[master] fix dispatch.c shutdown race

4952.	[bug]		A race condition on shutdown could trigger an
			assertion failure in dispatch.c. [RT #43822]

CHANGES

+4952.	[bug]		A race condition on shutdown could trigger an
+			assertion failure in dispatch.c. [RT #43822]
+
 4591.	[port]		Addressed some python 3 compatibility issues.
 			Thanks to Ville Skytta. [RT #44955] [RT #44956]
 

lib/dns/dispatch.c

@@ -3721,9 +3721,12 @@ dns_dispatch_importrecv(dns_dispatch_t *disp, isc_event_t *event) {
 	REQUIRE((disp->attributes & DNS_DISPATCHATTR_NOLISTEN) != 0);
 	REQUIRE(event != NULL);
 
-	sevent = (isc_socketevent_t *)event;
+	if ((disp->attributes & DNS_DISPATCHATTR_NOLISTEN) == 0)
+		return;
 
+	sevent = (isc_socketevent_t *)event;
 	INSIST(sevent->n <= disp->mgr->buffersize);
+
 	newsevent = (isc_socketevent_t *)
 		    isc_event_allocate(disp->mgr->mctx, NULL,
 				      DNS_EVENT_IMPORTRECVDONE, udp_shrecv,

lib/dns/include/dns/dispatch.h

@@ -533,6 +533,9 @@ dns_dispatch_importrecv(dns_dispatch_t *disp, isc_event_t *event);
  * shared between dispatchers and clients.  If the dispatcher fails to copy
  * or send the event, nothing happens.
  *
+ * If the attribute DNS_DISPATCHATTR_NOLISTEN is not set, then
+ * the dispatch is already handling a recv; return immediately.
+ *
  * Requires:
  *\li 	disp is valid, and the attribute DNS_DISPATCHATTR_NOLISTEN is set.
  * 	event != NULL

[Andreas Hasenack]

Hi, any chance ticket #43822 can be made public? We got a report[2] in Ubuntu with a very similar crash signature

1. https://bugs.isc.org/Public/Bug/Display.html?id=43822
2. https://bugs.launchpad.net/ubuntu/+source/bind9/+bug/1833400