SOMETIMES readProcessStderr returns an empty string ignoring the output of the child

[igrep]

UPDATE:

• Modify the command to reproduce to make sure typed-process is installed.
• Replace .\example.exe with ./example in repro.hs for easier testing on other OSs.
• Make example.hs and repro.hs smaller.

Minimum code to reproduce

I'm not sure this is really the smallest code, but these reproduce the error relatively more often.

example.hs:

main :: IO ()
main = do
  _ <- getLine
  fail "Invalid input!!"

repro.hs:

{-# LANGUAGE OverloadedStrings #-}
import Control.Monad
import Control.Monad.IO.Class
import qualified Data.ByteString.Lazy as BL
import qualified Data.ByteString as B
import System.Exit
import System.IO
import System.Process.Typed

main :: IO ()
main = do
  replicateM_ 300 $ do
    (CommandResult _ out) <- runCommand
    if (out == "")
      then fail "Reproduced!"
      else putStrLn "."

data CommandResult =
  CommandResult
    !ExitCode
    !B.ByteString
    deriving Show

runCommand :: IO CommandResult
runCommand = do
  let prc = setStdin (byteStringInput "\n")
        $ proc "./example" []
  (ecode, out) <- readProcessStderr prc
  return . CommandResult ecode $ BL.toStrict out

Steps to reproduce

stack --resolver=lts-16.1 exec ghc example.hs
stack --resolver=lts-16.1 runghc --package typed-process repro.hs

Expected result

stack --resolver=lts-16.1 runghc repro.hs should print only ., without any error.

Actual result

stack --resolver=lts-16.1 runghc repro.hs exits with an error after running the command several times.
For example:

> stack --resolver=lts-16.1 runghc repro.hs
.
.
.
.
.
repro.hs: user error (Reproduced!)

NOTE

Sometimes the error is not reproduced even after running 300 times.

My environment

Windows 10 Pro ver. 1809

[notogawa]

On MacOS 10.15.4, repro.hs (replace example.exe to ./example) generates expected result.

[igrep]

It wasn't reproduced also on my Debian 10 VM.
It actually seems a Windows specific bug.

[snoyberg]

Can you try compiling with the multithreaded runtime instead? And does this occur with readProcess?

[igrep]

Unfortunately, -threaded didn't solve... 😞

> stack --resolver=lts-16.1 ghc --package typed-process -- -threaded repro.hs
[1 of 1] Compiling Main             ( repro.hs, repro.o )
Linking repro.exe ...
> .\repro.exe
.
.
.
.
.
.
.
.
.
.
.
.
.
.
repro.exe: user error (Reproduced!)

[igrep]

And found readProcess also causes the same problem.

repro-readProcess.hs:

{-# LANGUAGE OverloadedStrings #-}
import Control.Monad
import qualified Data.ByteString.Lazy as BL
import qualified Data.ByteString as B
import System.Exit
import System.IO
import System.Process.Typed

main :: IO ()
main = do
  hSetBuffering stdout NoBuffering
  replicateM_ 300 $ do
    (CommandResult _ _out err) <- runCommand
    if err == ""
      then fail "Reproduced!"
      else putStrLn "."

data CommandResult =
  CommandResult
    !ExitCode
    !B.ByteString
    !B.ByteString
    deriving Show

runCommand :: IO CommandResult
runCommand = do
  let prc = setStdin (byteStringInput "bbbbb\nbbbbbbbbbddd\nccc\nkjkjk\ndafadfae\n\np\n")
        $ proc "./example" []
  (ecode, out, err) <- readProcess prc
  return $ CommandResult ecode (BL.toStrict out) (BL.toStrict err)

> stack --resolver=lts-16.1 ghc --package typed-process -- -threaded .\repro-readProcess.hs
[1 of 1] Compiling Main             ( repro-readProcess.hs, repro-readProcess.o )
Linking repro-readProcess.exe ...
> .\repro-readProcess.exe
.
.
.
.
.
.
.
.
.
.
.
repro-readProcess.exe: user error (Reproduced!)

[igrep]

One point more: Deleting all getLines from example.hs makes the problem stop happening (both in repro-readProcess.hs and repro.hs).

[snoyberg]

I don't really have any thoughts on what may be causing this, sorry.