Using withArgs concurrently segfaults

Summary

Calling withArgs simultaneously from multiple threads crashes the program.

Steps to reproduce

Compile the following program with -threaded and run it with +RTS -N.

import Control.Concurrent ( forkIO )
import Control.Monad ( replicateM_ )
import System.Environment ( withArgs )

main = replicateM_ 64 . forkIO . withArgs [] $ pure ()

I can reproduce this bug by spawning fewer than 64 threads, so that number doesn't seem to matter too much. Similarly I can reproduce the bug when running with -Nx for all x > 1. But as those numbers go down, the likelihood of reproducing this bug seems to go down as well.

At any rate, I see output like this:

double free or corruption (fasttop)
free(): double free detected in tcache 2
Aborted (core dumped)

Segmentation fault (core dumped)

double free or corruption (out)
Aborted (core dumped)

free(): double free detected in tcache 2
Segmentation fault (core dumped)

Expected behavior

I don't really know what multiple calls to withArgs from separate threads should do, but it probably shouldn't segfault 😆

Environment

• GHC version used: 8.10.1, 8.8.3, 8.6.5

Optional:

• Operating System: Ubuntu 20.04
• System Architecture: x86_64