waitForProcess throws a NoSuchThing exception if the process completes too quickly.

[ProofOfKeags]

I have observed that if you call waitForProcess and the process has already completed, an exception is raised. This can be rather surprising since if you launch a process and it doesn't take a lot of time you can be unable to check whether or not it completed successfully by looking at the exit code.

[snoyberg]

I've never seen this, despite having test cases that include very short lived processes like true. Can you provide a repo, including exact versions of libraries and GHC, as well as OS? Ideally a Stack script would lock everything down.

[ProofOfKeags]

It's possible that it isn't related to it being short lived, that part is speculation. The observed behavior is that if I launch several (4) processes using typed-process with conduit output streams via async actions, every so often one of them will fail on withProcessTerm or the variant withProcessTerm_. When I switch to withProcessWait, it never causes this particular issue.

When I drilled down, the exception being raised was that it was blocked indefinitely on an STM transaction and I figured out that this was due to waitExitCodeSTM blocking on a TMVar that would never be filled. The root cause of this is because waitForProcess raises a NoSuchThing variant of IOError and this would cascade to the ExitCode not being retrieved and consequently not fed to the TMVar. I was able to confirm this when I used the non-exit code checking variant. In this case the exception changed from STM blocking to waitForProcess "No child processes", which is called from within the various withProcessTerm calls.

I filed this bug here because I think the underlying issue is that waitForProcess seems like it should always succeed in this context, but it is possible the bug is in typed-process instead.

I will put together a small repo that can still produce this error today.

[bgamari]

Any further updates on this, @ProofOfKeags ?

[ProofOfKeags]

I completely forgot about this. No updates as of right now. Not sure when I'll be able to get to it.