api: don't download files as root when Homebrew's not owned by root.

This was mentioned in a random comment. While we're here, make some
helper functions to query this a bit more nicely elsewhere when we do
it.

Library/Homebrew/api.rb

@@ -41,6 +41,11 @@ module Homebrew
       url = "#{Homebrew::EnvConfig.api_domain}/#{endpoint}"
       default_url = "#{HOMEBREW_API_DEFAULT_DOMAIN}/#{endpoint}"
 
+      if Homebrew.running_as_root_but_not_owned_by_root? &&
+         (!target.exist? || target.empty?)
+        odie "Need to download #{url} but cannot as root! Try again without `sudo`."
+      end
+
       # TODO: consider using more of Utils::Curl
       curl_args = %W[
         --compressed
@@ -55,6 +60,7 @@ module Homebrew
                       !target.empty? &&
                       (Homebrew::EnvConfig.no_auto_update? ||
                       ((Time.now - Homebrew::EnvConfig.api_auto_update_secs.to_i) < target.mtime))
+      skip_download ||= Homebrew.running_as_root_but_not_owned_by_root?
 
       json_data = begin
         begin

Library/Homebrew/brew.rb

@@ -120,12 +120,12 @@ begin
     # Unset HOMEBREW_HELP to avoid confusing the tap
     with_env HOMEBREW_HELP: nil do
       tap_commands = []
-      if File.exist?("/.dockerenv") ||
-         Process.uid.zero? ||
+      if (File.exist?("/.dockerenv") ||
+         Homebrew.running_as_root? ||
          ((cgroup = Utils.popen_read("cat", "/proc/1/cgroup").presence) &&
-          %w[azpl_job actions_job docker garden kubepods].none? { |type| cgroup.include?(type) })
-        brew_uid = HOMEBREW_BREW_FILE.stat.uid
-        tap_commands += %W[/usr/bin/sudo -u ##{brew_uid}] if Process.uid.zero? && !brew_uid.zero?
+          %w[azpl_job actions_job docker garden kubepods].none? { |type| cgroup.include?(type) })) &&
+         Homebrew.running_as_root_but_not_owned_by_root?
+        tap_commands += %W[/usr/bin/sudo -u ##{Homebrew.owner_uid}]
       end
       quiet_arg = args.quiet? ? "--quiet" : nil
       tap_commands += [HOMEBREW_BREW_FILE, "tap", *quiet_arg, possible_tap.name]

Library/Homebrew/global.rb

@@ -111,6 +111,19 @@ module Homebrew
     def auditing?
       @auditing == true
     end
+
+    def running_as_root?
+      @process_uid ||= Process.uid
+      @process_uid.zero?
+    end
+
+    def owner_uid
+      @owner_uid ||= HOMEBREW_BREW_FILE.stat.uid
+    end
+
+    def running_as_root_but_not_owned_by_root?
+      running_as_root? && !owner_uid.zero?
+    end
   end
 end