Guard fiber target across fiber_store. (rubyGH-17957) [Backport #22196]

Author: samuel-williams-shopify

cont.c

@@ -2791,6 +2791,13 @@ fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int kw_splat, rb_fi
 
     VM_ASSERT(FIBER_RUNNABLE_P(fiber));
 
+    /*
+     * Keep the target fiber object alive across fiber_store.  The raw
+     * rb_fiber_t pointer is used after the coroutine switch, and GC may run
+     * while this C frame is suspended.
+     */
+    VALUE fiber_value = fiber->cont.self;
+
     rb_fiber_t *current_fiber = fiber_current();
 
     VM_ASSERT(!current_fiber->resuming_fiber);
@@ -2824,6 +2831,7 @@ fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int kw_splat, rb_fi
         }
     }
 #endif
+    RB_GC_GUARD(fiber_value);
 
     if (fiber_current()->blocking) {
         th->blocking += 1;

test/ruby/test_fiber.rb

@@ -497,6 +497,40 @@ def test_create_fiber_in_new_thread
     assert_equal :ok, ret, '[Bug #14642]'
   end
 
+  def test_gc_during_nested_resume_yield
+    assert_normal_exit <<-'RUBY', '[Bug #22196]', timeout: 10
+      parent = child1 = child2 = nil
+
+      parent = Fiber.new do
+        child1 = Fiber.new do
+          parent.resume
+        end
+
+        child2 = Fiber.new do
+          GC.start
+          parent.resume
+        end
+
+        Fiber.yield
+
+        child1 = nil
+
+        Fiber.yield
+      end
+
+      parent.resume
+
+      child = child1
+      child1 = nil
+      child.resume
+      child = nil
+
+      child = child2
+      child2 = nil
+      child.resume
+    RUBY
+  end
+
   def test_machine_stack_gc
     assert_normal_exit <<-RUBY, '[Bug #14561]', timeout: 60
       enum = Enumerator.new { |y| y << 1 }