Thanks for seeing this through, and for adding the buffer-permission test in response to the review.

The implementation is nicely minimal — reusing the existing chumask path (now that #4836 fixed it reaching ServerEngine) and keeping umask as a string so it flows through the same to_i(8) handling is the right call. The command-line precedence via !@cl_opt.key?(:chumask) is correct too, since cmd_opts only gains :chumask when --umask is actually passed.

For future readers, it's worth being explicit that <system> umask takes effect in exactly the same modes as the existing --umask option:

• --no-supervisor (standalone): applied — Fluentd calls File.umask directly in the worker.
• Supervised + --daemon (daemonize, e.g. fluent-package): applied — ServerEngine sets the umask before spawning workers, so worker-created files inherit it.
• Supervised without --daemon (foreground): not applied to worker-created files — ServerEngine's non-daemonize path never calls File.umask, and Fluentd doesn't set worker_chumask.

So the common packaged deployment (--daemon) does honor it; only the foreground-supervisor case is uncovered. That's existing --umask behavior rather than anything this PR changes, so it's not a blocker.

For the foreground-supervisor gap, let's track it in a separate follow-up issue rather than widening this PR — keeping this one scoped to "the config-file equivalent of --umask" is cleanest, and matches what you already suggested.

For the docs follow-up, it'd help to spell out which modes the option takes effect in, so users on a foreground supervisor don't expect it to change buffer file permissions.

Thanks.