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|
{- Persistent sqlite database handles.
-
- Copyright 2015 Joey Hess <id@joeyh.name>
-
- Licensed under the GNU GPL version 3 or higher.
-}
{-# LANGUAGE BangPatterns #-}
module Database.Handle (
DbHandle,
initDb,
openDb,
queryDb,
closeDb,
Size,
queueDb,
flushQueueDb,
commitDb,
) where
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
import Control.Concurrent.Async
import Control.Exception (throwIO)
import qualified Data.Text as T
import Control.Monad.Trans.Resource (runResourceT)
import Control.Monad.Logger (runNoLoggingT)
import Data.List
{- A DbHandle is a reference to a worker thread that communicates with
- 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 ()
initDb f migration = do
let db = T.pack f
enableWAL db
runSqlite db migration
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
{- 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 -> TableName -> IO DbHandle
openDb db tablename = do
jobs <- newEmptyMVar
worker <- async (workerThread (T.pack db) tablename jobs)
q <- newMVar emptyDbQueue
return $ DbHandle worker jobs q
data Job
= QueryJob (SqlPersistM ())
| ChangeJob ((SqlPersistM () -> IO ()) -> IO ())
| CloseJob
type TableName = String
workerThread :: T.Text -> TableName -> MVar Job -> IO ()
workerThread db tablename jobs = catchNonAsync (run loop) showerr
where
showerr e = liftIO $ warningIO $
"sqlite worker thread crashed: " ++ show e
loop = do
job <- liftIO $ takeMVar jobs
case job of
QueryJob a -> a >> loop
-- 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
ChangeJob a -> liftIO (a run) >> loop
CloseJob -> return ()
-- like runSqlite, but calls settle on the raw sql Connection.
run a = do
conn <- Sqlite.open db
settle conn
runResourceT $ runNoLoggingT $
withSqlConn (wrapConnection conn) $
runSqlConn a
-- Work around a bug in sqlite: New database connections can
-- sometimes take a while to become usable; select statements will
-- fail with ErrorBusy for some time. So, loop until a select
-- succeeds; once one succeeds the connection will stay usable.
-- <http://thread.gmane.org/gmane.comp.db.sqlite.general/93116>
settle conn = do
r <- tryNonAsync $ do
stmt <- Sqlite.prepare conn nullselect
void $ Sqlite.step stmt
void $ Sqlite.finalize stmt
case r of
Right _ -> return ()
Left e -> do
if "ErrorBusy" `isInfixOf` show e
then do
threadDelay 1000 -- 1/1000th second
settle conn
else throwIO e
-- This should succeed for any table.
nullselect = T.pack $ "SELECT null from " ++ tablename ++ " limit 1"
{- Makes a query using the DbHandle. This should not be used to make
- changes to the database!
-
- Note that the action is not run by the calling thread, but by a
- worker thread. Exceptions are propigated to the calling thread.
-
- Only one action can be run at a time against a given DbHandle.
- If called concurrently in the same process, this will block until
- it is able to run.
-}
queryDb :: DbHandle -> SqlPersistM a -> IO a
queryDb (DbHandle _ jobs _) a = do
res <- newEmptyMVar
putMVar jobs $ QueryJob $
liftIO . putMVar res =<< tryNonAsync a
either throwIO return =<< takeMVar res
closeDb :: DbHandle -> IO ()
closeDb h@(DbHandle worker jobs _) = do
flushQueueDb h
putMVar jobs CloseJob
wait worker
type Size = Int
{- A queue of actions to perform, with a count of the number of actions
- queued. -}
data DbQueue = DbQueue Size (SqlPersistM ())
emptyDbQueue :: DbQueue
emptyDbQueue = DbQueue 0 (return ())
{- Queues a change to be made to the database. It will be buffered
- to be committed later, unless the queue gets larger than the specified
- size.
-
- (Be sure to call closeDb or flushQueueDb to ensure the change
- gets committed.)
-
- Transactions built up by queueDb are sent to sqlite all at once.
- If sqlite fails due to another change being made concurrently by another
- process, the transaction is put back in the queue. This solves
- the sqlite multiple writer problem.
-}
queueDb :: DbHandle -> Size -> SqlPersistM () -> IO ()
queueDb h@(DbHandle _ _ qvar) maxsz a = do
DbQueue sz qa <- takeMVar qvar
let !sz' = sz + 1
let qa' = qa >> a
let enqueue newsz = putMVar qvar (DbQueue newsz qa')
if sz' > maxsz
then do
r <- commitDb h qa'
case r of
Left _ -> enqueue 0
Right _ -> putMVar qvar emptyDbQueue
else enqueue sz'
{- If flushing the queue fails, this could be because there is another
- writer to the database. Retry repeatedly for up to 10 seconds. -}
flushQueueDb :: DbHandle -> IO ()
flushQueueDb h@(DbHandle _ _ qvar) = do
DbQueue sz qa <- takeMVar qvar
when (sz > 0) $
robustly Nothing 100 (commitDb h qa)
where
robustly :: Maybe SomeException -> Int -> IO (Either SomeException ()) -> IO ()
robustly e 0 _ = error $ "failed to commit changes to sqlite database: " ++ show e
robustly _ n a = do
r <- a
case r of
Right _ -> return ()
Left e -> do
threadDelay 100000 -- 1/10th second
robustly (Just e) (n-1) a
commitDb :: DbHandle -> SqlPersistM () -> IO (Either SomeException ())
commitDb (DbHandle _ jobs _) a = do
res <- newEmptyMVar
putMVar jobs $ ChangeJob $ \runner ->
liftIO $ putMVar res =<< tryNonAsync (runner a)
takeMVar res
|
