If I run this script on macOS, after about 15 minutes, a ghc-pkg process will deadlock.

#!/usr/bin/env bash

out=`pwd`/testing-package-db

rm -rf $out

cabal update
cabal install random

mkdir -p $out
ghc-pkg init $out/package.conf.d

while true; do
    ghc-pkg --package-db $HOME/.ghc/x86_64-darwin-8.6.3/package.conf.d dump | ghc-pkg --force --package-db $out/package.conf.d register -
done

Output from sw_vers:

ProductName:    Mac OS X
ProductVersion: 10.12.6
BuildVersion:   16G29

This GHC build comes from the latest nixpkgs-18.09. I have also attached a nix file to build test scripts.

We noticed that if we edited the ghc-pkg binary to link with the threaded RTS, then we could not reproduce the deadlock (after running the test script for 24 hours).

Attached is the output from /usr/bin/sample for the stuck process, and the otool -L ghc-pkg output.