add CredPair cache

Note that this does not yet use SecureMem. It would probably make sense for
the Password part of a CredPair to use SecureMem, and making that change
is better than passing in a String and having it converted to SecureMem in
this code.

Assistant/CredPairCache.hs

@@ -0,0 +1,53 @@
+{- git-annex assistant CredPair cache.
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# LANGUAGE BangPatterns #-}
+
+module Assistant.CredPairCache (
+	cacheCred,
+	getCachedCred,
+	expireCachedCred,
+) where
+
+import Assistant.Types.CredPairCache
+import Types.Creds
+import Assistant.Common
+import Utility.ThreadScheduler
+
+import qualified Data.Map as M
+import Control.Concurrent
+
+{- Caches a CredPair, but only for a limited time, after which it
+ - will expire.
+ -
+ - Note that repeatedly caching the same CredPair
+ - does not reset its expiry time.
+ -}
+cacheCred :: CredPair -> Seconds -> Assistant ()
+cacheCred (login, password) expireafter = do
+	cache <- getAssistant credPairCache
+	liftIO $ do
+		changeStrict cache $ M.insert login password
+		void $ forkIO $ do
+			threadDelaySeconds expireafter
+			changeStrict cache $ M.delete login
+
+getCachedCred :: Login -> Assistant (Maybe Password)
+getCachedCred login = do
+	cache <- getAssistant credPairCache
+	liftIO $ M.lookup login <$> readMVar cache
+
+expireCachedCred :: Login -> Assistant ()
+expireCachedCred login = do
+	cache <- getAssistant credPairCache
+	liftIO $ changeStrict cache $ M.delete login
+
+{- Update map strictly to avoid keeping references to old creds in memory. -}
+changeStrict :: CredPairCache -> (M.Map Login Password -> M.Map Login Password) -> IO ()
+changeStrict cache a = modifyMVar_ cache $ \m -> do
+	let !m' = a m
+	return m'