fix: forbid same server-port and admin-server-port

Forbids server-port and admin-server-port from being equal altogether,
despite they might not conflict at all in case admin and app are bound
to different addresses. Implemented as per the discussion at
#3508 (comment)

CHANGELOG.md

@@ -42,6 +42,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
  - #3414, Force listener to connect to read-write instances using `target_session_attrs` - @steve-chavez
  - #3255, Fix incorrect `413 Request Entity Too Large` on pg errors `54*` - @taimoorzaeem
  - #3549, Remove verbosity from error logs starting with "An error occurred..." and replacing it with "Failed to..." - @laurenceisla
+ - #3508, Server port and admin server port can end up with the same value - @develop7
+    + The app is fails to start when the server port and admin server port are the same
 
 ### Deprecated
 

src/PostgREST/Config.hs

@@ -223,12 +223,21 @@ readAppConfig dbSettings optPath prevDbUri roleSettings roleIsolationLvl = do
     Left err ->
       return . Left $ "Error in config " <> err
     Right parsedConfig ->
-      Right <$> decodeLoadFiles parsedConfig
+      case processForbiddenValues parsedConfig of
+        Left err' ->
+          return . Left $ "Error in config " <> err'
+        Right config ->
+          Right <$> decodeLoadFiles config
   where
     -- Both C.ParseError and IOError are shown here
     loadConfig :: FilePath -> IO (Either SomeException C.Config)
     loadConfig = try . C.load
 
+    processForbiddenValues :: AppConfig -> Either Text AppConfig
+    processForbiddenValues cfg@AppConfig{..}
+      | configAdminServerPort == Just configServerPort = Left "admin-server-port cannot be the same as server-port"
+      | otherwise = Right cfg
+
     decodeLoadFiles :: AppConfig -> IO AppConfig
     decodeLoadFiles parsedConfig =
       decodeJWKS <$>