Preserve locale during brew startup

[MikeMcQuaid]

• Validate the locale charmap instead of trusting its name.
• Reuse filtered Linux locale settings to avoid locale -a.

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OpenAI Codex GPT 5.6 Sol xhigh with local review and (macOS) testing

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[Copilot]

Pull request overview

This PR adjusts Homebrew’s early startup environment handling to better preserve a user’s locale (while still forcing a safe UTF-8 fallback when needed), and to avoid expensive locale enumeration on Linux by reusing values already captured during bin/brew’s environment filtering.

Changes:

• Update setup-locale to validate the active locale via locale charmap and avoid locale -a when the filtered locale values are already UTF-8.
• Add bash-level specs covering setup-locale behavior (macOS charmap validation and Linux variable restoration).
• Preserve LC_ALL/LC_CTYPE through bin/brew filtering by copying them into HOMEBREW_* variables.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/utils/os.sh	Restores locale from filtered copies and enforces UTF-8 by validating locale charmap.
Library/Homebrew/test/bash_spec.rb	Adds tests for setup-locale behavior on macOS and Linux paths.
bin/brew	Copies LC_ALL and LC_CTYPE into HOMEBREW_* before environment filtering.

Comments suppressed due to low confidence (2)

Library/Homebrew/utils/os.sh:110

• On macOS, setup-locale never restores LANG/LC_* from the HOMEBREW_LANG/HOMEBREW_LC_* copies that bin/brew now exports, so after environment filtering the locale remains unset and the macOS branch will always force LC_ALL=en_US.UTF-8. This prevents preserving a valid user UTF-8 locale during startup and makes the new LC_ALL/LC_CTYPE exports in bin/brew ineffective on macOS.

  if [[ -z "${HOMEBREW_MACOS}" ]]
  then
    [[ -z "${HOMEBREW_LANG:-}" ]] || export LANG="${HOMEBREW_LANG}"
    [[ -z "${HOMEBREW_LC_CTYPE:-}" ]] || export LC_CTYPE="${HOMEBREW_LC_CTYPE}"
    [[ -z "${HOMEBREW_LC_ALL:-}" ]] || export LC_ALL="${HOMEBREW_LC_ALL}"

Library/Homebrew/utils/os.sh:120

• locale charmap can print warnings to stderr when the locale variables are invalid (the broken-locale scenario this code is trying to handle). Because setup-locale runs during brew startup, those warnings can leak into user output. Suppressing stderr for the charmap probe keeps startup quiet while still letting you detect non-UTF-8.

    if [[ -z "${LC_ALL:-${LC_CTYPE:-${LANG:-}}}" ]] || [[ "$(locale charmap)" != "UTF-8" ]]
    then
      export LC_ALL="en_US.UTF-8"
    fi
  else

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