[ruby-core:102242] [Ruby master Bug#17581] Ruby 3.0 backtrace sometimes returns empty array

Issue #17581 has been reported by HParker (Adam Hess).

----------------------------------------
Bug #17581: Ruby 3.0 backtrace sometimes returns empty array
https://bugs.ruby-lang.org/issues/17581

* Author: HParker (Adam Hess)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0dev (2020-09-04T16:41:35Z master de30450d91) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
```ruby
class Foo
  def bar
    p caller(1,1)
  end
end
[Foo.new].group_by(&:bar)
```

3.0: []

2.7: ["test2.rb:6:in `each'"]

A similar thing happens when asking for a larger part of the backtrace:

```ruby
class Foo
  def bar
    p caller(0,4)
  end
end
[Foo.new].group_by(&:bar)
```

3.0: ["test2.rb:3:in `bar'", "test2.rb:6:in `each'", "test2.rb:6:in `group_by'"]

2.7: ["test2.rb:3:in `bar'", "test2.rb:6:in `each'", "test2.rb:6:in `group_by'", "test2.rb:6:in `<main>'"]

I suspect it was introduced by: https://github.com/ruby/ruby/commit/3b24b7914c16930bfadc89d6aff6326a51c54295
Since it seems to have to do with which frames are returned, though I haven't verified since the commit is difficult to revert.



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