What happens to child threads when their parent dies in GHC Haskell?

Question

The documentation for forkIO says

GHC note: the new thread inherits the masked state of the parent (see mask).

The newly created thread has an exception handler that discards the exceptions
BlockedIndefinitelyOnMVar, BlockedIndefinitelyOnSTM, and ThreadKilled, and passes
all other exceptions to the uncaught exception handler.

Why does the child exception handler discard ThreadKilled? Is there some connection between the two threads after they're created?

What exactly happens when the parent thread dies? Does the child get any exception raised? Or is there any way at all to see from the child's perspective that the parent died? Is there anything else that happens except that the parent thread just stops running?

The reason why I'm asking this is that in many cases I'm forced to create a thread in a context where I can't access the parent's scope. Just imagine you're somewhere deep in a library and need to call forkIO, and have that thread die when the parent dies. Is it necessary to restructure the program and propagate the child's ThreadId up to the parent and explicitly kill it? Or is there any other workaround for this?

Answer 1

What exactly happens when the parent thread dies?

Nothing. That's actually also true for POSIX threads. Threads don't share the parent-child relationship you might know from fork in C or similar languages. There is, however, one main thread, and its termination will usually lead to the termination of the whole program:

Note that the thread in which main() was originally invoked differs from this. When it returns from main(), the effect is as if there was an implicit call to exit() using the return value of main() as the exit status.

Does the child get any exception raised? Or is there any way at all to see from the child's perspective that the parent died? Is there anything else that happens except that the parent thread just stops running?

No. No. And no. For the same reason as with usual OS threads. You can try this pretty easily:

import Control.Concurrent (forkIO, threadDelay)

delaySeconds n = threadDelay $ n * (10^6)

main = do
  forkIO $ do
    forkIO $ delaySeconds 1 >> print "Pseudo child 1"
    forkIO $ delaySeconds 1 >> print "Pseudo child 2"
    print "Pseudo parent says goodbye"
  delaySeconds 10
  print "Exiting main"

The "parent" will say goodbye, and the "children" will print a second later. Remember, there is no actual parent in thread programming. There are only siblings. One of them is a little bit special, yes, but that is just how it's been specified.

Is it necessary to restructure the program and propagate the child's ThreadId up to the parent and explicitly kill it?

At least a little bit, since forkIO doesn't provide this. Also, if there was a forkIOKillAutomatically, what type should it have? And why?

Or is there any other workaround for this?

Well, you could provide the rest of your parent as another action, and therefore use a helper:

forkRunDie :: IO () -> IO () -> IO ()
forkRunDie p s = forkIO p >>= \tid -> s >> killThread tid

The above example would then become

main = do
  forkIO $ do
    forkRunDie (delaySeconds 1 >> print "Pseudo child 1") $ do
      forkRunDie (delaySeconds 1 >> print "Pseudo child 2") $ do
        print "Pseudo parent says goodbye"
  delaySeconds 10
  print "Exiting main"

In this case the only output will be

"Pseudo parent says goodbye"
"Exiting main"

See also:

• Smarter termination for thread racing by Conal Elliott (provides a very similar function to forkRunDie with finally).

Answer 2

In a standalone GHC program, only the main thread is required to terminate in order for the process to terminate. Thus all other forked threads will simply terminate at the same time as the main thread (the terminology for this kind of behaviour is "daemonic threads").

https://hackage.haskell.org/package/base-4.7.0.0/docs/Control-Concurrent.html#g:12