Re-submit wakeup read immediately on completion (simulated multi-shot).

Instead of waiting until the next select() call to re-register the
wakeup eventfd read, re-submit it immediately in select_process_completions
while still on the owner thread. This keeps a read always pending and
removes one SQE submission from the hot blocking-wait path.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: samuel-williams-shopify

ext/io/event/selector/uring.c

@@ -1175,12 +1175,19 @@ unsigned select_process_completions(struct IO_Event_Selector_URing *selector) {
 			continue;
 		}
 
-		// Wakeup eventfd read completion — the read already consumed the counter,
-		// no separate drain needed. Clear the flag so the next blocking wait
-		// re-submits the read.
+		// Wakeup eventfd read completion — the read already consumed the counter.
+		// Immediately re-submit a new read so one is always pending (simulating
+		// multi-shot), avoiding an extra submission on the next blocking wait.
 		if (cqe->user_data == (uint64_t)(uintptr_t)&selector->wakeup_fd) {
-			selector->wakeup_registered = 0;
 			io_uring_cq_advance(ring, 1);
+			struct io_uring_sqe *wakeup_sqe = io_uring_get_sqe(ring);
+			if (wakeup_sqe) {
+				io_uring_prep_read(wakeup_sqe, selector->wakeup_fd, &selector->wakeup_value, sizeof(selector->wakeup_value), 0);
+				io_uring_sqe_set_data(wakeup_sqe, &selector->wakeup_fd);
+				selector->pending += 1;
+			} else {
+				selector->wakeup_registered = 0;
+			}
 			continue;
 		}