fix bug that prevented db being written to disk in SingleWriter mode

The bug occurred when closeDb was not called, and garbage collection of
the DbHandle didn't give the workerThread time to shut down. Fixed by
exiting the runSqlite action when a commit is made.

(MultiWriter mode already forked off a runSqlite action, so avoided the
problem.)

This commit was sponsored by Brock Spratlen on Patreon.

Database/Handle.hs

@@ -142,11 +142,15 @@ data Job
 	| CloseJob
 
 workerThread :: T.Text -> TableName -> MVar Job -> IO ()
-workerThread db tablename jobs =
+workerThread db tablename jobs = go
-	catchNonAsync (runSqliteRobustly tablename db loop) showerr
   where
-  	showerr e = hPutStrLn stderr $
+	go = do
+		v <- tryNonAsync (runSqliteRobustly tablename db loop)
+		case v of
+			Left e -> hPutStrLn stderr $
 				"sqlite worker thread crashed: " ++ show e
+			Right True -> go
+			Right False -> return ()
 	
 	getjob :: IO (Either BlockedIndefinitelyOnMVar Job)
 	getjob = try $ takeMVar jobs
@@ -157,15 +161,21 @@ workerThread db tablename jobs =
 			-- Exception is thrown when the MVar is garbage
 			-- collected, which means the whole DbHandle
 			-- is not used any longer. Shutdown cleanly.
-			Left BlockedIndefinitelyOnMVar -> return ()
+			Left BlockedIndefinitelyOnMVar -> return False
-			Right CloseJob -> return ()
+			Right CloseJob -> return False
 			Right (QueryJob a) -> a >> loop
-			Right (ChangeJob a) -> a >> loop
+			Right (ChangeJob a) -> do
+				a
+				-- Exit this sqlite transaction so the
+				-- database gets updated on disk.
+				return True
 			-- Change is run in a separate database connection
 			-- since sqlite only supports a single writer at a
 			-- time, and it may crash the database connection
 			-- that the write is made to.
-			Right (RobustChangeJob a) -> liftIO (a (runSqliteRobustly tablename db)) >> loop
+			Right (RobustChangeJob a) -> do
+				liftIO (a (runSqliteRobustly tablename db))
+				loop
 	
 -- like runSqlite, but calls settle on the raw sql Connection.
 runSqliteRobustly :: TableName -> T.Text -> (SqlPersistM a) -> IO a

doc/todo/export.mdwn

@@ -17,9 +17,6 @@ there need to be a new interface in supported remotes?
 
 Work is in progress. Todo list:
 
-* bug: export db update does not reash disk after Remote.Helper.Export calls
-  updateExportTree. 
-
 * tracking exports
 
 * Support configuring export in the assistant