upstream: clear io_want/io_ready flags at start of poll() cycle;

avoids plausible spin during rekeying if channel io_want flags are reused
across cycles. ok markus@ deraadt@

OpenBSD-Commit-ID: 91034f855b7c73cd2591657c49ac30f10322b967

channels.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: channels.c,v 1.415 2022/03/30 21:10:25 djm Exp $ */
+/* $OpenBSD: channels.c,v 1.416 2022/04/11 22:52:08 djm Exp $ */
 /*
  * Author: Tatu Ylonen <[email protected]>
  * Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
@@ -2601,7 +2601,12 @@ channel_prepare_poll(struct ssh *ssh, struct pollfd **pfdp, u_int *npfd_allocp,
 	u_int i, oalloc, p, npfd = npfd_reserved;
 
 	channel_before_prepare_io(ssh); /* might create a new channel */
-
+	/* clear out I/O flags from last poll */
+	for (i = 0; i < sc->channels_alloc; i++) {
+		if (sc->channels[i] == NULL)
+			continue;
+		sc->channels[i]->io_want = sc->channels[i]->io_ready = 0;
+	}
 	/* Allocate 4x pollfd for each channel (rfd, wfd, efd, sock) */
 	if (sc->channels_alloc >= (INT_MAX / 4) - npfd_reserved)
 		fatal_f("too many channels"); /* shouldn't happen */