1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
|
{- 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,
DbConcurrency(..),
openDb,
TableName,
queryDb,
closeDb,
commitDb,
commitDb',
) where
import Utility.Exception
import Utility.FileSystemEncoding
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, BlockedIndefinitelyOnMVar(..))
import qualified Data.Text as T
import Control.Monad.Trans.Resource (runResourceT)
import Control.Monad.Logger (runNoLoggingT)
import Data.List
import System.IO
{- 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 DbConcurrency (Async ()) (MVar Job)
{- Name of a table that should exist once the database is initialized. -}
type TableName = String
{- Sqlite only allows a single write to a database at a time; a concurrent
- write will crash.
-
- While a DbHandle serializes concurrent writes from
- multiple threads. But, when a database can be written to by
- multiple processes concurrently, use MultiWriter to make writes
- to the database be done robustly.
-
- The downside of using MultiWriter is that after writing a change to the
- database, the a query using the same DbHandle will not immediately see
- the change! This is because the change is actually written using a
- separate database connection, and caching can prevent seeing the change.
- Also, consider that if multiple processes are writing to a database,
- you can't rely on seeing values you've just written anyway, as another
- process may change them.
-
- When a database can only be written to by a single process, use
- SingleWriter. Changes written to the database will always be immediately
- visible then.
-}
data DbConcurrency = SingleWriter | MultiWriter
{- Opens the database, but does not perform any migrations. Only use
- once the database is known to exist and have the right tables. -}
openDb :: DbConcurrency -> FilePath -> TableName -> IO DbHandle
openDb dbconcurrency db tablename = do
jobs <- newEmptyMVar
worker <- async (workerThread (T.pack db) tablename jobs)
-- work around https://github.com/yesodweb/persistent/issues/474
liftIO $ fileEncoding stderr
return $ DbHandle dbconcurrency worker jobs
{- This is optional; when the DbHandle gets garbage collected it will
- auto-close. -}
closeDb :: DbHandle -> IO ()
closeDb (DbHandle _ worker jobs) = do
putMVar jobs CloseJob
wait worker
{- 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.
-
- Note that when the DbHandle was opened in MultiWriter mode, recent
- writes may not be seen by queryDb.
-}
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)
`catchNonAsync` (const $ error "sqlite query crashed")
{- Writes a change to the database.
-
- In MultiWriter mode, catches failure to write to the database,
- and retries repeatedly for up to 10 seconds, which should avoid
- all but the most exceptional problems.
-}
commitDb :: DbHandle -> SqlPersistM () -> IO ()
commitDb h wa = robustly Nothing 100 (commitDb' h wa)
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 MultiWriter _ jobs) a = do
res <- newEmptyMVar
putMVar jobs $ RobustChangeJob $ \runner ->
liftIO $ putMVar res =<< tryNonAsync (runner a)
takeMVar res
commitDb' (DbHandle SingleWriter _ jobs) a = do
res <- newEmptyMVar
putMVar jobs $ ChangeJob $
liftIO . putMVar res =<< tryNonAsync a
takeMVar res
`catchNonAsync` (const $ error "sqlite commit crashed")
data Job
= QueryJob (SqlPersistM ())
| ChangeJob (SqlPersistM ())
| RobustChangeJob ((SqlPersistM () -> IO ()) -> IO ())
| CloseJob
workerThread :: T.Text -> TableName -> MVar Job -> IO ()
workerThread db tablename jobs = go
where
go = do
v <- tryNonAsync (runSqliteRobustly tablename db loop)
case v of
Left e -> hPutStrLn stderr $
"sqlite worker thread crashed: " ++ show e
Right True -> go
Right False -> return ()
getjob :: IO (Either BlockedIndefinitelyOnMVar Job)
getjob = try $ takeMVar jobs
loop = do
job <- liftIO getjob
case job of
-- Exception is thrown when the MVar is garbage
-- collected, which means the whole DbHandle
-- is not used any longer. Shutdown cleanly.
Left BlockedIndefinitelyOnMVar -> return False
Right CloseJob -> return False
Right (QueryJob a) -> a >> loop
Right (ChangeJob a) -> do
a
-- Exit this sqlite transaction so the
-- database gets updated on disk.
return True
-- 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
-- that the write is made to.
Right (RobustChangeJob a) -> do
liftIO (a (runSqliteRobustly tablename db))
loop
-- like runSqlite, but calls settle on the raw sql Connection.
runSqliteRobustly :: TableName -> T.Text -> (SqlPersistM a) -> IO a
runSqliteRobustly tablename db a = do
conn <- Sqlite.open db
settle conn
runResourceT $ runNoLoggingT $
withSqlConn (wrapConnection conn) $
runSqlConn a
where
-- 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"
|
