[Haskell-cafe] timely shutdown of timer threads

Warren Harris warrensomebody at gmail.com
Tue Feb 1 20:58:20 CET 2011

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,


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