From: Evan Hunt <each@isc.org>
Date: Thu, 2 Dec 2021 20:04:42 +0000 (-0800)
Subject: prevent a shutdown hang on non-matching TCP responses
X-Git-Tag: v9.17.22~48^2~3
X-Git-Url: http://git.ipfire.org/gitweb/index.cgi?a=commitdiff_plain;h=5f82fc11a9cf2f847540df256584933f6402bfa2;p=thirdparty%2Fbind9.git

prevent a shutdown hang on non-matching TCP responses

When a non-matching DNS response is received by the resolver,
it calls dns_dispatch_getnext() to resume reading. This is necessary
for UDP but not for TCP, because TCP connections automatically
resume reading after any valid DNS response.

This commit adds a 'tcpreading' flag to TCP dispatches, so that
`dispatch_getnext()` can be called multiple times without subsequent
calls having any effect.
---

diff --git a/lib/dns/dispatch.c b/lib/dns/dispatch.c
index d5cbd07dcfd..ac0afb51dde 100644
--- a/lib/dns/dispatch.c
+++ b/lib/dns/dispatch.c
@@ -125,6 +125,7 @@ struct dns_dispatch {
 	isc_mutex_t lock; /*%< locks all below */
 	isc_socktype_t socktype;
 	atomic_uint_fast32_t state;
+	atomic_bool tcpreading;
 	isc_refcount_t references;
 	unsigned int shutdown_out : 1;
 
@@ -752,6 +753,8 @@ tcp_recv(isc_nmhandle_t *handle, isc_result_t result, isc_region_t *region,
 
 	REQUIRE(VALID_DISPATCH(disp));
 
+	atomic_store(&disp->tcpreading, false);
+
 	qid = disp->mgr->qid;
 
 	ISC_LIST_INIT(resps);
@@ -1531,11 +1534,14 @@ dispatch_getnext(dns_dispatch_t *disp, dns_dispentry_t *resp, int32_t timeout) {
 		break;
 
 	case isc_socktype_tcp:
-		dns_dispatch_attach(disp, &(dns_dispatch_t *){ NULL });
-		if (timeout > 0) {
-			isc_nmhandle_settimeout(disp->handle, timeout);
+		if (atomic_compare_exchange_strong(&disp->tcpreading,
+						   &(bool){ false }, true)) {
+			dns_dispatch_attach(disp, &(dns_dispatch_t *){ NULL });
+			if (timeout > 0) {
+				isc_nmhandle_settimeout(disp->handle, timeout);
+			}
+			isc_nm_read(disp->handle, tcp_recv, disp);
 		}
-		isc_nm_read(disp->handle, tcp_recv, disp);
 		break;
 
 	default: