refactor: Simplify schema cache loading triggering logic

DISCLAIMER:
This commit was authored entirely by a human without the assistance of LLMs.

Using debouncer to trigger schema cache loading makes it difficult to understand when exactly it is triggered.

(cherry picked from commit a4c1d945ee)

src/PostgREST/AppState.hs

@@ -1,6 +1,7 @@
 {-# LANGUAGE LambdaCase      #-}
 {-# LANGUAGE NamedFieldPuns  #-}
 {-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE RecursiveDo     #-}
 
 module PostgREST.AppState
   ( AppState
@@ -45,7 +46,6 @@ import           PostgREST.Version          (prettyVersion)
 
 import Control.AutoUpdate (defaultUpdateSettings, mkAutoUpdate,
                            updateAction)
-import Control.Debounce
 import Control.Retry      (RetryPolicy, RetryStatus (..), capDelay,
                            exponentialBackoff, retrying,
                            rsPreviousDelay)
@@ -117,15 +117,23 @@ init conf@AppConfig{configLogLevel, configDbPoolSize} = do
   pool <- initPool conf observer
   initWithPool pool conf loggerState metricsState observer --{ stateSocketREST = sock, stateSocketAdmin = adminSock}
 
+simpleDebounce :: IO () -> IO (IO ())
+simpleDebounce act = do
+  flag <- newEmptyMVar
+  void $ forkIO $ forever $ do
+    takeMVar flag
+    act
+  pure (void $ tryPutMVar flag ())
+
 initWithPool :: SQL.Pool -> AppConfig -> Logger.LoggerState -> Metrics.MetricsState -> ObservationHandler -> IO AppState
-initWithPool pool conf loggerState metricsState observer = do
+initWithPool pool conf loggerState metricsState observer = mdo
 
   appState <- AppState pool
     <$> newIORef minimumPgVersion -- assume we're in a supported version when starting, this will be corrected on a later step
     <*> newIORef Nothing
     <*> newIORef SCPending
     <*> newIORef False
-    <*> pure (pure ())
+    <*> simpleDebounce (retryingSchemaCacheLoad appState *> threadDelay 100000)  -- 100ms cooldown
     <*> newIORef conf
     <*> mkAutoUpdate defaultUpdateSettings { updateAction = getCurrentTime }
     <*> myThreadId
@@ -136,15 +144,7 @@ initWithPool pool conf loggerState metricsState observer = do
     <*> pure loggerState
     <*> pure metricsState
 
-  deb <-
-    let decisecond = 100000 in
-    mkDebounce defaultDebounceSettings
-       { debounceAction = retryingSchemaCacheLoad appState
-       , debounceFreq = decisecond
-       , debounceEdge = leadingEdge -- runs the worker at the start and the end
-       }
-
-  return appState { debouncedSCacheLoader = deb}
+  return appState
 
 destroy :: AppState -> IO ()
 destroy = destroyPool