[ruby-core:117712] [Ruby master Bug#20455] rb_errinfo() inconsistent with $! in the caller Ruby code

Issue #20455 has been updated by Eregon (Benoit Daloze).


@ko1 (and the code) pointed me to `$!` is `rb_ec_get_errinfo`, which walks the stack to find the first `rescue`/`ensure` (does it stop at the first ensure if it's an ensure without an active exception?) and uses `ec->errinfo` if not,
while `rb_errinfo` is just `GET_EC()->errinfo`.

I'm not sure what this means in terms of semantics, that they are the same but only if no rescue/ensure on the stack, and otherwise they are fully separate?
It sounds like very odd semantics.

BTW the header docs at https://github.com/ruby/ruby/blob/6f4f360fc46269eaba753cafe557519677a45a11/include/ruby/internal/error.h#L77-L84 seems not quite true and rather misleading.

----------------------------------------
Bug #20455: rb_errinfo() inconsistent with $! in the caller Ruby code
https://bugs.ruby-lang.org/issues/20455#change-108121

* Author: Eregon (Benoit Daloze)
* Status: Open
* ruby -v: ruby 3.4.0dev (2024-04-25T08:12:47Z master 64bd8e41df) [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
This (slightly modified for clarity) test in ruby/spec demonstrates the unexpected result:
```ruby
  describe "rb_errinfo" do
    def err
      $!
    end

    it "is cleared when entering a C method" do
      begin
        raise StandardError
      rescue
        $!.class.should == StandardError
        err.class.should == StandardError
        @s.rb_errinfo().should == nil
      end
    end
```

Why does `rb_errinfo()` return nil there, when $! is set in the caller (and $! isn't per frame but per thread, as shown with `err`)?

Is this bug?
If not, what is the logic and reason for clearing `$!` when calling a method defined in C?



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