[3.12] gh-110038: KqueueSelector must count all read/write events (GH-110039) (#110043)

[3.12] gh-110038: KqueueSelector must count all read/write events (GH-110039).
(cherry picked from commit b14f0ab51c)

Lib/test/test_selectors.py

@@ -279,6 +279,35 @@ class BaseSelectorTestCase:
 
         self.assertEqual([(wr_key, selectors.EVENT_WRITE)], result)
 
+    def test_select_read_write(self):
+        # gh-110038: when a file descriptor is registered for both read and
+        # write, the two events must be seen on a single call to select().
+        s = self.SELECTOR()
+        self.addCleanup(s.close)
+
+        sock1, sock2 = self.make_socketpair()
+        sock2.send(b"foo")
+        my_key = s.register(sock1, selectors.EVENT_READ | selectors.EVENT_WRITE)
+
+        seen_read, seen_write = False, False
+        result = s.select()
+        # We get the read and write either in the same result entry or in two
+        # distinct entries with the same key.
+        self.assertLessEqual(len(result), 2)
+        for key, events in result:
+            self.assertTrue(isinstance(key, selectors.SelectorKey))
+            self.assertEqual(key, my_key)
+            self.assertFalse(events & ~(selectors.EVENT_READ |
+                                        selectors.EVENT_WRITE))
+            if events & selectors.EVENT_READ:
+                self.assertFalse(seen_read)
+                seen_read = True
+            if events & selectors.EVENT_WRITE:
+                self.assertFalse(seen_write)
+                seen_write = True
+        self.assertTrue(seen_read)
+        self.assertTrue(seen_write)
+
     def test_context_manager(self):
         s = self.SELECTOR()
         self.addCleanup(s.close)