[ruby-core:123291] [Ruby Bug#21610] Use ec->interrupt_mask to prevent interrupts.

18 Sep 2025

      Issue #21610 has been updated by ioquatix (Samuel Williams).

A slightly different change may be required for 3.3 and 3.2 backports: https://github.com/ruby/ruby/commit/e900e9fc9ace5e36de5170c83df0f56167e4c8a9

----------------------------------------
Bug #21610: Use ec->interrupt_mask to prevent interrupts.
https://bugs.ruby-lang.org/issues/21610#change-114661

* Author: ioquatix (Samuel Williams)
* Status: Closed
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.2: REQUIRED, 3.3: REQUIRED, 3.4: REQUIRED
----------------------------------------
The following program can trigger `rb_bug` in `rb_fiber_scheduler_unblock`:

```ruby
require 'async/scheduler'

scheduler = Async::Scheduler.new
Fiber.set_scheduler(scheduler)

Signal.trap(:USR1) do
end

q = Thread::Queue.new

Thread.new do
  loop do
    Ractor.new do
      Process.kill(:USR1, $$)
    end.join
  end
end

Fiber.schedule do
  Fiber.schedule do
    1.upto(1000000) do |i|
      sleep 0.01
      q.pop
      q.push(1)
      puts "1 iter push/pop"
    end
  end
  Fiber.schedule do
    1.upto(1000000) do |i|
      sleep 0.01
      q.push(i)
      q.pop
      puts "1 iter push/pop#2"
    end
  end
  Fiber.schedule do
    gets
    exit!
  end
end
```

In brief, when `rb_fiber_scheduler_unblock` executes with `ec->interrupt_flag` set, the following sequence can occur:

```
Thread A (Signaling)              Thread B (Blocked Fiber)
--------------------              ---------------------
calls unblock(fiber_x)           
├─ enters scheduler user code     fiber_x sleeping in scheduler.block()
├─ ⚠️  SIGNAL INTERRUPT ⚠️       
├─ raises SignalException        
├─ unblock operation aborted     
├─ scheduler state inconsistent   
└─ returns believing success      └─ fiber_x waits forever ⚠️
```

Confirmed fix: https://github.com/ruby/ruby/pull/14588

-- 
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