Minimal example:

# config.ru
problem

# puma-config.rb
pidfile 'tmp/puma.pid'
state_path 'tmp/puma.state'
stdout_redirect 'log/puma.stdout', 'log/puma.stderr'

When run without -d option application error is logged to stderr.

$ puma -C puma-config.rb 
Puma 2.1.0 starting...
* Min threads: 0, max threads: 16
* Environment: development
* Listening on tcp://0.0.0.0:9292
config.ru:4:in `block in <main>': undefined local variable or method `problem' for #<Rack::Builder:0xa22c6f8 @map=nil, @run=nil, @use=[]> (NameError)

When run with -d option the application error is lost, neither written to stderr nor to the file specified in stdout_redirect.

In my opinion it should always log the application load errors to the file if stdout_redirect was specified.

Confirmed on current master.

Was mentioned in #253 (comment) by @sabcio.

I will come up with a pull request for this in the next few days.