[ruby-core:112781] [Ruby master Feature#19197] Add Exception#root_cause

Issue #19197 has been updated by mame (Yusuke Endoh).


Discussed at the dev meeting.

Please elaborate the use case a bit more. The proposed method is easy to implement in Ruby. To introduce it as a builtin feature, you need to prove that it is frequently needed in a variety of applications and libraries.

> You'd want to look at the root cause, and bucket all SQL-connection issues into a single report

Do you have in mind an application monitor service like DataDog or Sentry? If so, what you need is not a Ruby method, but a request to ask the service.

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Feature #19197: Add Exception#root_cause
https://bugs.ruby-lang.org/issues/19197#change-102279

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
* Priority: Normal
----------------------------------------
### Description

I would like to add a `#root_cause` method to `Exception`.

It returns the last exception in linked-list of causality chain, that is, the original exception (whose own `cause` is `nil`).

### Example

```ruby
e = begin
    raise 'A' # This is the root cause
  rescue => a
    begin
      raise 'B'
    rescue => B
      begin
        raise 'C' # This is the outermost cause assigned to `e`
      rescue => c
        c
      end
    end
  end

# Here's what the structure looks like:
# C -> B -> A -> nil  
p(e)                   # => #<RuntimeError: C>
p(e.cause)             # => #<RuntimeError: B>
p(e.cause.cause)       # => #<RuntimeError: A>
p(e.cause.cause.cause) # => nil

# Here's the proposed API, showing that A is the root cause of e
p(e.root_cause)        # => #<RuntimeError: A>
# And that the root_cause has no further cause
p(e.root_cause.cause)  # => nil
```

### Motivation

There are some kinds of exceptions that can occur all over the place (and might be wrapped by arbitrarily many middlemen), but are attributable to a singular global cause. For example, a database outage could raise exceptions in almost every line of business logic of an app that uses ActiveRecord models.

Fundamentally, you wouldn't want an error report for every one of these lines. You'd want to look at the root cause, and bucket all SQL-connection issues into a single report, regardless of where they surface.

### Implementation

Draft PR: https://github.com/ruby/ruby/pull/6913



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