I wrote:
Synchronising with signal handlers should be done using ordinary thread synchronisation. Have one big signal lock if you like, or one per signal, or one per handler - it's up to you.
It's true that you can't synchronise with other people's signal handlers this way. But I think that's a feature, or at least not a bug. Even using SIG_IGN or sigprocmask() you couldn't stop a handler that had already been started anyway.
It just occurred to me that it would be relatively easy to add back support for blocking signals in a way that makes sense. We could add one MVar per signal, and have every signal handler do something like withMVar sigINT_lock $ \_ -> do ... (implemented under the hood in the API). That would let us provide a way to temporarily disable handlers for particular signals. It would have the effect of serialising the handlers for each signal too, I'm not sure if that's a good thing. We could provide a back door too, perhaps. Cheers, Simon