[ruby-core:126592] [Ruby Bug#22106] Getting a segfault when using finalizers and/or WeakMap in Ractors

8 Sep 2026

      Issue #22106 has been updated by ko1 (Koichi Sasada).

Status changed from Open to Closed

Ractor local GC had solved this issue on Ruby 4.1dev.
If you have trouble again, please reopen this ticket.

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Bug #22106: Getting a segfault when using finalizers and/or WeakMap in Ractors
https://bugs.ruby-lang.org/issues/22106#change-118817

* Author: miles-georgi (Miles Georgi)
* Status: Closed
* Assignee: ractor
* ruby -v: ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [x86_64-linux]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
If I run the following script I get a segfault:

```ruby
def ractor_and_port_counts
  total_ractor_count = total_ractor_port_count = open_ractor_count = open_ractor_port_count = 0

  ObjectSpace.each_object do |object|
    case object
    when Ractor
      total_ractor_count += 1
      open_ractor_count += 1 unless object.default_port.closed?
    when Ractor::Port
      total_ractor_port_count += 1
      open_ractor_port_count += 1 unless object.closed?
    end
  end

  [total_ractor_count, open_ractor_count, total_ractor_port_count, open_ractor_port_count]
end

def show_ractor_counts
  GC.start
  pp ractor_and_port_counts
end

class Wrapper
  class << self
    def finalizer_proc
      proc do |id|
        wrapper_to_ractor = Ractor[:wrapper_to_ractor]

        if wrapper_to_ractor
          wrapper_to_ractor[id]&.each_key { |ractor| ractor << :close } rescue Ractor::ClosedError
        end
      end
    end

    def setup_finalizer(wrapper)
      wrapper_to_ractor = Ractor[:wrapper_to_ractor] ||= {}
      map = wrapper_to_ractor[wrapper.object_id] ||= ObjectSpace::WeakMap.new
      map[wrapper.ractor] = true

      ObjectSpace.define_finalizer(wrapper, &finalizer_proc)
    end
  end

  attr_reader :ractor

  def initialize(object)
    @ractor = Ractor.new do
      loop { break if receive == :close }

      o = nil
    end

    @ractor.send(object, move: true)

    self.class.setup_finalizer(self)
  end
end

class Outer
  def initialize = @inner = Wrapper.new(Inner.new)
end

class Inner; end

show_ractor_counts

outer = Wrapper.new(Outer.new)
show_ractor_counts

outer = nil
# Removing the following line makes it so it only segfaults sometimes instead of everytime
show_ractor_counts
```

I'll attach the script and the whole segfault but part of the segfault is:

```
<internal:gc>:44: [BUG] FL_FINALIZE flag is set, but finalizers are not found
ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [x86_64-linux]

-- Control frame information -----------------------------------------------
c:0004 p:0010 s:0023 e:000018 l:y b:0001 METHOD <internal:gc>:44
c:0003 p:0004 s:0011 e:000010 l:y b:0001 METHOD test_scripts/finalizer-in-ractor-test-segfault:23
c:0002 p:0075 s:0007 E:000f98 l:n b:---- EVAL   test_scripts/finalizer-in-ractor-test-segfault:72 [FINISH]
c:0001 p:0000 s:0003 E:000b40 l:y b:---- DUMMY  [FINISH]

-- Ruby level backtrace information ----------------------------------------
test_scripts/finalizer-in-ractor-test-segfault:72:in '<main>'
test_scripts/finalizer-in-ractor-test-segfault:23:in 'show_ractor_counts'
<internal:gc>:44:in 'start'
```

This happens in both a local build of 4.1-dev and 4.0.5

I suspect one might need to put GC.start at the end of the script if unable to reproduce.

---Files--------------------------------
segfault.txt (22 KB)
finalizer-in-ractor-test-segfault (1.66 KB)
finalizer-in-ractor-test-segfault-simplified (1009 Bytes)

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