Two variables, and each one is there for a measured reason.

LC_ALL=C so the output keeps one shape. tar -tv prints a date column, and untarEntries() reads the member name as everything after the time — a locale that spells dates differently would make that parse fail on a sound archive.

PATH, passed through from this process, because proc_open() given an environment replaces the whole of it rather than adding to it. And tar compresses nothing itself: --xz makes it spawn xz through a shell. With no PATH, that shell falls back to a built-in default — /usr/bin:/bin on a Mac — so gzip and bzip2 were found and xz and zstd, installed by Homebrew under /opt/homebrew/bin, were not. Measured on 2026-10-24:

/bin/sh: xz: command not found
tar: Child returned status 127

…on a host where tarCompressorExists() had just found xz on this process's own PATH and answered yes. The check looked at one environment and the work ran in another. That it went unnoticed until now is only because tar() refused xz before ever reaching the binary.