refactor: change stateSchemaCache to TMVar

This commit changes implementation of stateSchemaCache from IORef to TMVar, which introduces a schema cache lifecycle:
not initialized --> initialized (possibly empty). This in turn will allow implementing waiting for first schema cache load attempt at startup.

Author: mkleczek

src/PostgREST/AppState.hs

@@ -44,14 +44,16 @@ import           PostgREST.Observation
 import           PostgREST.TimeIt           (timeItT)
 import           PostgREST.Version          (prettyVersion)
 
-import Control.AutoUpdate (defaultUpdateSettings, mkAutoUpdate,
-                           updateAction)
-import Control.Retry      (RetryPolicy, RetryStatus (..), capDelay,
-                           exponentialBackoff, retrying,
-                           rsPreviousDelay)
-import Data.IORef         (IORef, atomicWriteIORef, newIORef,
-                           readIORef)
-import Data.Time.Clock    (UTCTime, getCurrentTime)
+import Control.AutoUpdate     (defaultUpdateSettings, mkAutoUpdate,
+                               updateAction)
+import Control.Concurrent.STM (TMVar, newEmptyTMVarIO, putTMVar,
+                               tryReadTMVar, tryTakeTMVar)
+import Control.Retry          (RetryPolicy, RetryStatus (..),
+                               capDelay, exponentialBackoff, retrying,
+                               rsPreviousDelay)
+import Data.IORef             (IORef, atomicWriteIORef, newIORef,
+                               readIORef)
+import Data.Time.Clock        (UTCTime, getCurrentTime)
 
 import PostgREST.Auth.JwtCache           (JwtCacheState, update)
 import PostgREST.Config                  (AppConfig (..),
@@ -76,7 +78,7 @@ data AppState = AppState
   -- | Database server version
   , statePgVersion         :: IORef PgVersion
   -- | Schema cache
-  , stateSchemaCache       :: IORef (Maybe SchemaCache)
+  , stateSchemaCache       :: TMVar (Maybe SchemaCache)
   -- | The schema cache status
   , stateSCacheStatus      :: SchemaCacheStatus
   -- | State of the LISTEN channel
@@ -123,7 +125,7 @@ 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
+    <*> newEmptyTMVarIO
     <*> newSchemaCacheStatus
     <*> newIORef False
     <*> makeDebouncer (retryingSchemaCacheLoad appState *> threadDelay 100000)  -- 100ms cooldown
@@ -237,10 +239,11 @@ putPgVersion :: AppState -> PgVersion -> IO ()
 putPgVersion = atomicWriteIORef . statePgVersion
 
 getSchemaCache :: AppState -> IO (Maybe SchemaCache)
-getSchemaCache = readIORef . stateSchemaCache
+getSchemaCache = pure . join <=< atomically . tryReadTMVar . stateSchemaCache
 
 putSchemaCache :: AppState -> Maybe SchemaCache -> IO ()
-putSchemaCache appState = atomicWriteIORef (stateSchemaCache appState)
+putSchemaCache AppState{stateSchemaCache} sc = atomically $
+  tryTakeTMVar stateSchemaCache *> putTMVar stateSchemaCache sc
 
 schemaCacheLoader :: AppState -> IO ()
 schemaCacheLoader = debouncedSCacheLoader