f4bdecc4ec
This removes a messy caveat that was easy to forget and caused at least one bug. The price paid is that, after a write to a MultiWriter db, it has to close the db connection that it had been using to read, and open a new connection. So it might be a little bit slower. But, writes are usually batched together, so there's often only a single write, and so there should not be much of a slowdown. Notice that SingleWriter already closed the db connection after a write, so paid the same overhead. This is the second try at fixing a bug: git-annex get when run as the first git-annex command in a new repo did not populate all unlocked files. (Reversion in version 8.20210621) Sponsored-by: Boyd Stephen Smith Jr. on Patreon
110 lines
3.1 KiB
Haskell
110 lines
3.1 KiB
Haskell
{- Persistent sqlite database queues
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-
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- Copyright 2015 Joey Hess <id@joeyh.name>
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-
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- Licensed under the GNU AGPL version 3 or higher.
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-}
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{-# LANGUAGE BangPatterns #-}
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module Database.Queue (
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DbQueue,
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DbConcurrency(..),
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openDbQueue,
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queryDbQueue,
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closeDbQueue,
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flushDbQueue,
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QueueSize,
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queueDb,
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) where
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import Utility.Monad
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import Utility.RawFilePath
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import Database.Handle
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import Database.Persist.Sqlite
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import Control.Concurrent
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import Data.Time.Clock
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import Control.Applicative
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import Prelude
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{- A DbQueue wraps a DbHandle, adding a queue of writes to perform.
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-
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- This is efficient when there are frequent writes, but
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- reads will not immediately have access to queued writes. -}
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data DbQueue = DQ DbHandle (MVar Queue)
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{- Opens the database queue, but does not perform any migrations. Only use
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- if the database is known to exist and have the right tables; ie after
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- running initDb. -}
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openDbQueue :: DbConcurrency -> RawFilePath -> TableName -> IO DbQueue
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openDbQueue dbconcurrency db tablename = DQ
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<$> openDb dbconcurrency db tablename
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<*> (newMVar =<< emptyQueue)
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{- This or flushDbQueue must be called, eg at program exit to ensure
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- queued changes get written to the database. -}
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closeDbQueue :: DbQueue -> IO ()
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closeDbQueue h@(DQ hdl _) = do
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flushDbQueue h
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closeDb hdl
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{- Blocks until all queued changes have been written to the database. -}
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flushDbQueue :: DbQueue -> IO ()
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flushDbQueue (DQ hdl qvar) = do
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q@(Queue sz _ qa) <- takeMVar qvar
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if sz > 0
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then do
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commitDb hdl qa
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putMVar qvar =<< emptyQueue
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else putMVar qvar q
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{- Makes a query using the DbQueue's database connection.
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- This should not be used to make changes to the database!
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-
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- Queries will not see changes that have been recently queued,
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- so use with care.
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-}
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queryDbQueue :: DbQueue -> SqlPersistM a -> IO a
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queryDbQueue (DQ hdl _) = queryDb hdl
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{- A queue of actions to perform, with a count of the number of actions
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- queued, and a last commit time. -}
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data Queue = Queue QueueSize LastCommitTime (SqlPersistM ())
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type QueueSize = Int
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type LastCommitTime = UTCTime
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emptyQueue :: IO Queue
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emptyQueue = do
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now <- getCurrentTime
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return $ Queue 0 now (return ())
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{- Queues a change to be made to the database. It will be queued
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- to be committed later, unless the commitchecker action returns true,
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- in which case any previously queued changes are also committed.
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-
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- Transactions built up by queueDb are sent to sqlite all at once.
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- If sqlite fails due to another change being made concurrently by another
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- process, the transaction is put back in the queue. This avoids
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- the sqlite multiple writer problem.
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-}
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queueDb
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:: DbQueue
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-> (QueueSize -> LastCommitTime -> IO Bool)
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-> SqlPersistM ()
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-> IO ()
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queueDb (DQ hdl qvar) commitchecker a = do
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Queue sz lastcommittime qa <- takeMVar qvar
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let !sz' = sz + 1
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let qa' = qa >> a
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let enqueue = putMVar qvar
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ifM (commitchecker sz' lastcommittime)
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( do
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r <- commitDb' hdl qa'
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case r of
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Left _ -> enqueue $ Queue sz' lastcommittime qa'
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Right _ -> enqueue =<< emptyQueue
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, enqueue $ Queue sz' lastcommittime qa'
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)
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