refactor: Remove unnecessary lazy initialization of stateLogDebouncePoolTimeout

stateLogDebouncePoolTimeout is an MVar initialized on the first logging of PoolAcqTimeoutObs. The code in logWithDebounce has race condition that could lead to creation of multiple debouncers.

This change simplifies logic by getting rid of lazy initialization of debouncer.

src/PostgREST/Logger.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE RecursiveDo     #-}
 {-|
 Module      : PostgREST.Logger
 Description : Logging based on the Observation.hs module. Access logs get sent to stdout and server diagnostic get sent to stderr.
@@ -39,29 +40,21 @@ import Protolude
 
 data LoggerState = LoggerState
   { stateGetZTime               :: IO ZonedTime  -- ^ Time with time zone used for logs
-  , stateLogDebouncePoolTimeout :: MVar (IO ())  -- ^ Logs with a debounce
+  , stateLogDebouncePoolTimeout :: IO ()         -- ^ Logs with a debounce
   }
 
 init :: IO LoggerState
-init = do
+init = mdo
+  let
+    oneSecond = 1000000
+    loggerState = LoggerState zTime debouncePoolTimeout
   zTime <- mkAutoUpdate defaultUpdateSettings { updateAction = getZonedTime }
-  LoggerState zTime <$> newEmptyMVar
-
-logWithDebounce :: LoggerState -> IO () -> IO ()
-logWithDebounce loggerState action = do
-  debouncer <- tryReadMVar $ stateLogDebouncePoolTimeout loggerState
-  case debouncer of
-    Just d -> d
-    Nothing -> do
-      newDebouncer <-
-        let oneSecond = 1000000 in
-        mkDebounce defaultDebounceSettings
-           { debounceAction = action
-           , debounceFreq = 5*oneSecond
-           , debounceEdge = leadingEdge -- logs at the start and the end
-           }
-      putMVar (stateLogDebouncePoolTimeout loggerState) newDebouncer
-      newDebouncer
+  debouncePoolTimeout <- mkDebounce defaultDebounceSettings
+          { debounceAction = logWithZTime loggerState $ observationMessage PoolAcqTimeoutObs
+          , debounceFreq = 5*oneSecond
+          , debounceEdge = leadingEdge -- logs at the start and the end
+          }
+  pure loggerState
 
 -- TODO stop using this middleware to reuse the same "observer" pattern for all our logs
 middleware :: LogLevel -> (Wai.Request -> Maybe BS.ByteString) -> Wai.Middleware
@@ -88,10 +81,9 @@ shouldLogResponse logLevel = case logLevel of
 -- All observations are logged except some that depend on the log-level
 observationLogger :: LoggerState -> LogLevel -> ObservationHandler
 observationLogger loggerState logLevel obs = case obs of
-  o@PoolAcqTimeoutObs -> do
-    when (logLevel >= LogError) $ do
-      logWithDebounce loggerState $
-        logWithZTime loggerState $ observationMessage o
+  PoolAcqTimeoutObs -> do
+    when (logLevel >= LogError) $
+      stateLogDebouncePoolTimeout loggerState
   o@(QueryErrorCodeHighObs _) -> do
     when (logLevel >= LogError) $ do
       logWithZTime loggerState $ observationMessage o