Add URing io_wait unmatched poll repro

Author: samuel-williams-shopify

ext/io/event/selector/uring.c

@@ -581,6 +581,16 @@ int events_from_poll_flags(short flags) {
 	return events;
 }
 
+#ifdef RUBY_DEBUG
+VALUE IO_Event_Selector_URing_test_io_wait_result(VALUE self, VALUE _result, VALUE _events) {
+	int result = NUM2INT(_result);
+	int events = NUM2INT(_events);
+	short flags = poll_flags_from_events(events);
+
+	return RB_INT2NUM(events_from_poll_flags(result & flags));
+}
+#endif
+
 struct io_wait_arguments {
 	struct IO_Event_Selector_URing *selector;
 	struct IO_Event_Selector_URing_Waiting *waiting;
@@ -1373,6 +1383,10 @@ void Init_IO_Event_Selector_URing(VALUE IO_Event_Selector) {
 
 	rb_define_method(IO_Event_Selector_URing, "io_wait", IO_Event_Selector_URing_io_wait, 3);
 
+#ifdef RUBY_DEBUG
+	rb_define_singleton_method(IO_Event_Selector_URing, "__test_io_wait_result", IO_Event_Selector_URing_test_io_wait_result, 2);
+#endif
+
 #ifdef HAVE_RUBY_IO_BUFFER_H
 	rb_define_method(IO_Event_Selector_URing, "io_read", IO_Event_Selector_URing_io_read_compatible, -1);
 	rb_define_method(IO_Event_Selector_URing, "io_write", IO_Event_Selector_URing_io_write_compatible, -1);

test/io/event/selector/uring.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2026, by Samuel Williams.
+
+require "io/event"
+require "io/event/selector"
+
+describe IO::Event::Selector do
+	with "URing" do
+		it "does not expose unmatched poll completions as integer zero from io_wait" do
+			skip "URing is not available" unless subject.const_defined?(:URing)
+			
+			uring = subject.const_get(:URing)
+			skip "URing debug repro hook is not available" unless uring.respond_to?(:__test_io_wait_result)
+			
+			# io_uring poll completions can include flags we did not request. The
+			# current implementation filters the raw poll result before translating
+			# it to Ruby IO events, which can produce Integer(0). Ruby's socket
+			# connect path treats any non-false, non-negative result as success.
+			result = uring.__test_io_wait_result(IO::PRIORITY, IO::READABLE | IO::WRITABLE)
+			
+			expect(result).not.to be == 0
+		end
+	end
+end