How to skip logging uncaught exceptions in Rails?

Question

When using custom exceptions_app and rescue_responses, an application has more control of uncaught exceptions and an excessive logging from DebugExceptions middleware becomes noise.

As an example, the application knows how to process ActionPolicy::Unauthorized, renders proper page in exceptions_app and thus the following log is redundant:

FATAL -- :   
FATAL -- : ActionPolicy::Unauthorized (Not Authorized):
FATAL -- :   
FATAL -- : app/controllers/topics_suggest_controller.rb:47:in `topic_load' 

What would be the most idiomatic way to skip logging only those exceptions listed in rescue_responses?

Answer 1

Some historical notes are below. As of June 25, 2021, my PR to Rails https://github.com/rails/rails/pull/42592 has been accepted and this functionality will be available in Rails 6.1.5 7.0.0

---

Approach 1

It's tempting to delete DebugExceptions middleware from the application's Rails stack.

config/application.rb:

config.middleware.delete ActionDispatch::DebugExceptions

The problem is that it is exactly the middleware that determines Rails cannot find a route and throws ActionController::RoutingError in such cases. Hence, if you would like to react to this exception in your exceptions_app, this approach is a no-go for you.

Go ahead and use this approach if it's fine for you to see HTTP status 404 and plain text response Not found when no route was found.

Approach 2

Monkey patch or change ActionDispatch::DebugExceptions in some way. In fact, there was a proposal in 2013 to change DebugExceptions behavior so that it skips logging exceptions registered in rescue_responses: 9343, there is an example code in there.

I think it's too much to overwrite the whole class, so I chose to override its method log_error responsible for logging errors.

lib/ext/suppress_exceptions.rb:

module Ext
  module SuppressExceptions
    private

    def log_error(_request, wrapper)
      exception = wrapper.exception
      return if ActionDispatch::ExceptionWrapper.rescue_responses.key? exception.class.name
      super
    end
  end
end

config/initializers/error_handling.rb:

require_dependency 'ext/suppress_exceptions'

ActiveSupport.on_load(:action_controller) do
  ActionDispatch::DebugExceptions.prepend Ext::SuppressExceptions
end