Hack to work around nix duplicate -L flags causing `argument list too long` for GCC, see https://github.com/NixOS/nixpkgs/issues/41340

nix-env-flags-deduper.py

#!/usr/bin/env python3

# Script to deduplicate linker flags in nix variables.
# Workaround for:
#     https://github.com/NixOS/nixpkgs/issues/41340
#
# This is obviously a hack and it should be fixed in nixpkgs.
#
# Usage (from bash to update env vars in the current shell context):
#     eval <(nix-env-flags-deduper.py)
#
# Note that currently no effort is taken to deduplicate flags *across*
# different env vars, e.g.
#     NIX_LDFLAGS
#     NIX_TARGET_LDFLAGS
# may both be passed to the same compiler/linker and contain overlapping
# entries, but we don't deduplicate across those here.
#
# This script maintains flag ordering but assumes that space-separated
# LDFLAGS are independent of each other.
# Any usage like `--option A --option B` will result in `--option A B`
# and thus probably break things.

import os
import re
import sys

# Check minimum Python version.
# See https://stackoverflow.com/questions/7961363/removing-duplicates-in-lists/7961425#7961425
MIN_PYTHON = (3, 6)
if sys.version_info < MIN_PYTHON:
  sys.exit("Python %s.%s or later is required for dict.fromkeys() order-preserving list deduplication.\n" % MIN_PYTHON)

# For each relevant nix variable, write an `export VAR=...deduped contents...`
# to stdout.
# We only dedupe `-L` flags because the multiple `-l` flags can actually be
# important for some use cases on some linkers (like GNU ld and gold with their
# "symbol hole filling" approach).
for var_name, val in os.environ.items():
  if re.match(r'NIX_(.*_)?LDFLAGS', var_name): # e.g. NIX_LFAGS, NIX_TARGET_LDFLAGS
    flags = val.split(' ')

    deduped_flags = []
    seen_flags_set = set()

    for f in flags:
      if f.startswith('-L') and f not in seen_flags_set:
        deduped_flags.append(f)
      seen_flags_set.add(f)

    print("export {}='{}'".format(var_name, ' '.join(deduped_flags)))