refactor: make sure proper sockets cleanup is performed in App.run

Right now code in App.run does not properly use bracket/finally to close sockets and clean-up mainSocketRef. This is not a big problem at the moment because the application is going to exit enyway but introducting proper resource handling will make future refactorings safer.

src/PostgREST/App.hs

@@ -24,7 +24,6 @@ import GHC.IO.Exception (IOErrorType (..))
 import System.IO.Error  (ioeGetErrorType)
 
 import Control.Monad.Except     (liftEither)
-import Control.Monad.Extra      (whenJust)
 import Data.Either.Combinators  (mapLeft, whenLeft)
 import Data.IORef               (atomicWriteIORef, newIORef,
                                  readIORef)
@@ -83,11 +82,14 @@ run appState = do
   conf <- AppState.getConfig appState
 
   mainSocketRef <- newIORef Nothing
-  adminSocket <- initAdminServerSocket conf
+  let setMainSocketRef = atomicWriteIORef mainSocketRef . Just
+      clearMainSocketRef = atomicWriteIORef mainSocketRef Nothing
+
+  bracket (initAdminServerSocket conf) ensureSocketClosed $ \adminSocket -> do
 
     let closeSockets = do
-        whenJust adminSocket NS.close
+          ensureSocketClosed adminSocket
-        readIORef mainSocketRef >>= foldMap NS.close
+          ensureSocketClosed =<< readIORef mainSocketRef
     Unix.installSignalHandlers observer closeSockets (AppState.schemaCacheLoader appState) (AppState.readInDbConfig False appState)
 
     Admin.runAdmin appState adminSocket (checkMainAppLive (readIORef mainSocketRef)) (serverSettings conf)
@@ -99,8 +101,7 @@ run appState = do
     AppState.schemaCacheLoader appState
     AppState.waitForSchemaCacheInit appState
 
-  mainSocket <- initServerSocket conf
+    bracket (initServerSocket conf) NS.close $ \mainSocket -> do
-  atomicWriteIORef mainSocketRef $ Just mainSocket
 
       let app = postgrest appState (AppState.schemaCacheLoader appState)
 
@@ -110,12 +111,15 @@ run appState = do
         appServerSettings = serverSettings conf
           & setPort (configServerPort conf)
           & setOnException onWarpException
-      & setBeforeMainLoop (observer $ AppServerAddressObs address)
+          & setBeforeMainLoop (setMainSocketRef mainSocket *> observer (AppServerAddressObs address))
 
       Warp.runSettingsSocket appServerSettings mainSocket app
+        `finally` clearMainSocketRef
   where
     observer = AppState.getObserver appState
 
+    ensureSocketClosed = foldMap NS.close
+
     onWarpException :: Maybe Wai.Request -> SomeException -> IO ()
     onWarpException _ ex =
       when (shouldDisplayException ex) $