I have an application that forks a thread to run an activity on a timer. (The
activity happens to be Berkeley DB checkpointing, but that's actually beside
the point here.) The problem is that when the application wants to quit, I
would like my main thread to be able to tell the timer thread to shut down in a
timely way. However, I don't see a primitive in haskell that allows me to both
wait for a timeout, or a notification. (If I were to do this in java, I would
use wait/notify.)
Here's the code I wrote to attempt this shutdown procedure:
import Control.Exception
import Database.Berkeley.Db
import Data.IORef
import System.Posix.Unistd
...
closeEnv :: Env -> IO ()
closeEnv env = do
threadId <- readIORef (envCheckpointThread env)
case threadId of
Just tid -> killThread tid
Nothing -> return ()
dbEnv_close [] (envDbEnv env)
startCheckpointing :: Env -> IO ThreadId
startCheckpointing env = do
forkOS run
where run = catch checkpoint handler
checkpoint = checkpoint1 >> checkpoint
checkpoint1 = unblock $ do
let dbenv = envDbEnv env
_ <- sleep checkpointInterval
putStrLn "# checkpoint"
dbEnv_txn_checkpoint [] dbenv 0 0
handler ThreadKilled = return ()
handler exn = throw exn
However, there are several problems here: First, the checkpointInterval is 15
sec, so it takes at least 15 seconds for the thread to wake up and be
interrupted. Second, it isn't clear to me whether the killThread call should
interrupt the sleep or not. In practice, several checkpoints occur before the
ThreadKilled message is delivered, and this can take up to 2 minutes to shut
down.
Is there a better way to do this? Suggestions would be greatly appreciated,
Warren
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