Cleanup HOMEBREW_TEMP handling

- Ensure that `HOMEBREW_TEMP` is only displayed in `brew config` when
  it's non-default.
- Attempt to create a missing `HOMEBREW_TEMP` directory rather than
  failing to `realpath`. Note this will still fail on permissions errors
  which is to be expected.

Library/Homebrew/brew.sh

@@ -105,8 +105,7 @@ then
   fi
 
   HOMEBREW_CACHE="${HOMEBREW_CACHE:-${HOME}/Library/Caches/Homebrew}"
-
-  HOMEBREW_TEMP="${HOMEBREW_TEMP:-/private/tmp}"
+  HOMEBREW_SYSTEM_TEMP="/private/tmp"
 else
   HOMEBREW_PROCESSOR="$(uname -m)"
   HOMEBREW_PRODUCT="${HOMEBREW_SYSTEM}brew"
@@ -116,10 +115,11 @@ else
 
   CACHE_HOME="${XDG_CACHE_HOME:-${HOME}/.cache}"
   HOMEBREW_CACHE="${HOMEBREW_CACHE:-${CACHE_HOME}/Homebrew}"
-
-  HOMEBREW_TEMP="${HOMEBREW_TEMP:-/tmp}"
+  HOMEBREW_SYSTEM_TEMP="/tmp"
 fi
 
+HOMEBREW_TEMP="${HOMEBREW_TEMP:-${HOMEBREW_SYSTEM_TEMP}}"
+
 if [[ -n "$HOMEBREW_FORCE_BREWED_CURL" &&
       -x "$HOMEBREW_PREFIX/opt/curl/bin/curl" ]] &&
          "$HOMEBREW_PREFIX/opt/curl/bin/curl" --version >/dev/null
@@ -147,6 +147,7 @@ export HOMEBREW_BREW_FILE
 export HOMEBREW_PREFIX
 export HOMEBREW_REPOSITORY
 export HOMEBREW_LIBRARY
+export HOMEBREW_SYSTEM_TEMP
 export HOMEBREW_TEMP
 
 # Declared in brew.sh

Library/Homebrew/config.rb

@@ -39,7 +39,11 @@ HOMEBREW_CACHE_FORMULA = HOMEBREW_CACHE/"Formula"
 HOMEBREW_LOGS = Pathname.new(ENV["HOMEBREW_LOGS"] || "~/Library/Logs/Homebrew/").expand_path
 
 # Must use /tmp instead of $TMPDIR because long paths break Unix domain sockets
-HOMEBREW_TEMP = Pathname.new(ENV["HOMEBREW_TEMP"]).realpath
+HOMEBREW_TEMP = begin
+  tmp = Pathname.new(ENV["HOMEBREW_TEMP"])
+  tmp.mkpath unless tmp.exist?
+  tmp.realpath
+end
 
 unless defined? HOMEBREW_LIBRARY_PATH
   # Root of the Homebrew code base

Library/Homebrew/dev-cmd/update-test.rb

@@ -31,14 +31,15 @@ module Homebrew
 
     ENV["HOMEBREW_UPDATE_TEST"] = "1"
 
-    if args.to_tag?
+    branch = if args.to_tag?
       ENV["HOMEBREW_UPDATE_TO_TAG"] = "1"
-      branch = "stable"
+      "stable"
     else
-      branch = "master"
+      "master"
     end
 
-    cd HOMEBREW_REPOSITORY
+    start_commit, end_commit = nil
+    cd HOMEBREW_REPOSITORY do
       start_commit = if commit = args.commit
         commit
       elsif date = args.before
@@ -68,12 +69,12 @@ module Homebrew
 
       end_commit = Utils.popen_read("git", "rev-parse", "HEAD").chomp
       odie "Could not find end commit!" if end_commit.empty?
+    end
 
     puts "Start commit: #{start_commit}"
     puts "End   commit: #{end_commit}"
 
-    mktemp("update-test") do |staging|
-      staging.retain! if args.keep_tmp?
+    mkdir "update-test" do
       curdir = Pathname.new(Dir.pwd)
 
       oh1 "Setup test environment..."
@@ -107,5 +108,7 @@ module Homebrew
         EOS
       end
     end
+  ensure
+    FileUtils.rm_r "update-test" unless args.keep_tmp?
   end
 end

Library/Homebrew/system_config.rb

@@ -116,6 +116,7 @@ class SystemConfig
         HOMEBREW_CELLAR: "/usr/local/Cellar",
         HOMEBREW_CACHE: "#{ENV["HOME"]}/Library/Caches/Homebrew",
         HOMEBREW_RUBY_WARNINGS: "-W0",
+        HOMEBREW_TEMP: ENV["HOMEBREW_SYSTEM_TEMP"],
       }.freeze
       boring_keys = %w[
         HOMEBREW_BROWSER
@@ -134,6 +135,7 @@ class SystemConfig
         HOMEBREW_MACOS_VERSION
         HOMEBREW_RUBY_PATH
         HOMEBREW_SYSTEM
+        HOMEBREW_SYSTEM_TEMP
         HOMEBREW_OS_VERSION
         HOMEBREW_PATH
         HOMEBREW_PROCESSOR
@@ -155,6 +157,9 @@ class SystemConfig
       if defaults_hash[:HOMEBREW_RUBY_WARNINGS] != ENV["HOMEBREW_RUBY_WARNINGS"].to_s
         f.puts "HOMEBREW_RUBY_WARNINGS: #{ENV["HOMEBREW_RUBY_WARNINGS"]}"
       end
+      if defaults_hash[:HOMEBREW_TEMP] != HOMEBREW_TEMP.to_s
+        f.puts "HOMEBREW_TEMP: #{HOMEBREW_TEMP}"
+      end
       unless ENV["HOMEBREW_ENV"]
         ENV.sort.each do |key, value|
           next unless key.start_with?("HOMEBREW_")