assistant: Fix race in v6 mode that caused downloaded file content to sometimes not replace pointer files.

The keys database handle needs to be closed after merging, because the
smudge filter, in another process, updates the database. Old cached info
can be read for a while from the open database handle; closing it ensures
that the info written by the smudge filter is available.

This is pretty horribly ad-hoc, and it's especially nasty that the
transferrer closes the database every time.

Database/Keys.hs

@@ -9,6 +9,7 @@
 
 module Database.Keys (
 	DbHandle,
+	closeDb,
 	addAssociatedFile,
 	getAssociatedFiles,
 	getAssociatedKey,
@@ -137,6 +138,16 @@ openDb createdb _ = catchPermissionDenied permerr $ withExclusiveLock gitAnnexKe
 		False -> return DbUnavailable
 		True -> throwM e
 
+{- Closes the database if it was open. Any writes will be flushed to it.
+ -
+ - This does not normally need to be called; the database will auto-close
+ - when the handle is garbage collected. However, this can be used to
+ - force a re-read of the database, in case another process has written
+ - data to it.
+ -}
+closeDb :: Annex ()
+closeDb = liftIO . closeDbHandle =<< getDbHandle
+
 addAssociatedFile :: Key -> TopFilePath -> Annex ()
 addAssociatedFile k f = runWriterIO $ SQL.addAssociatedFile (toIKey k) f
 

Database/Keys/Handle.hs

@@ -11,6 +11,7 @@ module Database.Keys.Handle (
 	DbState(..),
 	withDbState,
 	flushDbQueue,
+	closeDbHandle,
 ) where
 
 import qualified Database.Queue as H
@@ -38,8 +39,7 @@ newDbHandle = DbHandle <$> newMVar DbClosed
 withDbState
 	:: (MonadIO m, MonadCatch m)
 	=> DbHandle
-	-> (DbState
-	-> m (v, DbState))
+	-> (DbState -> m (v, DbState))
 	-> m v
 withDbState (DbHandle mvar) a = do
 	st <- liftIO $ takeMVar mvar
@@ -55,3 +55,11 @@ flushDbQueue (DbHandle mvar) = go =<< readMVar mvar
   where
 	go (DbOpen qh) = H.flushDbQueue qh
 	go _ = return ()
+
+closeDbHandle :: DbHandle -> IO ()
+closeDbHandle h = withDbState h go
+  where
+	go (DbOpen qh) = do
+		H.closeDbQueue qh
+		return ((), DbClosed)
+	go st = return ((), st)