fix: Start listening after schema cache load

This change ensures PostgREST starts listening on a server socket only after it loaded the schema cache and is ready to handle requests. It is no longer going to return 503 errors during startup until the schema cache is loaded.

src/PostgREST/Admin.hs

@@ -22,20 +22,20 @@ import qualified PostgREST.AppState as AppState
 import qualified Network.Socket as NS
 import           Protolude
 
-runAdmin :: AppState -> Maybe NS.Socket -> NS.Socket -> Warp.Settings -> IO ()
-runAdmin appState maybeAdminSocket socketREST settings = do
+runAdmin :: AppState -> Maybe NS.Socket -> IO (Maybe NS.Socket) -> Warp.Settings -> IO ()
+runAdmin appState maybeAdminSocket getSocketREST settings = do
   whenJust maybeAdminSocket $ \adminSocket -> do
     address <- resolveSocketToAddress adminSocket
     observer $ AdminStartObs address
     void . forkIO $ Warp.runSettingsSocket settings adminSocket adminApp
   where
-    adminApp = admin appState socketREST
+    adminApp = admin appState getSocketREST
     observer = AppState.getObserver appState
 
 -- | PostgREST admin application
-admin :: AppState.AppState -> NS.Socket -> Wai.Application
-admin appState socketREST req respond  = do
-  isMainAppReachable  <- isRight <$> reachMainApp socketREST
+admin :: AppState.AppState -> IO (Maybe NS.Socket) -> Wai.Application
+admin appState getSocketREST req respond = do
+  isMainAppReachable <- getSocketREST >>= maybe (pure False) (fmap isRight . reachMainApp)
   isLoaded <- AppState.isLoaded appState
   isPending <- AppState.isPending appState
 
@@ -44,8 +44,8 @@ admin appState socketREST req respond  = do
       respond $ Wai.responseLBS (if isMainAppReachable then HTTP.status200 else HTTP.status500) [] mempty
     ["ready"] ->
       let
-        status | not isMainAppReachable = HTTP.status500
-               | isPending              = HTTP.status503
+        status | isPending              = HTTP.status503
+               | not isMainAppReachable = HTTP.status500
                | isLoaded               = HTTP.status200
                | otherwise              = HTTP.status500
       in