Using push_named_tags for MDC-like logging

[KazWolfe]

I'd like to look at using SemanticLogger#push_named_tags to support MDC-like log contexts:

class ApplicationController < ActionController::Base
  before_action :set_log_context

  def set_log_context
    SemanticLogger.push_named_tags(user: { id: current_user.id }) if user_signed_in?
  end
end

This in and of itself works, but has a few quirks:

1. When using push_named_tags, the log context will leak between requests. As an example, a logged out request following a logged in request will still receive the named tags.
2. The push_named_tags method in general does not perform a deep merge on provided context, meaning if multiple push_named_tags were to set user, only the last one would win.

As is, both of these can be resolved with some tricks that ultimately come down to a special tag named _mdc, a lot of (inefficient) juggling, and a middleware to clear this after every request. This, however, is obviously inelegant and inefficient. A number of monkeypatches can also be applied, but that in turn has its own problems. It would be nicer to be able to use SemanticLogger.tagged, but I'm not sure how (or even if) it's possible to appropriately wrap Rails contexts in that.

Has anyone managed to set up something like this before? Or, better yet, is this something that would be beneficial to SemanticLogger overall? It seems relatively simple to tweak the requisite behavior, but I'm not sure it's desired.

I suspect all that would really be needed would be to change named_tags to properly deep merge, and then inject some middleware accordingly to set/clear things.