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
|
{- 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,
queryDb,
closeDb,
Size,
queueDb,
flushQueueDb,
commitDb,
) 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)
openDb :: FilePath -> IO DbHandle
openDb db = do
jobs <- newEmptyMVar
worker <- async (workerThread (T.pack db) jobs)
q <- newMVar emptyDbQueue
return $ DbHandle worker jobs q
data Job
= QueryJob (SqlPersistM ())
| ChangeJob ((SqlPersistM () -> IO ()) -> IO ())
| CloseJob
workerThread :: T.Text -> MVar Job -> IO ()
workerThread db jobs = catchNonAsync loop showerr
where
showerr e = liftIO $ warningIO $
"sqlite worker thread crashed: " ++ show e
run = runSqlite db
loop = do
r <- run queryloop
case r of
QueryJob _ -> 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 -> a run >> loop
CloseJob -> return ()
queryloop = do
job <- liftIO $ takeMVar jobs
case job of
QueryJob a -> a >> queryloop
_ -> return job
{- 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 e -> do
print ("commit deferred", e)
enqueue 0
Right _ -> do
print "commit made"
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
|
