Simpler approach.

Author: samuel-williams-shopify

io.c

@@ -5546,20 +5546,6 @@ fptr_finalize_flush(rb_io_t *fptr, int noraise, int keepgvl)
         // Need to keep FILE objects of stdin, stdout and stderr, so we are done:
         done = 1;
     }
-    // The check for Qnil here is purely defensive, we should never invoke the fiber scheduler if there is no IO object.
-    else if (fptr->self != Qnil && fptr->closing_count == 0) {
-        VALUE scheduler = rb_fiber_scheduler_current();
-        if (scheduler != Qnil) {
-            fptr->closing_count += 1;
-            VALUE result = rb_fiber_scheduler_io_close(scheduler, fptr->self);
-            fptr->closing_count -= 1;
-
-            if (!UNDEF_P(result)) {
-                // Scheduler handled it:
-                if (fptr->fd == -1) return;
-            }
-        }
-    }
 
     fptr->fd = -1;
     fptr->stdio_file = 0;
@@ -5579,6 +5565,15 @@ fptr_finalize_flush(rb_io_t *fptr, int noraise, int keepgvl)
         done = 1;
     }
 
+    VALUE scheduler = rb_fiber_scheduler_current();
+    if (!done && fd >= 0 && scheduler != Qnil) {
+        VALUE result = rb_fiber_scheduler_io_close(scheduler, RB_INT2NUM(fd));
+
+        if (!UNDEF_P(result)) {
+            done = 1;
+        }
+    }
+
     if (!done && fd >= 0) {
         // fptr->fd may be closed even if close fails. POSIX doesn't specify it.
         // We assumes it is closed.

test/fiber/scheduler.rb

@@ -257,8 +257,8 @@ def io_select(...)
 
   # This hook is invoked by `IO#close`. Using a separate IO object
   # demonstrates that the close operation is asynchronous.
-  def io_close(io)
-    io.close
+  def io_close(descriptor)
+    Fiber.blocking{IO.for_fd(descriptor.to_i).close}
   end
 
   # This hook is invoked by `Kernel#sleep` and `Thread::Mutex#sleep`.