George Russell wrote:
Posix.runProcess really should use it I think.
No, it's better to be able to choose the way to handle this. Maybe add a flag to Posix.runProcess. But the whole GHC.Conc.forkProcess isn't finished, yet, anyway. I don't really see the point of allowing the user to choose the old way. Posix.runProcess is supposed to fork off a new process outside of this runtime system. I don't see any way this can be helped by allowing other threads to continue in the child until the actual time of exec, unless for some reason the evaluation of the arguments to exec somehow relies on other threads. But this would surely at least require argument evaluation to unsafely conceal IO, and in any case could be better fixed by forcing the arguments to be fully evaluated before the fork.
Let's not forget that the new (child) process may never do exec(). I have an interpreter that forks to replicate itself, relying on the runtime system to carry over. My application uses multiple (GHC) threads as well as multiple (Unix) processes. I use an MVar to mutually exclude operations that are externally visible, so that in a new child process I can kill preexisting auxiliary threads. In my case, the cost in coding is modest; I'm not yet at the point where I can gauge the execution cost. The version of `forkProcess` that doesn't retain preexisting auxiliary threads is attractive, except that those threads are apparently not (currently) garbage collected properly in the child process. In my experience, I've not found a use for retaining auxiliary threads in the child process after a fork. On the other hand, I'm not yet convinced that couldn't be useful in some application. Dean