From 6bebcedb80470698987eb1f8b5ac1a39371a887f Mon Sep 17 00:00:00 2001
From: Aram Sargsyan <aram@isc.org>
Date: Mon, 14 Nov 2022 12:18:06 +0000
Subject: [PATCH] Cancel all fetch events in dns_resolver_cancelfetch()

Although 'dns_fetch_t' fetch can have two associated events, one for
each of 'DNS_EVENT_FETCHDONE' and 'DNS_EVENT_TRYSTALE' types, the
dns_resolver_cancelfetch() function is designed in a way that it
expects only one existing event, which it must cancel, and when it
happens so that 'stale-answer-client-timeout' is enabled and there
are two events, only one of them is canceled, and it results in an
assertion in dns_resolver_destroyfetch(), when it finds a dangling
event.

Change the logic of dns_resolver_cancelfetch() function so that it
cancels both the events (if they exist), and in the right order.

Conflict: change UNREACHABLE() to INSIST(0)
Reference: https://gitlab.isc.org/isc-projects/bind9/-/commit/b4a65aaea19762a3712932aa2270e8a833fbde22
(cherry picked from commit ec2098ca35039e4f81fd0aa7c525eb960b8f47bf)
---
 lib/dns/resolver.c | 52 +++++++++++++++++++++++++++++++++++-----------
 lib/ns/query.c     |  4 +++-
 2 files changed, 43 insertions(+), 13 deletions(-)

diff --git a/lib/dns/resolver.c b/lib/dns/resolver.c
index bfd5967..b1a842c 100644
--- a/lib/dns/resolver.c
+++ b/lib/dns/resolver.c
@@ -11200,8 +11200,9 @@ void
 dns_resolver_cancelfetch(dns_fetch_t *fetch) {
 	fetchctx_t *fctx;
 	dns_resolver_t *res;
-	dns_fetchevent_t *event, *next_event;
-	isc_task_t *etask;
+	dns_fetchevent_t *event = NULL;
+	dns_fetchevent_t *event_trystale = NULL;
+	dns_fetchevent_t *event_fetchdone = NULL;
 
 	REQUIRE(DNS_FETCH_VALID(fetch));
 	fctx = fetch->private;
@@ -11213,32 +11214,59 @@ dns_resolver_cancelfetch(dns_fetch_t *fetch) {
 	LOCK(&res->buckets[fctx->bucketnum].lock);
 
 	/*
-	 * Find the completion event for this fetch (as opposed
+	 * Find the events for this fetch (as opposed
 	 * to those for other fetches that have joined the same
-	 * fctx) and send it with result = ISC_R_CANCELED.
+ 	 * fctx) and send them with result = ISC_R_CANCELED.
 	 */
-	event = NULL;
 	if (fctx->state != fetchstate_done) {
+		dns_fetchevent_t *next_event = NULL;	
 		for (event = ISC_LIST_HEAD(fctx->events); event != NULL;
 		     event = next_event) {
 			next_event = ISC_LIST_NEXT(event, ev_link);
 			if (event->fetch == fetch) {
 				ISC_LIST_UNLINK(fctx->events, event, ev_link);
-				break;
+				switch (event->ev_type) {
+				case DNS_EVENT_TRYSTALE:
+					INSIST(event_trystale == NULL);
+					event_trystale = event;
+					break;
+				case DNS_EVENT_FETCHDONE:
+					INSIST(event_fetchdone == NULL);
+					event_fetchdone = event;
+					break;
+				default:
+					INSIST(0);
+				}
+				if (event_trystale != NULL &&
+				    event_fetchdone != NULL)
+				{
+					break;
+				}			
 			}
 		}
 	}
-	if (event != NULL) {
-		etask = event->ev_sender;
-		event->ev_sender = fctx;
-		event->result = ISC_R_CANCELED;
-		isc_task_sendanddetach(&etask, ISC_EVENT_PTR(&event));
+	/*
+	 * The "trystale" event must be sent before the "fetchdone" event,
+	 * because the latter clears the "recursing" query attribute, which is
+	 * required by both events (handled by the same callback function).
+	 */
+	if (event_trystale != NULL) {
+		isc_task_t *etask = event_trystale->ev_sender;
+		event_trystale->ev_sender = fctx;
+		event_trystale->result = ISC_R_CANCELED;
+		isc_task_sendanddetach(&etask, ISC_EVENT_PTR(&event_trystale));
 	}
+	if (event_fetchdone != NULL) {
+		isc_task_t *etask = event_fetchdone->ev_sender;
+		event_fetchdone->ev_sender = fctx;
+		event_fetchdone->result = ISC_R_CANCELED;
+		isc_task_sendanddetach(&etask, ISC_EVENT_PTR(&event_fetchdone));
+        }
+
 	/*
 	 * The fctx continues running even if no fetches remain;
 	 * the answer is still cached.
 	 */
-
 	UNLOCK(&res->buckets[fctx->bucketnum].lock);
 }
 
diff --git a/lib/ns/query.c b/lib/ns/query.c
index 1944a0b..1444de5 100644
--- a/lib/ns/query.c
+++ b/lib/ns/query.c
@@ -6073,7 +6073,9 @@ fetch_callback(isc_task_t *task, isc_event_t *event) {
 	CTRACE(ISC_LOG_DEBUG(3), "fetch_callback");
 
 	if (event->ev_type == DNS_EVENT_TRYSTALE) {
-		query_lookup_stale(client);
+		if (devent->result != ISC_R_CANCELED) {
+			query_lookup_stale(client);
+		}		
 		isc_event_free(ISC_EVENT_PTR(&event));
 		return;
 	}
-- 
2.33.0

