chore: Ensure newly created files have a reasonable owner (#2465)

script/synth.rb

@@ -15,8 +15,16 @@ end
 # so any previously created files don't pollute this and subsequent synth runs.
 execute "git clean -df"
 
-Dir.chdir "google-apis-generator"
+# This script is generally run as a superuser in a container. As a result,
+# newly generated files will have a superuser owner, and cannot easily be
+# removed by the caller. Thus, we must ensure any files generated during this
+# run are given a reasonable owner before the script exits.
+at_exit do
+  user_group = ENV["USER_GROUP"]
+  execute "chown -R #{user_group} #{DIR}/generated" if user_group
+end
 
+Dir.chdir "google-apis-generator"
 execute "bundle install"
 
 if ARGV.empty?

synth.py

@@ -31,6 +31,7 @@ command = [
     f"-v{os.getcwd()}:/workspace",
     "-v/var/run/docker.sock:/var/run/docker.sock",
     "-w", "/workspace",
+    "-e", f"USER_GROUP={os.getuid()}:{os.getgid()}",
     "--entrypoint", "script/synth.rb",
     "gcr.io/cloud-devrel-kokoro-resources/yoshi-ruby/autosynth"]
 if extra_args():