(WIP) Force a timer thread when waiting for other threads to finish IO

If the initial SIGVTALRM doesn't actually hit the thread doing IO whilst
it's blocked in select(2), we have to keep spamming it so that a signal
_does_ land on it whilst it is blocked in read.

This is not actually a good solution but let's see if it works.

include/ruby/thread.h

@@ -59,6 +59,8 @@
  */
 #define RB_NOGVL_UBF_ASYNC_SAFE  (0x2)
 
+#define RB_NOGVL_FORCE_TIMER_THREAD (0x4)
+
 /** @} */
 
 RBIMPL_SYMBOL_EXPORT_BEGIN()

io.c

@@ -5476,7 +5476,7 @@ fptr_finalize_flush(rb_io_t *fptr, int noraise, int keepgvl,
     // Ensure waiting_fd users do not hit EBADF.
     if (busy) {
         // Wait for them to exit before we call close().
-        (void)rb_thread_call_without_gvl(call_close_wait_nogvl, busy, RUBY_UBF_IO, 0);
+        (void)rb_nogvl(call_close_wait_nogvl, busy, RUBY_UBF_IO, 0, RB_NOGVL_FORCE_TIMER_THREAD);
     }
 
     // Disable for now.

thread.c

@@ -1540,10 +1540,14 @@ rb_nogvl(void *(*func)(void *), void *data1,
             vm->ubf_async_safe = 1;
         }
         else {
-            ubf_th = rb_thread_start_unblock_thread();
+            flags |= RB_NOGVL_FORCE_TIMER_THREAD;
         }
     }
 
+    if (flags & RB_NOGVL_FORCE_TIMER_THREAD) {
+        ubf_th = rb_thread_start_unblock_thread();
+    }
+
     BLOCKING_REGION(th, {
         val = func(data1);
         saved_errno = errno;