thread: leave RB_WAITFD_PRI waits to the blocking path

waitfd_to_waiting_flag maps RB_WAITFD_PRI to no thread_sched_waiting_*
event, so the M:N scheduler registered nothing for it yet parked the
thread: a timeout-less IO#wait(IO::PRIORITY) on an M:N thread slept
forever (a VM_ASSERT in thread_io_wait_events on debug builds).
Exclude it from thread_io_mn_schedulable; poll(2) on the blocking path
handles POLLPRI.

The test lives in test/ruby/test_ractor.rb rather than bootstraptest
because it needs the socket extension, which btest runs without.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/ruby/test_ractor.rb

@@ -771,4 +771,27 @@ class TestRactor < Test::Unit::TestCase
       end
     RUBY
   end
+
+  def test_io_priority_wait_on_mn_thread
+    omit 'POLLPRI/MSG_OOB semantics differ on windows' if RUBY_PLATFORM =~ /mswin|mingw/
+    # A timeout-less IO#wait(IO::PRIORITY) on an M:N thread must take the
+    # blocking path: the M:N scheduler has no event for POLLPRI and used to
+    # register nothing yet park the thread forever.
+    assert_separately([], __FILE__, __LINE__, <<-'RUBY')
+      Warning[:experimental] = false
+      require 'socket'
+      r = Ractor.new do
+        serv = TCPServer.new("127.0.0.1", 0)
+        c = TCPSocket.new("127.0.0.1", serv.addr[1])
+        s = serv.accept
+        t = Thread.new { c.wait(IO::PRIORITY, nil) }
+        sleep 0.5
+        s.send("!", Socket::MSG_OOB)
+        woken = t.join(5)
+        [serv, c, s].each(&:close)
+        woken ? :ok : :timeout
+      end
+      assert_equal :ok, r.value
+    RUBY
+  end
 end

thread.c

@@ -2055,7 +2055,11 @@ static bool
 thread_io_mn_schedulable(rb_thread_t *th, int events, const struct timeval *timeout)
 {
 #if defined(USE_MN_THREADS) && USE_MN_THREADS
-    return !th_has_dedicated_nt(th) && (events || timeout) && th->blocking;
+    // RB_WAITFD_PRI has no thread_sched_waiting_* event: the scheduler would
+    // register nothing and park the thread forever.  POLLPRI works on the
+    // blocking path.
+    return !th_has_dedicated_nt(th) && (events || timeout) && th->blocking &&
+        !(events & ~(RB_WAITFD_IN | RB_WAITFD_OUT));
 #else
     return false;
 #endif