Keep blocked scheduler fibers alive during GC

fixtures/io/event/test_scheduler.rb

@@ -41,6 +41,7 @@ def initialize(selector: nil, worker_pool: nil, maximum_worker_count: nil)
 
 			# Track the number of fibers that are blocked.
 			@blocked = 0
+			@blocking = {}
 		end
 
 		# @attribute [WorkerPool] The worker pool used for executing blocking operations.
@@ -80,8 +81,10 @@ def block(blocker, timeout = nil)
 
 			begin
 				@blocked += 1
+				@blocking[fiber] = true
 				@selector.transfer
 			ensure
+				@blocking.delete(fiber)
 				@blocked -= 1
 			end
 		ensure

test/io/event/worker_pool.rb

@@ -123,6 +123,34 @@
 			expect(result[:duration]).to be == 0.05
 		end
 
+		it "keeps blocked fibers alive during garbage collection" do
+			result = nil
+
+			Thread.new do
+				Fiber.set_scheduler(scheduler)
+
+				gc_thread = Thread.new do
+					sleep(0.02)
+					GC.start(full_mark: true, immediate_sweep: true)
+				end
+
+				Fiber.schedule do
+					result = IO::Event::WorkerPool.busy(duration: 0.1)
+				end
+
+				scheduler.run
+				gc_thread.value
+			ensure
+				Fiber.set_scheduler(nil)
+				scheduler.close
+			end.value
+
+			expect(result).to have_keys(
+				cancelled: be == false,
+				result: be == :completed
+			)
+		end
+
 		it "can cancel a busy operation using unblock function" do
 			# This tests the cancellation mechanism through rb_thread_call_without_gvl
 			completed = false