thread sanitizer (tsan) data races with .send()

[jaredh159]

i have some code in a reducer similar to this example from a recent episode:

case .loadResponse:
  state.isLoading = false
  return .send(.delegate(.goToCounter(state.count)))

where a reducer uses .send() to immediately feed an effect back into the system. i always develop with thread sanitizer enabled, and i started getting fairly consistent (maybe 80% of the time) access race warnings about multiple threads accessing the view stores send functionality.

when i changed the code to this:

case .loadResponse:
  state.isLoading = false
  return .run { send in
    await send(.delegate(.goToCounter(state.count)))
  }

then the tsan warnings went away.

now... my app is really complex with a bunch of hairy entitlements, and it's not open source, so it's difficult to show you guys a minimal repro. i'm willing to try to isolate it and reproduce it, but i thought before i went through the effort to do that i would ask a couple things:

1. is this a known issue at all?
2. do you guys test in CI or develop with TSAN enabled?
3. could it be a false positive?
4. is it more likely to be something about the surrounding code that i'm doing wrong?
5. would a stack trace or more info about the TSAN warning be helpful on it's own, independent of a full repro?

thanks so much!

EDIT: oh, forgot to mention, i'm running the very latest commit of the prerelease/1.0 branch, fwiw

[mbrandonw]

Hi @jaredh159, some answers to your questions:

is this a known issue at all?

It is not, as far as we know.

do you guys test in CI or develop with TSAN enabled?

We do sometimes, and haven't come across this issue. I also just turned it on for the demo we have been building in episodes and wasn't able to reproduce.

could it be a false positive?

I personally doubt that.

is it more likely to be something about the surrounding code that i'm doing wrong?

That is a possibility. The only thing that comes to mind is if you somehow have an effect that emits or completes on the non-main thread. This is really only possible when using Combine-based effects rather than async effects, so if you have any of those that would be where to look.

would a stack trace or more info about the TSAN warning be helpful on it's own, independent of a full repro?

That could help, or if you are able to reproduce the problem in any of our demo apps.

[jaredh159]

thanks for the reply @mbrandonw that's helpful. i don't have any combine-based effects in that area of the app, but i dug around a bit more, and will continue to investigate and possibly work on a full repro, or a repro with a demo app, if i can pull it off.