feature request: tear-down of child threads

[mf59816]

Hi everybody,

I just figured out how to send ThreadKilled signals to all child threads. The use case is that we have lots of parallel threads that work on a database, and if they are killed they want to record that fact to the database before being destroyed. Since the child threads don't get notified if the main thread receives a user interrupt, I did something like this:

worker :: ThreadId -> MVar (Set ThreadId) -> Int -> IO ()
worker mainTid globalState _ =
  do
    workerThreadId <- myThreadId
    modifyMVar_ globalState (\ s -> return $ S.insert workerThreadId s)
    finally
      (sequence_ $ replicate 7 (print workerThreadId >> threadDelay (1 * 10^6)))
      (print $ "done: " ++ show workerThreadId)

main = do
  mainTid <- myThreadId
  globalState :: MVar (Set ThreadId) <- newMVar S.empty
  catch
    (do
       exceptions <- parallelE_ (L.map (worker mainTid globalState) [1..3])
       print exceptions)
    (\ (e :: SomeException) -> do
       workerThreadIds <- S.toList <$> readMVar globalState
       mapM_ (\ tid -> throwTo tid ThreadKilled) workerThreadIds
       print e)
  stopGlobalPool

Without this, if the main thread dies, the child threads are just silenlty destroyed without having a chance to do anything about it (write the fact to a log file, roll back a transaction, ...). I suspect that many people have been forced to figure this out and implement it by hand, and it would be cool if it could be stashed away in parallel-io under the hood.

Questions: (a) would a patch that does this implicitly be welcome? (b) would it be done like in the code I just hacked together, or would you like to see it done differently? (c) is there any concievable reason anybody would want to switch this feature off?