cs & io: fix UDP receive poller registration

Closes #3590

pkgs/racket-test-core/tests/racket/udp.rktl

@@ -195,6 +195,16 @@
 (test #f sync/timeout 0.05 udp2-r)
 (test #f sync/timeout 0.05 (udp-receive!-evt udp2 us1))
 (test #f sync/timeout 0.05 (udp-receive!-evt udp2 (make-bytes 0)))
+(when (run-unreliable-tests? 'timing)
+  (let loop ([retries 5])
+    (if (zero? retries)
+        (test #t 'udp-receive-evt-does-not-spin #f)
+        (let ([start (current-process-milliseconds)]
+              [wait 0.05])
+          (test #f sync/timeout wait (udp-receive!-evt udp2 (make-bytes 0)))
+          (or (< (- (current-process-milliseconds) start)
+                 (/ wait 5))
+              (loop (sub1 retries)))))))
 
 ;; break behavior
 (let ([t (parameterize-break #f

racket/src/cs/schemified/io.scm

@@ -37414,7 +37414,7 @@
 (define cell.2 (unsafe-make-place-local ""))
 (define struct:udp-receiving-evt
   (make-record-type-descriptor* 'udp-receive-evt #f #f #f #f 2 0))
-(define effect_2721
+(define effect_2847
   (struct-type-install-properties!
    struct:udp-receiving-evt
    'udp-receive-evt
@@ -37449,7 +37449,7 @@
                        (unsafe-place-local-ref cell.1)
                        (udp-s (|#%app| udp-receiving-evt-u self_0))
                        ps_0
-                       2)))
+                       1)))
                    (values #f self_0))))))))))))
    (current-inspector)
    #f

racket/src/io/network/udp-receive.rkt

@@ -149,7 +149,7 @@
              (sandman-poll-ctx-add-poll-set-adder!
               poll-ctx
               (lambda (ps)
-                (rktio_poll_add rktio (udp-s (udp-receiving-evt-u self)) ps RKTIO_POLL_WRITE)))
+                (rktio_poll_add rktio (udp-s (udp-receiving-evt-u self)) ps RKTIO_POLL_READ)))
              (values #f self)])]))))
   #:reflection-name 'udp-receive-evt
   #:authentic)