better approach to enabling WAL mode

The old approach opened the database an extra time to enable WAL mode,
but more recent persistent-sqlite has a better API to enable it.

Database/Handle.hs

@@ -1,6 +1,6 @@
 {- Persistent sqlite database handles.
  -
- - Copyright 2015 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}

Database/Init.hs

@@ -1,6 +1,6 @@
 {- Persistent sqlite database initialization
  -
- - Copyright 2015-2017 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -10,18 +10,17 @@ module Database.Init where
 import Annex.Common
 import Annex.Perms
 import Utility.FileMode
+import Database.Handle
 
 import Database.Persist.Sqlite
 import qualified Database.Sqlite as Sqlite
 import Control.Monad.IO.Class (liftIO)
 import qualified Data.Text as T
+import Lens.Micro
 
 {- Ensures that the database is freshly initialized. Deletes any
  - existing database. Pass the migration action for the database.
  -
- - The database is initialized using WAL mode, to prevent readers
- - from blocking writers, and prevent a writer from blocking readers.
- -
  - The permissions of the database are set based on the
  - core.sharedRepository setting. Setting these permissions on the main db
  - file causes Sqlite to always use the same permissions for additional
@@ -34,9 +33,7 @@ initDb db migration = do
 	let tmpdb = tmpdbdir </> "db"
 	liftIO $ do
 		createDirectoryIfMissing True tmpdbdir
-		let tdb = T.pack tmpdb
-		enableWAL tdb
-		runSqlite tdb migration
+		runSqliteInfo (mkConnInfo tmpdb) migration
 	setAnnexDirPerm tmpdbdir
 	-- Work around sqlite bug that prevents it from honoring
 	-- less restrictive umasks.
@@ -46,10 +43,14 @@ initDb db migration = do
 		void $ tryIO $ removeDirectoryRecursive dbdir
 		rename tmpdbdir dbdir
 
-enableWAL :: T.Text -> IO ()
-enableWAL db = do
-	conn <- Sqlite.open db
-	stmt <- Sqlite.prepare conn (T.pack "PRAGMA journal_mode=WAL;")
-	void $ Sqlite.step stmt
-	void $ Sqlite.finalize stmt
-	Sqlite.close conn
+{- Make sure that the database uses WAL mode, to prevent readers
+ - from blocking writers, and prevent a writer from blocking readers.
+ -
+ - This is the default in persistent-sqlite currently, but force it on just
+ - in case. 
+ -
+ - Note that once WAL mode is enabled, it will persist whenever the
+ - database is opened. -}
+mkConnInfo :: FilePath -> SqliteConnectionInfo
+mkConnInfo db = over walEnabled (const True) $ 
+	mkSqliteConnectionInfo (T.pack db)

debian/control

@@ -50,6 +50,7 @@ Build-Depends:
 	libghc-persistent-template-dev,
 	libghc-persistent-sqlite-dev,
 	libghc-esqueleto-dev,
+	libghc-microlens-dev,
 	libghc-securemem-dev,
 	libghc-byteable-dev,
 	libghc-stm-chans-dev,

git-annex.cabal

@@ -336,9 +336,10 @@ Executable git-annex
    time,
    old-locale,
    esqueleto,
-   persistent-sqlite, 
+   persistent-sqlite (>= 2.1.3), 
    persistent,
    persistent-template,
+   microlens,
    aeson,
    vector,
    tagsoup,