Use io_uring_sq_ready to count pending SQEs

Rather than account them ourselves with `pending`, just rely on liburing
itself to keep proper track.

Author: tavianator

ext/io/event/selector/uring.c

@@ -32,7 +32,6 @@ struct IO_Event_Selector_URing
 {
 	struct IO_Event_Selector backend;
 	struct io_uring ring;
-	size_t pending;
 
 	// Flag indicating whether the selector is currently blocked in a system call.
 	// Set to 1 when blocked in io_uring_wait_cqe_timeout() without GVL, 0 otherwise.
@@ -239,7 +238,6 @@ VALUE IO_Event_Selector_URing_allocate(VALUE self) {
 	IO_Event_Selector_initialize(&selector->backend, self, Qnil);
 	selector->ring.ring_fd = -1;
 
-	selector->pending = 0;
 	selector->blocked = 0;
 	selector->interrupt.descriptor = -1;
 	selector->wakeup_registered = 0;
@@ -421,15 +419,14 @@ void IO_Event_Selector_URing_dump_completion_queue(struct IO_Event_Selector_URin
 // Flush the submission queue, optionally yielding if unsuccessful.
 static
 int io_uring_submit_all(struct IO_Event_Selector_URing *selector, bool yield) {
-	while (selector->pending > 0) {
+	struct io_uring *ring = &selector->ring;
+
+	while (io_uring_sq_ready(ring) > 0) {
 		int result = io_uring_submit(&selector->ring);
 
-		if (result >= 0) {
-			// io_uring_submit() returns the number of submitted SQEs
-			selector->pending -= result;
-		} else if (result == -EBUSY || result == -EAGAIN) {
+		if (result == -EBUSY || result == -EAGAIN) {
 			if (yield) IO_Event_Selector_yield(&selector->backend);
-		} else {
+		} else if (result < 0) {
 			rb_syserr_fail(-result, "io_uring_submit_all:io_uring_submit");
 			return result;
 		}
@@ -442,23 +439,32 @@ int io_uring_submit_all(struct IO_Event_Selector_URing *selector, bool yield) {
 // Flush the submission queue if pending operations are present.
 static
 int io_uring_submit_flush(struct IO_Event_Selector_URing *selector) {
-	if (DEBUG) fprintf(stderr, "io_uring_submit_now(pending=%ld)\n", selector->pending);
+	if (DEBUG) {
+		unsigned pending = io_uring_sq_ready(&selector->ring);
+		fprintf(stderr, "io_uring_submit_flush(pending=%u)\n", pending);
+	}
 
 	return io_uring_submit_all(selector, false);
 }
 
 // Immediately flush the submission queue, yielding to the event loop if it was not successful.
 static
 int io_uring_submit_now(struct IO_Event_Selector_URing *selector) {
-	if (DEBUG) fprintf(stderr, "io_uring_submit_now(pending=%ld)\n", selector->pending);
-	
+	if (DEBUG) {
+		unsigned pending = io_uring_sq_ready(&selector->ring);
+		fprintf(stderr, "io_uring_submit_now(pending=%u)\n", pending);
+	}
+
 	return io_uring_submit_all(selector, true);
 }
 
 // Submit a pending operation. This does not submit the operation immediately, but instead defers it to the next call to `io_uring_submit_flush` or `io_uring_submit_now`. This is useful for operations that are not urgent, but should be used with care as it can lead to a deadlock if the submission queue is not flushed.
 static
 void io_uring_submit_pending(struct IO_Event_Selector_URing *selector) {
-	if (DEBUG) fprintf(stderr, "io_uring_submit_pending(ring=%p, pending=%ld)\n", &selector->ring, selector->pending);
+	if (DEBUG) {
+		unsigned pending = io_uring_sq_ready(&selector->ring);
+		fprintf(stderr, "io_uring_submit_pending(ring=%p, pending=%u)\n", &selector->ring, pending);
+	}
 }
 
 struct io_uring_sqe * io_get_sqe(struct IO_Event_Selector_URing *selector) {
@@ -471,7 +477,6 @@ struct io_uring_sqe * io_get_sqe(struct IO_Event_Selector_URing *selector) {
 		sqe = io_uring_get_sqe(&selector->ring);
 	}
 
-	selector->pending += 1;
 	return sqe;
 }