Preserve locale during brew startup

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

Library/Homebrew/test/bash_spec.rb

@@ -39,6 +39,59 @@ RSpec.describe "Bash" do
     end
   end
 
+  describe "setup-locale" do
+    it "uses the macOS locale charmap rather than the locale name", :needs_macos do
+      setup_locale = [
+        "/bin/bash", "-c", <<~BASH, "bash", (HOMEBREW_LIBRARY_PATH/"utils/os.sh").to_s
+          source "$1"
+          locale() {
+            [[ "${LC_CTYPE:-${LANG:-}}" == "UTF-8" ]] && printf "UTF-8" || printf "US-ASCII"
+          }
+          setup-locale
+          printf "%s" "${LC_ALL-unset}"
+        BASH
+      ]
+      invalid_stdout, invalid_stderr, invalid_status = Open3.capture3(
+        { "LANG" => "C.utf8", "LC_CTYPE" => nil, "LC_ALL" => nil }, *setup_locale
+      )
+      valid_stdout, valid_stderr, valid_status = Open3.capture3(
+        { "LANG" => nil, "LC_CTYPE" => "UTF-8", "LC_ALL" => nil }, *setup_locale
+      )
+
+      expect([invalid_stdout, invalid_stderr, invalid_status.success?,
+              valid_stdout, valid_stderr, valid_status.success?])
+        .to eq(["en_US.UTF-8", "", true, "unset", "", true])
+    end
+
+    it "restores filtered Linux locale variables and removes their copies" do
+      stdout, stderr, status = Open3.capture3(
+        { "LANG" => nil, "LC_CTYPE" => nil, "LC_ALL" => nil },
+        "/bin/bash", "-c", <<~'BASH', "bash", (HOMEBREW_LIBRARY_PATH/"utils/os.sh").to_s
+          source "$1"
+          HOMEBREW_MACOS=
+          HOMEBREW_LANG=C
+          HOMEBREW_LC_CTYPE=C
+          HOMEBREW_LC_ALL=C.UTF-8
+          locale() {
+            if [[ "$1" == "charmap" ]]
+            then
+              [[ "${LC_ALL:-}" == "C.UTF-8" ]] && printf "UTF-8" || printf "US-ASCII"
+            else
+              printf "locale -a called\n" >&2
+              printf "C.UTF-8\n"
+            fi
+          }
+          setup-locale
+          printf "%s\n" "${LANG-unset}" "${LC_CTYPE-unset}" "${LC_ALL-unset}" \
+            "${HOMEBREW_LANG-unset}" "${HOMEBREW_LC_CTYPE-unset}" "${HOMEBREW_LC_ALL-unset}"
+        BASH
+      )
+
+      expect([stdout, stderr, status.success?])
+        .to eq(["C\nC\nC.UTF-8\nunset\nunset\nunset\n", "", true])
+    end
+  end
+
   describe "every `.sh` file" do
     it "has valid Bash syntax" do
       Pathname.glob("#{HOMEBREW_LIBRARY_PATH}/**/*.sh").each do |path|

Library/Homebrew/utils/os.sh

@@ -99,24 +99,30 @@ then
 fi
 
 # Force UTF-8 to avoid encoding issues for users with broken locale settings.
-# The locale is checked by name to avoid forking `locale charmap`. When
-# bin/brew has filtered the environment no locale variables survive so a UTF-8
-# one must be set.
+# Validate the active locale's charmap rather than trusting its name before
+# selecting a usable fallback.
 setup-locale() {
-  local utf8_locale_regex='\.([Uu][Tt][Ff]-?8)(@|$)'
   local locales c_utf_regex en_us_regex utf_regex
-  if [[ "${LC_ALL:-${LC_CTYPE:-${LANG:-}}}" =~ ${utf8_locale_regex} ]]
+  if [[ -z "${HOMEBREW_MACOS}" ]]
   then
-    # The locale is already UTF-8.
-    :
-  elif [[ -n "${HOMEBREW_MACOS}" ]]
+    [[ -z "${HOMEBREW_LANG:-}" ]] || export LANG="${HOMEBREW_LANG}"
+    [[ -z "${HOMEBREW_LC_CTYPE:-}" ]] || export LC_CTYPE="${HOMEBREW_LC_CTYPE}"
+    [[ -z "${HOMEBREW_LC_ALL:-}" ]] || export LC_ALL="${HOMEBREW_LC_ALL}"
+  fi
+  unset HOMEBREW_LANG HOMEBREW_LC_CTYPE HOMEBREW_LC_ALL
+
+  if [[ -n "${HOMEBREW_MACOS}" ]]
   then
-    export LC_ALL="en_US.UTF-8"
+    if [[ -z "${LC_ALL:-${LC_CTYPE:-${LANG:-}}}" ]] || [[ "$(locale charmap)" != "UTF-8" ]]
+    then
+      export LC_ALL="en_US.UTF-8"
+    fi
   else
     if ! command -v locale >/dev/null
     then
       export LC_ALL=C
-    else
+    elif [[ "$(locale charmap)" != "UTF-8" ]]
+    then
       locales="$(locale -a)"
       c_utf_regex='\bC\.(utf8|UTF-8)\b'
       en_us_regex='\ben_US\.(utf8|UTF-8)\b'

bin/brew

@@ -235,6 +235,8 @@ USED_BY_HOMEBREW_VARS=(
   GOBIN
   GOPATH
   LANG
+  LC_ALL
+  LC_CTYPE
   NODENV_ROOT
   PATH
   PYENV_ROOT