[ruby-core:121300] [Ruby master Bug#21146] VM_ASSERT(expr) gives bad bug report results when another ractor fails an assertion during printing of report

Issue #21146 has been updated by luke-gru (Luke Gruber).


Thanks for your comment. I can make it simpler, but I am a bit confused as to what I should do instead. If the first thread gets to the global variable first and enters `rb_vm_bugreport`, then other threads that also try to enter this function should be blocked, but there are only a few ways to block them:

1) use a mutex, like the VM lock in my patch
2) use a blocking system call like sleep (which is also safe in a signal handler in case someone uses `VM_ASSERT` or `rb_bug` in a signal handler
3) busy-wait until the process ends, which is going to be soon anyway

Would you prefer #2?

Also when you say use a global variable, do you mean an atomic global? Because if not, a write could be made that goes to the CPU's write buffer and is not flushed, so two or more threads could enter the critical section at once. I'm open to doing whatever you want, because maybe I'm overthinking it for just a debug case anyway.

Thanks again!


----------------------------------------
Bug #21146: VM_ASSERT(expr) gives bad bug report results when another ractor fails an assertion during printing of report
https://bugs.ruby-lang.org/issues/21146#change-112263

* Author: luke-gru (Luke Gruber)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
test.rb:
```ruby
rs = 100.times.map do
  Ractor.new do
    cnt = rand 3
    cnt += 1 if cnt.zero?
    sleep cnt
    100.times do |i|
      if i != 0 && i % 50 == 0
        Ractor.fail_assert
      end
    end
  end
end
```

ractor.rb:

```ruby
def self.fail_assert
  __builtin_cexpr! %q{
    VM_ASSERT(0), Qfalse
  }
end
```

> make run

I would like to be able to see the bug report for the first failed assertion, without any output from the other ractors.



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