Describe the idea

Suppose you have a Rails middleware that does this:

def call(env)
  return [200, { 'Content-Type' => 'text/plain' }, 'hi']
end

Since 'hi' does not respond to each as specified in the Rack spec (https://github.com/rack/rack/blob/master/SPEC.rdoc#label-The+Body), this fails quietly in the Web server with this message:

#<NoMethodError: undefined method `each' for "hi":String>

Other users have reported this in puma/puma#1782.

Why do you think it's beneficial to most of the users

While this problem is clearly a bug in the middleware, we've come across it enough times that sometimes it takes a long to troubleshoot. The backtrace is not always available, and the log messages are hidden away in an unstructured stderr output. It's surprising that this exception isn't reported via Sentry.

Possible implementation

Capturing this exception might depend on integration with the Web server in question. For example, if you use Thin, the exception is encountered here: https://github.com/macournoyer/thin/blob/0712d603a31d97b9fa8a0260da400da2e4217d60/lib/thin/connection.rb#L114-L123

For Puma, there is this low-level error handler: https://github.com/puma/puma#error-handling. Perhaps it makes sense to hook Sentry up to the Puma ErrorLogger? https://github.com/puma/puma/blob/v5.3.2/lib/puma/error_logger.rb

Note that the exception is:

1. Hit on this line: https://github.com/puma/puma/blob/bda19f8225a36f83bb2671fc4baef870df9f76b4/lib/puma/request.rb#L149.
2. Rescued here: https://github.com/puma/puma/blob/f4766ce46976ec623f163a0428515d157f53e420/lib/puma/server.rb#L476
3. Reported here: https://github.com/puma/puma/blob/f4766ce46976ec623f163a0428515d157f53e420/lib/puma/server.rb#L517
4. Logged here: https://github.com/puma/puma/blob/f4766ce46976ec623f163a0428515d157f53e420/lib/puma/events.rb#L123

The backtrace shows:

lib/ruby/gems/2.7.0/gems/rack-2.2.3/lib/rack/body_proxy.rb:41:in `method_missing'
lib/ruby/gems/2.7.0/gems/rack-2.2.3/lib/rack/body_proxy.rb:41:in `method_missing'
lib/ruby/gems/2.7.0/gems/rack-2.2.3/lib/rack/body_proxy.rb:41:in `method_missing'
lib/ruby/gems/2.7.0/gems/rack-2.2.3/lib/rack/body_proxy.rb:41:in `method_missing'
lib/ruby/gems/2.7.0/gems/rack-2.2.3/lib/rack/body_proxy.rb:41:in `method_missing'
lib/ruby/gems/2.7.0/gems/puma-5.3.2/lib/puma/request.rb:149:in `handle_request'
lib/ruby/gems/2.7.0/gems/puma-5.3.2/lib/puma/server.rb:438:in `process_client'
lib/ruby/gems/2.7.0/gems/puma-5.3.2/lib/puma/thread_pool.rb:145:in `block in spawn_thread'

Similar issues:

1. SignalException: SIGTERM not caught by sentry-raven #917.
2. https://forum.sentry.io/t/puma-exception-handling/979.