[ruby-core:84245] [Ruby trunk Bug#14179] Forwardable Segmentation fault ruby 2.2.8p477 (2017-09-14 revision 59906)

Issue #14179 has been reported by dadah89 (Igor Justino).

----------------------------------------
Bug #14179: Forwardable Segmentation fault ruby 2.2.8p477 (2017-09-14 revision 59906)
https://bugs.ruby-lang.org/issues/14179

* Author: dadah89 (Igor Justino)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.2.8p477 (2017-09-14 revision 59906)
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Hello,

Delegating `backtrace` causes a segmentation fault on ruby 2.2.8.

Seems to be related to this change: https://github.com/ruby/ruby/commit/a14ba6e3d58d6f63f47c0549f60306c4989478ba

Looking at that change, seems like the `$@` call on line 207 would cause an infinite loop (assuming $@ would execute something similar $!.backtrace, but I'm not sure how $@ is implemented).

Example:
```
irb(main):052:0> class WrappedException < StandardError
irb(main):053:1>   extend Forwardable
irb(main):054:1> 
irb(main):055:1*   def_delegators :@err, :backtrace
irb(main):056:1> 
irb(main):057:1*   def initialize(err)
irb(main):058:2>     @err = err
irb(main):059:2>   end
irb(main):060:1> end
=> :initialize
irb(main):061:0> 
irb(main):062:0* begin
irb(main):063:1*   raise WrappedException, StandardError.new('test')
irb(main):064:1> rescue => e
irb(main):065:1>   e.backtrace
irb(main):066:1> end
Segmentation fault
```

```
irb(main):031:0> class WrappedException < StandardError
irb(main):032:1>   extend Forwardable
irb(main):033:1> 
irb(main):034:1*   def_delegators :@err, :backtrace
irb(main):035:1> 
irb(main):036:1*   def initialize(err)
irb(main):037:2>     @err = err
irb(main):038:2>   end
irb(main):039:1> end
=> :initialize
irb(main):040:0> 
irb(main):041:0* begin
irb(main):042:1*   raise WrappedException, StandardError.new('test')
irb(main):043:1> rescue => e
irb(main):044:1>   e.backtrace
irb(main):045:1> end
RuntimeError: can't modify frozen SystemStackError
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
... 5481 levels...
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from /opt/.rbenv/versions/2.2.8/lib/ruby/2.2.0/forwardable.rb:207:in `backtrace'
	from (irb):44:in `rescue in irb_binding'
	from (irb):41
	from /opt/.rbenv/versions/2.2.8/bin/irb:11:in `<main>'
irb(main):046:0> 
```



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