Switch to MonadCatchIO-transformers for better handling of state while catching exceptions.

As seen in this bug report, the lifted exception handling using the StateT
monad throws away state changes when an action throws an exception.
http://git-annex.branchable.com/bugs/git_annex_fork_bombs_on_gpg_file/
  .. Which can result in cached values being redundantly calculated, or other
     possibly worse bugs when the annex state gets out of sync with reality.

This switches from a StateT AnnexState to a ReaderT (MVar AnnexState).
All changes to the state go via the MVar. So when an Annex action is
running inside an exception handler, and it makes some changes, they
immediately go into affect in the MVar. If it then throws an exception
(or even crashes its thread!), the state changes are still in effect.

The MonadCatchIO-transformers change is actually only incidental.
I could have kept on using lifted-base for the exception handling.
However, I'd have needed to write a new instance of MonadBaseControl
for the new monad.. and I didn't write the old instance.. I begged Bas
and he kindly sent it to me. Happily, MonadCatchIO-transformers is
able to derive a MonadCatchIO instance for my monad.

This is a deep level change. It passes the test suite! What could it break?

Well.. The most likely breakage would be to code that runs an Annex action
in an exception handler, and *wants* state changes to be thrown away.
Perhaps the state changes leaves the state inconsistent, or wrong. Since
there are relatively few places in git-annex that catch exceptions in the
Annex monad, and the AnnexState is generally just used to cache calculated
data, this is unlikely to be a problem.

Oh yeah, this change also makes Assistant.Types.ThreadedMonad a bit
redundant. It's now entirely possible to run concurrent Annex actions in
different threads, all sharing access to the same state! The ThreadedMonad
just adds some extra work on top of that, with its own MVar, and avoids
such actions possibly stepping on one-another's toes. I have not gotten
rid of it, but might try that later. Being able to run concurrent Annex
actions would simplify parts of the Assistant code.

1 files changed, 36 insertions, 25 deletions

diff --git a/Annex.hs b/Annex.hs
index f9cbfef0d..b0a67899f 100644
--- a/Annex.hs
+++ b/Annex.hs
@@ -1,6 +1,6 @@
 {- git-annex monad
  -
- - Copyright 2010-2012 Joey Hess <joey@kitenet.net>
+ - Copyright 2010-2013 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -16,7 +16,6 @@ module Annex (
 	newState,
 	run,
 	eval,
-	exec,
 	getState,
 	changeState,
 	setFlag,
@@ -35,10 +34,10 @@ module Annex (
 	withCurrentState,
 ) where
 
-import "mtl" Control.Monad.State.Strict
-import Control.Monad.Trans.Control (StM, MonadBaseControl, liftBaseWith, restoreM)
-import Control.Monad.Base (liftBase, MonadBase)
+import "mtl" Control.Monad.Reader
+import "MonadCatchIO-transformers" Control.Monad.CatchIO
 import System.Posix.Types (Fd)
+import Control.Concurrent
 
 import Common
 import qualified Git
@@ -56,32 +55,24 @@ import Types.TrustLevel
 import Types.Group
 import Types.Messages
 import Types.UUID
-import Utility.State
 import qualified Utility.Matcher
 import qualified Data.Map as M
 import qualified Data.Set as S
 
--- git-annex's monad
-newtype Annex a = Annex { runAnnex :: StateT AnnexState IO a }
+{- git-annex's monad is a ReaderT around an AnnexState stored in a MVar.
+ - This allows modifying the state in an exception-safe fashion.
+ - The MVar is not exposed outside this module.
+ -}
+newtype Annex a = Annex { runAnnex :: ReaderT (MVar AnnexState) IO a }
 	deriving (
 		Monad,
 		MonadIO,
-		MonadState AnnexState,
+		MonadReader (MVar AnnexState),
+		MonadCatchIO,
 		Functor,
 		Applicative
 	)
 
-instance MonadBase IO Annex where
-	liftBase = Annex . liftBase
-
-instance MonadBaseControl IO Annex where
-	newtype StM Annex a = StAnnex (StM (StateT AnnexState IO) a)
-	liftBaseWith f = Annex $ liftBaseWith $ \runInIO ->
-		f $ liftM StAnnex . runInIO . runAnnex
-	restoreM = Annex . restoreM . unStAnnex
-	  where
-		unStAnnex (StAnnex st) = st
-
 type Matcher a = Either [Utility.Matcher.Token a] (Utility.Matcher.Matcher a)
 
 data FileInfo = FileInfo
@@ -156,13 +147,32 @@ newState gitrepo = AnnexState
 new :: Git.Repo -> IO AnnexState
 new = newState <$$> Git.Config.read
 
-{- performs an action in the Annex monad -}
+{- Performs an action in the Annex monad from a starting state,
+ - returning a new state. -}
 run :: AnnexState -> Annex a -> IO (a, AnnexState)
-run s a = runStateT (runAnnex a) s
+run s a = do
+	mvar <- newMVar s
+	r <- runReaderT (runAnnex a) mvar
+	s' <- takeMVar mvar
+	return (r, s')
+
+{- Performs an action in the Annex monad from a starting state, 
+ - and throws away the new state. -}
 eval :: AnnexState -> Annex a -> IO a
-eval s a = evalStateT (runAnnex a) s
-exec :: AnnexState -> Annex a -> IO AnnexState
-exec s a = execStateT (runAnnex a) s
+eval s a = do
+	mvar <- newMVar s
+	runReaderT (runAnnex a) mvar
+
+getState :: (AnnexState -> v) -> Annex v
+getState selector = do
+	mvar <- ask
+	s <- liftIO $ readMVar mvar
+	return $ selector s
+
+changeState :: (AnnexState -> AnnexState) -> Annex ()
+changeState modifier = do
+	mvar <- ask
+	liftIO $ modifyMVar_ mvar $ return . modifier
 
 {- Sets a flag to True -}
 setFlag :: String -> Annex ()
@@ -204,6 +214,7 @@ inRepo a = liftIO . a =<< gitRepo
 fromRepo :: (Git.Repo -> a) -> Annex a
 fromRepo a = a <$> gitRepo
 
+{- Calculates a value from an annex's git repository and its GitConfig. -}
 calcRepo :: (Git.Repo -> GitConfig -> IO a) -> Annex a
 calcRepo a = do
 	s <- getState id