[ruby-core:104151] [Ruby master Feature#15567] Allow ensure to match specific situations

Issue #15567 has been updated by Eregon (Benoit Daloze).


> A timeout error in my mind is more similar to a TERM signal, and Ruby handles that using an exception.

I think that's a good point.
I agree for similar things like Ctrl+C/SIGINT which is an `Interrupt` exception (< SignalException < Exception), and `Kernel#exit` which is a `SystemExit` exception (< Exception).

Nothing can guarantee to bubble through user code without stopping in between (it's always possible to hang/loop via `ensure`),
except a fatal error that would immediately end the process or skip even `ensure` blocks and always reach the C main().
So proper exception handling (e.g. no `rescue Exception; # ignore`, `ensure` code must be bounded in time/steps) is always needed for correctness, and if Ctrl+C/exit work with an exception, so should Timeout.timeout.
Maybe we should open a new issue so that Timeout.timeout uses an Exception and not `throw`?

---

I'm not sure if `throw` is always used as non-local control flow, and not as some sort of exception (e.g., to indicate some 403).

BTW I guess we could replace throw with a non-local `return` using `handle_request(proc { return })` instead of `catch`, but it wouldn't change anything related to this issue, both are considered non-local exits and non-distinguishable.
That's an argument for treating `throw` as control flow much like non-local `return` and not as exceptional though.

----------------------------------------
Feature #15567: Allow ensure to match specific situations
https://bugs.ruby-lang.org/issues/15567#change-92323

* Author: ioquatix (Samuel Williams)
* Status: Rejected
* 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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