Issue #15650 has been reported by vincentvanbush (Micha Buszkiewicz).

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Bug #15650: Segmentation fault when accessing $! in at_exit within a forked process
https://bugs.ruby-lang.org/issues/15650

* Author: vincentvanbush (Micha Buszkiewicz)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
In the following piece of code, `break` is erroneously used inside a block passed to `Process.fork`, which would normally result in a ``fork': unexpected break` message.
It is not entirely clear to me whether this should be accessible as an exception object or not - if not, I would expect this code to just print the error out and terminate, so $! would just contain `nil` in the `at_exit` block.
```
fork do
  at_exit do
    puts $!
  end

  break
end
```
However, what occurs is a segmentation fault, which can be found in an attachment to this issue.

Historical behavior:

* `ruby 1.8.7 (2011-06-30 patchlevel 352) [x86_64-linux]`:
```
nil
foo.rb:7:in `fork': unexpected break
```
* `ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-linux]`:
```
foo.rb:3:in `block (2 levels) in <main>': method `method_missing' called on unexpected T_NODE object (0x0055a2323cfc88 flags=0x381c klass=0x0) (NotImplementedError)
        from foo.rb:1:in `fork'
        from foo.rb:1:in `<main>'
foo.rb:1:in `fork': unexpected break
```
* `ruby 2.0.0p648 (2015-12-16 revision 53162) [x86_64-linux]`:
foo.rb:3:in `block (2 levels) in <main>': undefined method `inspect' for #<Object:0x005648577df7f8> (NoMethodError)
        from foo.rb:1:in `fork'
        from foo.rb:1:in `<main>'
foo.rb:1: unexpected break
* `ruby 2.1.9p490 (2016-03-30 revision 54437) [x86_64-linux]` - segmentation fault
* `ruby 2.2.10p489 (2018-03-28 revision 63023) [x86_64-linux]` - ditto
* `ruby 2.3.8p459 (2018-10-18 revision 65136) [x86_64-linux]` - ditto
* `ruby 2.4.5p335 (2018-10-18 revision 65137) [x86_64-linux]` - ditto
* `ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux]` - ditto

There is clearly something funny going on since 1.9, and 2.1 goes totally nuts.

When `break` is replaced with `raise 'foo'`, all of these versions catch the exception under `$!` correctly.

Tried compiling 2.6.1 under Ubuntu with GCC 4, 5, 6, 7 and 8, and Clang and the exception is not different. Also tried one of the rubies (2.3.8) compiled in CentOS 7 - no difference in result.

---Files--------------------------------
backtrace.txt (17.3 KB)


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