Bug #21401: ObjectSpace can't count Fibers after using a Ractor

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: ruby 3.5.0dev (2025-06-09T14:38:29Z master f9966b9b76) +PRISM [x86_64-darwin22]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[rmosolgo (Robert Mosolgo)]

Hi, thanks for all your great work, Ruby crew!

I have a test in my project that uses ObjectSpace.each_object(Fiber).count to see how many Fibers have been started by my code. This test started failing when I added other tests using Ractors. I was able to isolate it with this script:

require "objspace"

f = Fiber.new { 1 + 1 }
GC.start

p ObjectSpace.each_object(Fiber).count
# => 2 

Ractor.new { 1 + 1 }.take

p ObjectSpace.each_object(Fiber).count
# => 0 (should be 2) 

# The fiber _is_ there!
p f 
# #<Fiber:0x0000000100c568a8 ractor_fibers.rb:4 (created)>

It seems like ObjectSpace can't count fibers for some reason, but I expect it to return the number of Fibers in memory. Other classes like Hash and String are not affected.

It fails on Ruby 3.4.1 and 3.5-dev. I used:

• ruby 3.5.0dev (2025-06-09T14:38:29Z master f9966b9b76) +PRISM [x86_64-darwin22]
• ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-darwin22]

Please let me know if I can help with anything else in tracking down this bug. Thanks again!

[jhawthorn (John Hawthorn)]

• Is duplicate of Bug #19387: ObjectSpace.each_objects only returns shareable objects after starting a Ractor added

[jhawthorn (John Hawthorn)]

• Status changed from Open to Rejected

👋 Hi Robert! This is a known issue. Once a Ractor is started each_objects will only return shareable objects. I expect ko1's Ractor-local GC may change this. Closing as rejected in favour of #19387.