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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,
openDb,
runDb,
commitDb,
closeDb,
Size,
queueDb,
flushQueueDb,
) where
import Utility.Exception
import Messages
import Database.Persist.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
{- 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)
data Job = RunJob (SqlPersistM ()) | CommitJob | CloseJob
openDb :: FilePath -> IO DbHandle
openDb db = do
jobs <- newEmptyMVar
worker <- async (workerThread (T.pack db) jobs)
q <- newMVar emptyDbQueue
return $ DbHandle worker jobs q
workerThread :: T.Text -> MVar Job -> IO ()
workerThread db jobs = catchNonAsync go showerr
where
showerr e = liftIO $ warningIO $ "sqlite worker thread crashed: " ++ show e
go = do
r <- runSqlite db transaction
case r of
CloseJob -> return ()
_ -> go
transaction = do
job <- liftIO $ takeMVar jobs
case job of
RunJob a -> a >> transaction
CommitJob -> return CommitJob
CloseJob -> return CloseJob
{- Runs an action using the DbHandle. The action may be a query, or it may
- make a change. Changes are bundled up in a transaction, which does not
- complete until commitDb is called.
-
- 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.
-
- Note that if multiple processes are trying to change the database
- at the same time, sqlite will only let one build a transaction at a
- time.
-}
runDb :: DbHandle -> SqlPersistM a -> IO a
runDb (DbHandle _ jobs _) a = do
res <- newEmptyMVar
putMVar jobs $ RunJob $
liftIO . putMVar res =<< tryNonAsync a
either throwIO return =<< takeMVar res
{- Commits any transaction that was created by the previous calls to runDb,
- and starts a new transaction. -}
commitDb :: DbHandle -> IO ()
commitDb (DbHandle _ jobs _) = putMVar jobs CommitJob
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 committed 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 flushQueue 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 = putMVar qvar (DbQueue sz' qa')
if sz' > maxsz
then do
r <- tryNonAsync $ do
runDb h qa'
commitDb h
case r of
Left _ -> enqueue
Right _ -> putMVar qvar emptyDbQueue
else enqueue
{- 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 100 $ runDb h qa
where
robustly :: Int -> IO () -> IO ()
robustly 0 _ = error "failed to commit changes to sqlite database"
robustly n a = catchNonAsync a $ \_ -> do
threadDelay 100000 -- 1/10th second
robustly (n-1) a
|
