fix: purge JWT cache asynchronously in a separate thread

Otherwise performance was reduced unnecessarily.

src/PostgREST/Auth.hs

@@ -44,8 +44,9 @@ import System.Clock            (TimeSpec (..))
 import System.IO.Unsafe        (unsafePerformIO)
 import System.TimeIt           (timeItT)
 
-import PostgREST.AppState (AppState, AuthResult (..), getConfig,
-                           getJwtCache, getTime)
+import PostgREST.AppState (AppState, AuthResult (..),
+                           JwtCacheState (..), getConfig,
+                           getJwtCacheState, getTime)
 import PostgREST.Config   (AppConfig (..), JSPath, JSPathExp (..))
 import PostgREST.Error    (Error (..))
 
@@ -131,7 +132,8 @@ middleware appState app req respond = do
 -- | Used to retrieve and insert JWT to JWT Cache
 getJWTFromCache :: AppState -> ByteString -> Int -> IO (Either Error AuthResult) -> UTCTime -> IO (Either Error AuthResult)
 getJWTFromCache appState token maxLifetime parseJwt utc = do
-  checkCache <- C.lookup (getJwtCache appState) token
+  let JwtCacheState{..} = getJwtCacheState appState
+  checkCache <- C.lookup jwtCache token
   authResult <- maybe parseJwt (pure . Right) checkCache
 
   case (authResult,checkCache) of
@@ -151,17 +153,17 @@ getJWTFromCache appState token maxLifetime parseJwt utc = do
 
       let timeSpec = getTimeSpec res maxLifetime utc
 
-      -- purge expired cache entries
-      C.purgeExpired jwtCache
-
       -- insert new cache entry
       C.insert' jwtCache timeSpec token res
 
+      -- Execute IO action to purge the cache
+      -- It is assumed this action returns immidiately
+      -- so that request processing is not blocked.
+      purgeCache
+
     _                    -> pure ()
 
   return authResult
-    where
-      jwtCache = getJwtCache appState
 
 -- Used to extract JWT exp claim and add to JWT Cache
 getTimeSpec :: AuthResult -> Int -> UTCTime -> Maybe TimeSpec