Restore sync=true on global stdout/stderr streams

[cjlarose]

Description

Fixes #2545. In Puma 5.2, Puma stopped mutating the sync flag on the global STDOUT and STDERR objects. This can cause user application logs to be not flushed on every write if users were depending on this behavior. This PR restores sync behavior for those streams to the pre-5.2 behavior. We may re-introduce the change again in 6.0 with better documentation.

To verify this change, I used a Rack app that writes to stdout on every request

#stdout_test.ru
app = lambda do |env|
  $stdout.write "hello\n"
  [200, {"Content-Type" => "text/plain"}, ["Hello World"]]
end

run app

Start puma, piping its stdout to something that's not a TTY:

bundle exec ruby -Ilib bin/puma stdout_test.ru | tee puma.out

Then, send a bunch of requests

seq 100 | xargs -n 1 -I{} curl http://localhost:9292

On Puma 5.2, you can observe that no output is visible from Puma until many more requests complete.

This change introduces a config option to the config DSL, so that users can opt into making it so that puma doesn't set sync flag on the stdout and stderr streams. The default behavior is consistent with Puma's behavior prior to 5.2. In a future release (maybe 6.0), we may change the default or remove the option altogether.

mutate_stdout_and_stderr_to_sync_on_write false

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[nateberkopec]

Thanks. I started on a similar PR with some more stuff about deprecation/logging a warning, but I can just layer that on top of this.

[MSP-Greg]

The result of this I'm in favor of.

Re the name mutate_global_stdout_and_stderr_to_flush_on_write, when I see 'global', I think $stdout & $stderr. Also, consider removing mutate and add sync just to clarify the settings?

Maybe I'm getting too sensitive about naming, as I often choose poor names myself...

[cjlarose]

Re the name mutate_global_stdout_and_stderr_to_flush_on_write, when I see 'global', I think $stdout & $stderr. Also, consider removing mutate and add sync just to clarify the settings?

Good point! I renamed the setting to mutate_stdout_and_stderr_to_sync_on_write.