slightly more efficient checking of versionUsesKeysDatabase

It's a mvar lookup either way, but I think this way will be slightly more
efficient. And it reduces the number of places where it's checked to 1.

Database/Keys.hs

@@ -54,12 +54,9 @@ import Database.Esqueleto hiding (Key)
  - Any queued writes will be flushed before the read.
  -}
 runReader :: Monoid v => (SQL.ReadHandle -> Annex v) -> Annex v
-runReader a = ifM versionUsesKeysDatabase
+runReader a = do
-	( do
+	h <- getDbHandle
-		h <- getDbHandle
+	withDbState h go
-		withDbState h go
-	, return mempty
-	)
   where
 	go DbUnavailable = return (mempty, DbUnavailable)
 	go st@(DbOpen qh) = do
@@ -81,7 +78,7 @@ runReaderIO a = runReader (liftIO . a)
  -
  - The database is created if it doesn't exist yet. -}
 runWriter :: (SQL.WriteHandle -> Annex ()) -> Annex ()
-runWriter a = whenM versionUsesKeysDatabase $ do
+runWriter a = do
 	h <- getDbHandle
 	withDbState h go
   where
@@ -105,7 +102,10 @@ getDbHandle = go =<< Annex.getState Annex.keysdbhandle
   where
 	go (Just h) = pure h
 	go Nothing = do
-		h <- liftIO newDbHandle
+		h <- ifM versionUsesKeysDatabase
+			( liftIO newDbHandle
+			, liftIO unavailableDbHandle
+			)
 		Annex.changeState $ \s -> s { Annex.keysdbhandle = Just h }
 		return h
 
@@ -150,8 +150,7 @@ openDb createdb _ = catchPermissionDenied permerr $ withExclusiveLock gitAnnexKe
  - data to it.
  -}
 closeDb :: Annex ()
-closeDb = whenM versionUsesKeysDatabase $
+closeDb = liftIO . closeDbHandle =<< getDbHandle
-	liftIO . closeDbHandle =<< getDbHandle
 
 addAssociatedFile :: Key -> TopFilePath -> Annex ()
 addAssociatedFile k f = runWriterIO $ SQL.addAssociatedFile (toIKey k) f

Database/Keys/Handle.hs

@@ -8,6 +8,7 @@
 module Database.Keys.Handle (
 	DbHandle,
 	newDbHandle,
+	unavailableDbHandle,
 	DbState(..),
 	withDbState,
 	flushDbQueue,
@@ -33,6 +34,9 @@ data DbState = DbClosed | DbOpen H.DbQueue | DbUnavailable
 newDbHandle :: IO DbHandle
 newDbHandle = DbHandle <$> newMVar DbClosed
 
+unavailableDbHandle :: IO DbHandle
+unavailableDbHandle = DbHandle <$> newMVar DbUnavailable
+
 -- Runs an action on the state of the handle, which can change its state.
 -- The MVar is empty while the action runs, which blocks other users
 -- of the handle from running.