Use IO::Buffer#copy (>=1MiB) to trigger blocking_operation_wait reliably

IO::Buffer#copy with a large buffer calls rb_nogvl(RB_NOGVL_OFFLOAD_SAFE)
in all Ruby versions that have blocking_operation_wait support, rather than
relying on a specific version of io_close_fptr. Verified that the hook is
called on 3.4 and master.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: samuel-williams-shopify

test/fiber/test_blocking_operation_gc.rb

@@ -69,7 +69,7 @@ def blocking_operation_wait(work)
 class TestBlockingOperationGC < Test::Unit::TestCase
   def omit_unless_supported
     omit "GC.compact not supported" unless GC.respond_to?(:compact)
-    omit "TCPServer not available" unless defined?(TCPServer)
+    omit "IO::Buffer not available" unless defined?(IO::Buffer)
   end
 
   def run_with_scheduler(trigger:)
@@ -78,19 +78,14 @@ def run_with_scheduler(trigger:)
       scheduler = BlockingOperationGCScheduler.new(trigger: trigger)
       Fiber.set_scheduler(scheduler)
 
-      server = TCPServer.new("localhost", 0)
-      port = server.addr[1]
-      client = TCPSocket.new("localhost", port)
-      peer = server.accept
-
+      # IO::Buffer#copy with >= 1 MiB triggers rb_nogvl(RB_NOGVL_OFFLOAD_SAFE)
+      # which calls rb_fiber_scheduler_blocking_operation_wait when a fiber
+      # scheduler is active. This is the code path we are testing.
       Fiber.schedule do
-        # IO#close calls maygvl_close -> IO_WITHOUT_GVL -> rb_nogvl(RB_NOGVL_OFFLOAD_SAFE)
-        # -> rb_fiber_scheduler_blocking_operation_wait when a fiber scheduler is active.
-        server.close
+        src = IO::Buffer.for("x" * (2 * 1024 * 1024))
+        dst = IO::Buffer.new(2 * 1024 * 1024)
+        dst.copy(src)
       end
-    ensure
-      client&.close rescue nil
-      peer&.close rescue nil
     end.join
     scheduler
   end
@@ -104,7 +99,7 @@ def test_blocking_operation_gc_compact
     omit_unless_supported
     assert_nothing_raised do
       scheduler = run_with_scheduler(trigger: :compact)
-      omit "blocking_operation_wait was not called (Ruby version does not route IO#close through it)" if scheduler.blocking_operation_wait_call_count == 0
+      omit "blocking_operation_wait was not called" if scheduler.blocking_operation_wait_call_count == 0
     end
   end
 
@@ -116,7 +111,7 @@ def test_blocking_operation_gc_fiber_switch
     omit_unless_supported
     assert_nothing_raised do
       scheduler = run_with_scheduler(trigger: :fiber_switch)
-      omit "blocking_operation_wait was not called (Ruby version does not route IO#close through it)" if scheduler.blocking_operation_wait_call_count == 0
+      omit "blocking_operation_wait was not called" if scheduler.blocking_operation_wait_call_count == 0
     end
   end
 end