use WAL mode to ensure read from db always works, even when it's being written to

Also, moved the database to a subdir, as there are multiple files.

This seems to work well with concurrent fscks, although they still do
redundant work due to the commit granularity. Occasionally two writes will
conflict, and one is then deferred and happens later.

Except, with 3 concurrent fscks, I got failures:

git-annex: user error (SQLite3 returned ErrorBusy while attempting to perform prepare "SELECT \"fscked\".\"key\"\nFROM \"fscked\"\nWHERE \"fscked\".\"key\" = ?\n": database is locked)

Argh!!!

Database/Handle.hs

@@ -9,6 +9,7 @@
 
 module Database.Handle (
 	DbHandle,
+	initDb,
 	openDb,
 	queryDb,
 	closeDb,
@@ -22,6 +23,7 @@ import Utility.Exception
 import Messages
 
 import Database.Persist.Sqlite
+import qualified Database.Sqlite as Sqlite
 import Control.Monad
 import Control.Monad.IO.Class (liftIO)
 import Control.Concurrent
@@ -33,6 +35,29 @@ import qualified Data.Text as T
  - the database. It has a MVar which Jobs are submitted to. -}
 data DbHandle = DbHandle (Async ()) (MVar Job) (MVar DbQueue)
 
+{- Ensures that the database is initialized. Pass the migration action for
+ - the database.
+ -
+ - The database is put into WAL mode, to prevent readers from blocking
+ - writers, and prevent a writer from blocking readers.
+ -}
+initDb :: FilePath -> SqlPersistM () -> IO DbHandle
+initDb db migration = do
+	enableWAL db
+	h <- openDb db
+	either throwIO (const $ return ()) =<< commitDb h migration
+	return h
+	
+enableWAL :: FilePath -> IO ()
+enableWAL db = do
+	conn <- Sqlite.open (T.pack db)
+	stmt <- Sqlite.prepare conn (T.pack "PRAGMA journal_mode=WAL;")
+	void $ Sqlite.step stmt
+	void $ Sqlite.finalize stmt
+	Sqlite.close conn
+
+{- Opens the database, but does not perform any migrations. Only use
+ - if the database is known to exist and have the right tables. -}
 openDb :: FilePath -> IO DbHandle
 openDb db = do
 	jobs <- newEmptyMVar
@@ -120,12 +145,8 @@ queueDb h@(DbHandle _ _ qvar) maxsz a = do
 		then do
 			r <- commitDb h qa'
 			case r of
-				Left e -> do
-					print ("commit deferred", e)
-					enqueue 0
-				Right _ -> do
-					print "commit made"
-					putMVar qvar emptyDbQueue
+				Left e -> enqueue 0
+				Right _ -> putMVar qvar emptyDbQueue
 		else enqueue sz'
 
 {- If flushing the queue fails, this could be because there is another