Lock access to listener->connections

as it is accessed from multiple threads with libuv.

    WARNING: ThreadSanitizer: data race
    Write of size 8 at 0x000000000001 by thread T1:
    #0 conn_reset bin/named/controlconf.c:574
    #1 isc_nmhandle_detach netmgr/netmgr.c:1257
    #2 isc__nm_uvreq_put netmgr/netmgr.c:1389
    #3 tcp_send_cb netmgr/tcp.c:1030
    #4 <null> <null>
    #5 <null> <null>

    Previous read of size 8 at 0x000000000001 by thread T2:
    #0 conn_reset bin/named/controlconf.c:574
    #1 isc_nmhandle_detach netmgr/netmgr.c:1257
    #2 control_recvmessage bin/named/controlconf.c:556
    #3 recv_data lib/isccc/ccmsg.c:110
    #4 isc__nm_tcp_shutdown netmgr/tcp.c:1161
    #5 shutdown_walk_cb netmgr/netmgr.c:1511
    #6 uv_walk <null>
    #7 process_queue netmgr/netmgr.c:656
    #8 process_normal_queue netmgr/netmgr.c:582
    #9 process_queues netmgr/netmgr.c:590
    #10 async_cb netmgr/netmgr.c:548
    #11 <null> <null>
    #12 <null> <null>

    Location is heap block of size 265 at 0x000000000017 allocated by thread T3:
    #0 malloc <null>
    #1 default_memalloc lib/isc/mem.c:713
    #2 mem_get lib/isc/mem.c:622
    #3 isc___mem_get lib/isc/mem.c:1044
    #4 isc__mem_get lib/isc/mem.c:2432
    #5 add_listener bin/named/controlconf.c:1127
    #6 named_controls_configure bin/named/controlconf.c:1324
    #7 load_configuration bin/named/server.c:9181
    #8 run_server bin/named/server.c:9819
    #9 dispatch lib/isc/task.c:1152
    #10 run lib/isc/task.c:1344
    #11 <null> <null>

    Thread T1 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create pthreads/thread.c:73
    #2 isc_nm_start netmgr/netmgr.c:232
    #3 create_managers bin/named/main.c:909
    #4 setup bin/named/main.c:1223
    #5 main bin/named/main.c:1523

    Thread T2 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create pthreads/thread.c:73
    #2 isc_nm_start netmgr/netmgr.c:232
    #3 create_managers bin/named/main.c:909
    #4 setup bin/named/main.c:1223
    #5 main bin/named/main.c:1523

    Thread T3 (running) created by main thread at:
    #0 pthread_create <null>
    #1 isc_thread_create pthreads/thread.c:73
    #2 isc_taskmgr_create lib/isc/task.c:1434
    #3 create_managers bin/named/main.c:915
    #4 setup bin/named/main.c:1223
    #5 main bin/named/main.c:1523

    SUMMARY: ThreadSanitizer: data race bin/named/controlconf.c:574 in conn_reset

bin/named/controlconf.c

@@ -96,6 +96,7 @@ struct controllistener {
 	dns_acl_t *acl;
 	bool exiting;
 	controlkeylist_t keys;
+	isc_mutex_t connections_lock;
 	controlconnectionlist_t connections;
 	isc_socktype_t type;
 	uint32_t perm;
@@ -154,14 +155,19 @@ free_listener(controllistener_t *listener) {
 	if (listener->acl != NULL) {
 		dns_acl_detach(&listener->acl);
 	}
+	isc_mutex_destroy(&listener->connections_lock);
 
 	isc_mem_putanddetach(&listener->mctx, listener, sizeof(*listener));
 }
 
 static void
 maybe_free_listener(controllistener_t *listener) {
+	LOCK(&listener->connections_lock);
 	if (listener->exiting && ISC_LIST_EMPTY(listener->connections)) {
+		UNLOCK(&listener->connections_lock);
 		free_listener(listener);
+	} else {
+		UNLOCK(&listener->connections_lock);
 	}
 }
 
@@ -571,7 +577,9 @@ conn_reset(void *arg) {
 		return;
 	}
 
+	LOCK(&listener->connections_lock);
 	ISC_LIST_UNLINK(listener->connections, conn, link);
+	UNLOCK(&listener->connections_lock);
 #ifdef ENABLE_AFL
 	if (named_g_fuzz_type == isc_fuzz_rndc) {
 		named_fuzz_notify();
@@ -629,7 +637,9 @@ newconnection(controllistener_t *listener, isc_nmhandle_t *handle) {
 		goto cleanup;
 	}
 
+	LOCK(&listener->connections_lock);
 	ISC_LIST_APPEND(listener->connections, conn, link);
+	UNLOCK(&listener->connections_lock);
 	return (ISC_R_SUCCESS);
 
 cleanup:
@@ -1129,6 +1139,7 @@ add_listener(named_controls_t *cp, controllistener_t **listenerp,
 					 .address = *addr,
 					 .type = type };
 	isc_mem_attach(mctx, &listener->mctx);
+	isc_mutex_init(&listener->connections_lock);
 	ISC_LINK_INIT(listener, link);
 	ISC_LIST_INIT(listener->keys);
 	ISC_LIST_INIT(listener->connections);