Support being built with ghc 8.0.1 (MonadFail)

Tested on an older ghc by enabling MonadFailDesugaring globally.

In TransferQueue, the lack of a MonadFail for STM exposed what would
normally be a bug in the pattern matching, although in this case an
earlier check that the queue was not empty avoided a pattern match
failure.

Annex.hs

@@ -74,6 +74,7 @@ import "mtl" Control.Monad.Reader
 import Control.Concurrent
 import Control.Concurrent.Async
 import Control.Concurrent.STM
+import qualified Control.Monad.Fail as Fail
 import qualified Control.Concurrent.SSem as SSem
 import qualified Data.Map.Strict as M
 import qualified Data.Set as S
@@ -93,6 +94,7 @@ newtype Annex a = Annex { runAnnex :: ReaderT (MVar AnnexState) IO a }
 		MonadCatch,
 		MonadThrow,
 		MonadMask,
+		Fail.MonadFail,
 		Functor,
 		Applicative
 	)

Assistant/Monad.hs

@@ -27,6 +27,7 @@ module Assistant.Monad (
 
 import "mtl" Control.Monad.Reader
 import System.Log.Logger
+import qualified Control.Monad.Fail as Fail
 
 import Annex.Common
 import Assistant.Types.ThreadedMonad
@@ -49,6 +50,7 @@ newtype Assistant a = Assistant { mkAssistant :: ReaderT AssistantData IO a }
 		Monad,
 		MonadIO,
 		MonadReader AssistantData,
+		Fail.MonadFail,
 		Functor,
 		Applicative
 	)

Assistant/TransferQueue.hs

@@ -191,16 +191,17 @@ getNextTransfer acceptable = do
 		sz <- readTVar (queuesize q)
 		if sz < 1
 			then retry -- blocks until queuesize changes
-			else do
-				(r@(t,info):rest) <- readTList (queuelist q)
-				void $ modifyTVar' (queuesize q) pred
-				setTList (queuelist q) rest
-				if acceptable info
-					then do
-						adjustTransfersSTM dstatus $
-							M.insert t info
-						return $ Just r
-					else return Nothing
+			else readTList (queuelist q) >>= \case
+				[] -> retry -- blocks until something is queued
+				(r@(t,info):rest) -> do
+					void $ modifyTVar' (queuesize q) pred
+					setTList (queuelist q) rest
+					if acceptable info
+						then do
+							adjustTransfersSTM dstatus $
+								M.insert t info
+							return $ Just r
+						else return Nothing
 
 {- Moves transfers matching a condition from the queue, to the
  - currentTransfers map. -}

CHANGELOG

@@ -16,6 +16,7 @@ git-annex (7.20181212) UNRELEASED; urgency=medium
     and improved parsing and serialization of git-annex branch data.
   * The benchmark command, which only had some old benchmarking of the sqlite
     databases before, now allows benchmarking any other git-annex commands.
+  * Support being built with ghc 8.0.1 (MonadFail).
 
  -- Joey Hess <id@joeyh.name>  Tue, 18 Dec 2018 12:24:52 -0400