[ruby-core:111915] [Ruby master Bug#15507] Thread#raise is delivered to GC context

19 Jan 2023

      Issue #15507 has been updated by kjtsanaktsidis (KJ Tsanaktsidis).

❤️ Thank you for fixing this @jeremyevans - I was debugging some instances of our Ruby 2.7 app's test suite hanging on exit, and narrowed it down to this reproduction:

```
require 'tempfile'

def forget_about_a_tempfile
  Tempfile.new('garbage')
end

def tempfile_making_thread
  loop do
    forget_about_a_tempfile
    GC.start
  end
end

10.times do
  Thread.new { tempfile_making_thread }
end
sleep 0.5
```

I git bisect'd the fix to this commit. I guess the `eTerminateSignal` that kills threads on process exit has the same problem as an explicit `Thread#raise`. 

----------------------------------------
Bug #15507: Thread#raise is delivered to GC context
https://bugs.ruby-lang.org/issues/15507#change-101338

* Author: larskanis (Lars Kanis)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
Since I started the development of [Eventbox](https://github.com/larskanis/eventbox) I noticed sporadic failures of the Eventbox test suite on MRI. [One issue](https://github.com/larskanis/eventbox/issues/2) has been fixed in the meantime, but I was now able to minimize 3 others to a reproducible script. This is the first one:

This script combines a GC finalizer with Thread#raise:
```ruby
Thread.abort_on_exception = true

class Finalizer
  def self.define
    ObjectSpace.define_finalizer(String.new, self.method(:gc))
  end

  def self.gc(object_id)
    puts "gc #{object_id} #{caller[0]}"
  end
end

class Deadlocker
  class Stop < RuntimeError
  end

  def self.run
    th = Thread.handle_interrupt(Exception => :never) do
      Thread.new do
        begin
          Thread.handle_interrupt(Stop => :on_blocking) do
            100.times { String.new } # Trigger GC
            sleep 5                  # Wait for Stop exception
            raise "not interrupted"
          end
        rescue Stop
        end
      end
    end

    th.raise Stop
    th.join
  end
end

100.times do
  Finalizer.define   # This alone works well
  Deadlocker.run     # This alone works equally, but both interfere badly
end
```

The program output looks similar to:
```ruby
$ ruby -d --disable-gems interrupt-in-gc-error.rb
Exception `Deadlocker::Stop' at interrupt-in-gc-error.rb:23 - Deadlocker::Stop
Exception `Deadlocker::Stop' at interrupt-in-gc-error.rb:23 - Deadlocker::Stop
[...]
Exception `Deadlocker::Stop' at interrupt-in-gc-error.rb:9 - Deadlocker::Stop
gc 47133824012760 interrupt-in-gc-error.rb:22:in `call'
gc 47133824022800 interrupt-in-gc-error.rb:22:in `call'
[...]
Exception `RuntimeError' at interrupt-in-gc-error.rb:24 - not interrupted
#<Thread:0x000055bc65ac8f38@interrupt-in-gc-error.rb:19 run> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
	2: from interrupt-in-gc-error.rb:21:in `block (2 levels) in run'
	1: from interrupt-in-gc-error.rb:21:in `handle_interrupt'
interrupt-in-gc-error.rb:24:in `block (3 levels) in run': not interrupted (RuntimeError)
```

The debug output shows, that the `Stop` exception is delivered several times, so that the sleep call is properly interrupted. But if the interrupt is sent just in the moment when the finalizer is active, it is discarded and doesn't abort the `sleep` call.

IMHO interrupts shouldn't be delivered to any GC/trap context. At least they should be masked in this context.

This issue is present on all older MRI versions. However it doesn't appear on JRuby-9.2.5.0. So they seem to have solved the GC/interrupt relationship somehow.

-- 
https://bugs.ruby-lang.org/

kjtsanaktsidis (KJ Tsanaktsidis)

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