Race condition when multiple threads wait for a process

Consider the following code:

import System.Process
import Control.Concurrent.Async

main :: IO ()
main = do
    (_, _, _, ph) <- createProcess $ shell "sleep 1"
    let helper i = do
            ec <- waitForProcess ph
            print (i :: Int, ec)
    ((), ()) <- concurrently (helper 1) (helper 2)
    return ()

If I compile with the single threaded runtime, I get the output

(2,ExitSuccess)
(1,ExitSuccess)

But when compiling with the multithreaded runtime, I get:

bin: waitForProcess: does not exist (No child processes)

If you need to wait for a process from multiple threads, you can do so now by having a dedicated wait thread which writes to an MVar or TMVar, but the current default behavior can be surprising.

I discussed this in a blog post, and there was some conversation on Reddit.

At the least, I think we need better documentation, but if there's a better solution, that would be best.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/process
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