slave-thread-1.1: A fundamental solution to ghost threads and silent exceptions

Safe HaskellNone




Vanilla thread management in Haskell is low level and it does not approach the problems related to thread deaths. When it's used naively the following typical problems arise:

  • When a forked thread dies due to an uncaught exception, the exception does not get raised in the main thread, which is why the program continues to run as if nothing happened, i.e., with the presumption that the already dead thread is running normally. Naturally this may very well bring your program to a chaotic state.
  • Another issue is that one thread dying does not affect any of the threads forked from it. That's why your program may be accumulating ghost threads.
  • Ever dealt with your program ignoring the <Ctrl-C> strikes?

This library solves all the issues above with a concept of a slave thread. A slave thread has the following properties:

  1. When it dies for whatever reason (exception or finishing normally) it kills all the slave threads that were forked from it. This protects you from ghost threads.
  2. It waits for all slaves to die and execute their finalizers before executing its own finalizer and getting released itself. This gives you hierarchical releasing of resources.
  3. When a slave thread dies with an uncaught exception it reraises it in the master thread. This protects you from silent exceptions and lets you be sure of getting informed if your program gets brought to an erroneous state.


fork :: IO a -> IO ThreadId Source #

Fork a slave thread to run a computation on.

forkWithUnmask :: ((forall x. IO x -> IO x) -> IO a) -> IO ThreadId Source #

Like fork, but provides the computation a function that unmasks asynchronous exceptions. See Note [Unmask] at the bottom of this module.

forkFinally :: IO a -> IO b -> IO ThreadId Source #

Fork a slave thread with a finalizer action to run a computation on. The finalizer gets executed when the thread dies for whatever reason: due to being killed or an uncaught exception, or a normal termination.

Note the order of arguments:

forkFinally finalizer computation

forkFinallyWithUnmask :: IO a -> ((forall x. IO x -> IO x) -> IO b) -> IO ThreadId Source #

Like forkFinally, but provides the computation a function that unmasks asynchronous exceptions. See Note [Unmask] at the bottom of this module.

data SlaveThreadCrashed Source #

A slave thread crashed. This exception is classified as asynchronous, meaning it extends from SomeAsyncException.

In general,

  • Synchronous exceptions such as IOException are thrown by IO actions that are explicitly called by the thread that receives them, and may be caught, inspected, and handled by resuming execution.
  • Asynchronous exceptions such as ThreadKilled should normally only be caught temporarily in order to run finalizers, then re-thrown.

SlaveThreadCrashed being asynchronous means it should, by default, cause the entire thread hierarchy to come crashing down, ultimately terminating the program.

If you want more sophisticated behavior, such as a "supervisor" thread that monitors and restarts worker threads when they fail, you have to program that yourself.

N.B. Consider using a library like safe-exceptions or unliftio, which carefully distinguish synchronous and asynchronous exceptions, unlike base.



Threads forked by this library, unlike in base, already mask asynchronous exceptions internally, for bookkeeping purposes.

The *withUnmask variants of fork are thus different from the *withUnmask variants found in base and async, in that the unmasking function they provide restores the masking state to that of the calling context, as opposed to unmasked.

Put another way, the base code that you may have written as:

mask (\unmask -> forkIO (initialize >> unmask computation))

using this library would be instead written as:

forkWithUnmask (\unmask -> initialize >> unmask computation)

And the base code that you may have written as:

mask_ (forkIOWithUnmask (\unmask -> initialize >> unmask computation))

will instead have to manually call the low-level unmasking function called unsafeUnmask, as:

mask_ (forkWithUnmask (\_ -> initialize >> unsafeUnmask computation))

Note that we used forkWithUnmask (to guarantee initialize is run with asynchronous exceptions masked), but the unmasking function it provided does not guarantee asynchronous exceptions are actually unmasked, so we toss it and use unsafeUnmask instead.

This idiom is uncommon, but necessary when you need to fork a thread in library code that is unsure if it's being called with asynchronous exceptions masked (as in the "acquire" phase of a bracket call).