A helper function to `nix develop` on a package

dbg-nix-pkg.sh

# https://nixos.wiki/wiki/Nixpkgs/Create_and_debug_packages#Using_nix-shell_for_package_development
function debug_nix_package () {
        set -u
        packageName=${1:-default}
        flakeDir=${2:-$(pwd)}
        tmpdir=${3:-$(mktemp --directory --suffix=-nix-dbg)}
        system=${4:-$(uname --machine)-$(uname --kernel-name | tr '[:upper:]' '[:lower:]')}

        cd "$tmpdir" || exit
        ln -s "$flakeDir" flakeDir
        echo "$system" >system
        echo "$packageName" >packageName
        echo \
"try
        source ./setup.sh
        genericBuild >out 2>err
or
        source ./setup.sh
        runPhase unpackPhase
"

        cat - > "$tmpdir/setup.sh" <<EOF
eval "\$(typeset -f genericBuild | grep 'phases=')"
echo "phases=${phases[@]}"

set -x
set +e
export NIX_ENFORCE_PURITY=0
# squelch direnv
unset PROMPT_COMMAND
EOF
        nix develop "$flakeDir#packages.$system.$packageName"
}

debug-nix-drv.sh

script_content=$(cat <<'EOF'
set -x # Optional: it prints all commands, can be practical to debug
set +e # Optional: do not quit the shell on simple errors, Ctrl-C,...
export NIX_ENFORCE_PURITY=0

alias get-phases='eval echo $(typeset -f genericBuild | grep "phases=" | perl -pe "s/^\s+phases=//")'
alias set-phases='set $(get-phases); echo "$# phases: $@"'
alias run-next-phase='phaseName=$1; shift; realPhaseName="${!phaseName:-$phaseName}"; runPhase $phaseName; echo next phase: $1'

set-phases
echo next phase: $1

EOF
)


function debug_nix_drv() {
        set -u
        drvPath=$1

        pname=$(nix-store --query --binding pname "$drvPath")
        export out=$(mktemp -d -t "nix-debug-$pname.XXXXX")
        pushd "$out" || exit
        to_source=source-for-helpers.sh
        echo "$script_content" >"$out/$to_source"
        echo "#####################"
        echo "run source ./$to_source"
        echo "#####################"
        nix-shell "$drvPath"
        popd || exit
}