ \(Stats rounds) -> roundstats `deepseq`
       Stats (RoundStats (microsecs roundtime) dsroundstats: rounds)
 
   ifTrace f 1 $

haxl.cabal

@@ -31,6 +31,7 @@ library
     base == 4.*,
     bytestring >= 0.9 && < 0.11,
     containers == 0.5.*,
+    deepseq,
     directory >= 1.1 && < 1.3,
     filepath >= 1.3 && < 1.5,
     hashable == 1.2.*,