Introduce ActionDispatch::DebugExceptions interceptors

[gsamokovarov]

Plugins interacting with the exceptions caught and displayed by
ActionDispatch::DebugExceptions currently have to monkey patch it to get
the much needed exception for their calculation.

With DebugExceptions.register_interceptor, plugin authors can hook into
DebugExceptions and process the exception, before being rendered. They
can store it into the request and process it on the way back of the
middleware chain execution or act on it straight in the interceptor.

The interceptors can be plain blocks, procs, lambdas or any object that
responds to #call.

You can see web-console implemented through the interceptor API here.
Other plugin authors can benefit from this API as well. Most of the error
reporting plugins can use this.

[rails-bot]

r? @matthewd

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[maclover7]

Can we get rid of the transitory @interceptors, and just call self.class.interceptors here?

[gsamokovarov]

Injected it, so I can test it more easily – no need to keep track of the global state during the test run.

[maclover7]

🤷 I just think it would make sense to use the direct variable we will iterating on, instead of creating a shadow variable, that we won't even be changing.

[nateberkopec]

As the maintainer of an exception notifier (sentry-raven) I am wholeheartedly for this PR.

[gsamokovarov]

Actually, this breaks the render_exception interface, and plugin authors already depend on it. I'm going with the duplicated code approach. Yeah, I like DRY code, but sometimes the duplication is so much better than breaking interfaces or leaking abstractions.

[tgxworld]

@gsamokovarov Is web-console still monkey patching? At Discourse, we maintain logster which is monkey patching as well and being able to register callbacks in the middleware would be great for us.

@rafaelfranca @matthewd Is there anything I can do to help move this forward?

[gsamokovarov]

@tgxworld Yes, it is still monkey patching... You can make logster and web-console play together by requiring it after web-console, e.g. put logster after web-console it in the Gemfile.

I still stand by the interceptors, though. They are super simple and provide what most of the plugins monkey patching ActionDispatch::DebugExceptions need.

[rafaelfranca]

The idea looks good to me. @gsamokovarov can you rebase it?

[gsamokovarov]

@rafaelfranca I have rebased it. 👌

[tgxworld]

😍 Thank you @gsamokovarov and @rafaelfranca