manage logging via file handle

reason: `appendFile` combined with lazy evaluation lead to exhaustion of
open file descriptors, as each file is opened again for each write and
due to lazy evaluation is kept open multiple times.

src/Hash2Pub/PostService.hs

@@ -32,10 +32,11 @@ import           Data.Time.Clock.POSIX
 import           Data.Typeable             (Typeable)
 import qualified Network.HTTP.Client       as HTTP
 import qualified Network.HTTP.Types        as HTTPT
+import           System.IO
 import           System.Random
 import           Text.Read                 (readEither)
 
-import           Formatting                (float, format, (%), fixed)
+import           Formatting                (fixed, float, format, (%))
 import qualified Network.Wai.Handler.Warp  as Warp
 import           Servant
 import           Servant.Client
@@ -66,6 +67,7 @@ data PostService d = PostService
     , httpMan              :: HTTP.Manager
     , statsQueue           :: TQueue StatsEvent
     , loadStats            :: TVar RelayStats
+    , logFileHandle        :: Handle
     }
     deriving (Typeable)
 
@@ -96,6 +98,7 @@ instance DHT d => Service PostService d where
         httpMan' <- HTTP.newManager HTTP.defaultManagerSettings
         statsQueue' <- newTQueueIO
         loadStats' <- newTVarIO emptyStats
+        loggingFile <- openFile (confLogfilePath conf) WriteMode
         let
             thisService = PostService
               { serviceConf = conf
@@ -110,12 +113,13 @@ instance DHT d => Service PostService d where
               , httpMan = httpMan'
               , statsQueue = statsQueue'
               , loadStats = loadStats'
+              , logFileHandle = loggingFile
               }
             port' = fromIntegral (confServicePort conf)
             warpSettings = Warp.setPort port' . Warp.setHost (fromString . confServiceHost $ conf) $ Warp.defaultSettings
         -- log a start message, this also truncates existing files
-        TxtI.writeFile (confLogfilePath conf) $ Txt.unlines
-            [ "# Starting mock relay implementation\n"
+        TxtI.hPutStrLn loggingFile $ Txt.unlines
+            [ "# Starting mock relay implementation"
             , "#relay receive rate   ;relay delivery rate   ;instance publish rate ;instance fetch rate"
             ]
         -- Run 'concurrently_' from another thread to be able to return the
@@ -816,8 +820,8 @@ evaluateStatsThread serv statsAcc = getPOSIXTime >>= loop
           -- and now what? write a log to file
           -- format: total relayReceiveRates;total relayDeliveryRates;postFetchRate;postPublishRate
           -- later: current (reported) load, target load
-          TxtI.appendFile (confLogfilePath . serviceConf $ serv) $
-              format (fixed 20 % ";" % fixed 20 % ";" % fixed 20 % ";" % fixed 20 % "\n")
+          TxtI.hPutStrLn (logFileHandle serv) $
+              format (fixed 20 % ";" % fixed 20 % ";" % fixed 20 % ";" % fixed 20)
                   (sum . relayReceiveRates $ rateStats)
                   (sum . relayDeliveryRates $ rateStats)
                   (postPublishRate rateStats)