[ruby-core:104160] [Ruby master Feature#15567] Allow ensure to match specific situations

Issue #15567 has been updated by ioquatix (Samuel Williams).


Here are some examples of situations where `$!` is used inside `ensure` block. In every case, there is a potential buggy behaviour.

https://github.com/drbrain/net-http-persistent/blob/75574f2546a08aa2663b06a2e005bcf2ee304f13/lib/net/http/persistent.rb#L230

https://github.com/copiousfreetime/launchy/blob/80e723ebf2d9e73d35ae5de31a73268eb6c46ecc/lib/launchy.rb#L36-L40

https://github.com/rails/spring/blob/577cf01f232bb6dbd0ade7df2df2ac209697e741/lib/spring/application.rb#L296-L301

https://github.com/ahoward/open4/blob/2bc378285dcec9c88613fd4def57a58606c0680b/lib/open4.rb#L189-L192

I can find more, but I think it's sufficient to show the problem.

----------------------------------------
Feature #15567: Allow ensure to match specific situations
https://bugs.ruby-lang.org/issues/15567#change-92335

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
----------------------------------------
There are some situations where `rescue Exception` or `ensure` are not sufficient to correctly, efficiently and easily handle abnormal flow control.

Take the following program for example:

```
def doot
	yield
ensure
	# Did the function run to completion?
	return "abnormal" if $!
end

puts doot{throw :foo}
puts doot{raise "Boom"}
puts doot{"Hello World"}

catch(:foo) do
	puts doot{throw :foo}
end
```

Using `rescue Exception` is not sufficient as it is not invoked by `throw`.

Using `ensure` is inefficient because it's triggered every time, even though exceptional case might never happen or happen very infrequently.

I propose some way to limit the scope of the ensure block:

```
def doot
	yield
ensure when raise, throw
	return "abnormal"
end
```

The scope should be one (or more) of `raise`, `throw`, `return`, `next`, `break`, `redo`, `retry` (everything in `enum ruby_tag_type` except all except for `RUBY_TAG_FATAL`).

Additionally, it might be nice to support the inverted pattern, i.e.

```
def doot
	yield
ensure when not return
	return "abnormal"
end
```

Inverted patterns allow user to specify the behaviour without having problems if future scopes are introduced.

`return` in this case matches both explicit and implicit.




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