Fix EVDNS_BASE_DISABLE_WHEN_INACTIVE (#1493)

I faced with strange problem: event loop doesn't exit after dns resolving with
`EVDNS_BASE_DISABLE_WHEN_INACTIVE`.

Stand:
- Ubuntu 22;
- libevent release-2.1.12-stable
- `resolve.conf` contains 2 nameservers;
- I use `evdns_base_new` with `EVDNS_BASE_DISABLE_WHEN_INACTIVE | EVDNS_BASE_INITIALIZE_NAMESERVERS` to avoid OS specific code.

After small investigation, look like events related with dns sockets added to
event_base before `evdns->disable_when_inactive` was initialized. `libevent`
did epoll_ctl(DEL) after resolving completed on the first socket, but the
second socket remained in the `epoll` interest list.

evdns.c

@@ -4880,6 +4880,10 @@ evdns_base_new(struct event_base *event_base, int flags)
 	}
 #undef EVDNS_BASE_ALL_FLAGS
 
+	if (flags & EVDNS_BASE_DISABLE_WHEN_INACTIVE) {
+		base->disable_when_inactive = 1;
+	}
+
 	if (flags & EVDNS_BASE_INITIALIZE_NAMESERVERS) {
 		int r;
 		int opts = DNS_OPTIONS_ALL;
@@ -4890,16 +4894,13 @@ evdns_base_new(struct event_base *event_base, int flags)
 #ifdef _WIN32
 		r = evdns_base_config_windows_nameservers(base);
 #else
-		r = evdns_base_resolv_conf_parse(base, opts, "/etc/resolv.conf");
+		r = evdns_base_resolv_conf_parse(base, opts, evutil_resolvconf_filename_());
 #endif
 		if (r && (EVDNS_ERROR_NO_NAMESERVERS_CONFIGURED != r)) {
 			evdns_base_free_and_unlock(base, 0);
 			return NULL;
 		}
 	}
-	if (flags & EVDNS_BASE_DISABLE_WHEN_INACTIVE) {
-		base->disable_when_inactive = 1;
-	}
 
 	EVDNS_UNLOCK(base);
 	return base;

evutil.c

@@ -1849,6 +1849,23 @@ evutil_set_evdns_getaddrinfo_cancel_fn_(evdns_getaddrinfo_cancel_fn fn)
 		evdns_getaddrinfo_cancel_impl = fn;
 }
 
+static const char *evutil_custom_resolvconf_filename = NULL;
+
+void
+evutil_set_resolvconf_filename_(const char *filename)
+{
+	evutil_custom_resolvconf_filename = filename;
+}
+
+const char *
+evutil_resolvconf_filename_(void)
+{
+	if (evutil_custom_resolvconf_filename)
+		return evutil_custom_resolvconf_filename;
+
+	return "/etc/resolv.conf";
+}
+
 /* Internal helper function: act like evdns_getaddrinfo if dns_base is set;
  * otherwise do a blocking resolve and pass the result to the callback in the
  * way that evdns_getaddrinfo would.

test/regress_dns.c

@@ -1185,6 +1185,80 @@ end:
 	if (dns)
 		evdns_base_free(dns, 0);
 }
+
+static const char *dns_resolvconf_with_one_nameserver =
+	"nameserver 127.0.0.53\n";
+
+static void
+dns_initialize_inactive_one_nameserver_test(void *arg)
+{
+	struct basic_test_data *data = arg;
+	struct event_base *base = data->base;
+	struct evdns_base *dns = NULL;
+
+	char *filename = NULL;
+
+	tt_int_op(regress_make_tmpfile(dns_resolvconf_with_one_nameserver,
+				  strlen(dns_resolvconf_with_one_nameserver), &filename),
+		!=, -1);
+
+	tt_assert(filename);
+
+	evutil_set_resolvconf_filename_(filename);
+
+	dns = evdns_base_new(base,
+		EVDNS_BASE_INITIALIZE_NAMESERVERS | EVDNS_BASE_DISABLE_WHEN_INACTIVE);
+	tt_assert(dns);
+
+	tt_int_op(event_base_loop(base, EVLOOP_NONBLOCK), ==, 1);
+
+end:
+	if (dns)
+		evdns_base_free(dns, 0);
+	if (filename) {
+		unlink(filename);
+		free(filename);
+	}
+	evutil_set_resolvconf_filename_(NULL);
+}
+
+static const char *dns_resolvconf_with_two_nameservers =
+	"nameserver 127.0.0.53\n"
+	"nameserver 127.0.0.53\n";
+
+static void
+dns_initialize_inactive_two_nameservers_test(void *arg)
+{
+	struct basic_test_data *data = arg;
+	struct event_base *base = data->base;
+	struct evdns_base *dns = NULL;
+
+	char *filename = NULL;
+
+	tt_int_op(regress_make_tmpfile(dns_resolvconf_with_two_nameservers,
+				  strlen(dns_resolvconf_with_two_nameservers), &filename),
+		!=, -1);
+
+	tt_assert(filename);
+
+	evutil_set_resolvconf_filename_(filename);
+
+	dns = evdns_base_new(base,
+		EVDNS_BASE_INITIALIZE_NAMESERVERS | EVDNS_BASE_DISABLE_WHEN_INACTIVE);
+	tt_assert(dns);
+
+	tt_int_op(event_base_loop(base, EVLOOP_NONBLOCK), ==, 1);
+
+end:
+	if (dns)
+		evdns_base_free(dns, 0);
+	if (filename) {
+		unlink(filename);
+		free(filename);
+	}
+	evutil_set_resolvconf_filename_(NULL);
+}
+
 #ifndef _WIN32
 #define RESOLV_FILE "empty-resolv.conf"
 static void
@@ -3036,6 +3110,10 @@ struct testcase_t dns_testcases[] = {
 	{ "initialize_nameservers", dns_initialize_nameservers_test,
 	  TT_FORK|TT_NEED_BASE, &basic_setup, NULL },
 #ifndef _WIN32
+	{"initialize_with_one_inactive_nameserver", dns_initialize_inactive_one_nameserver_test,
+	  TT_FORK | TT_NEED_BASE, &basic_setup, NULL},
+	{"initialize_with_two_inactive_nameservers", dns_initialize_inactive_two_nameservers_test,
+ 	  TT_FORK | TT_NEED_BASE, &basic_setup, NULL},
 	{ "nameservers_no_default", dns_nameservers_no_default_test,
 	  TT_FORK|TT_NEED_BASE, &basic_setup, NULL },
 	{ "no_nameservers_configured", dns_nameservers_no_nameservers_configured_test,

util-internal.h

@@ -417,6 +417,12 @@ typedef void (*evdns_getaddrinfo_cancel_fn)(
 EVENT2_EXPORT_SYMBOL
 void evutil_set_evdns_getaddrinfo_cancel_fn_(evdns_getaddrinfo_cancel_fn fn);
 
+/* Customization point to override "/etc/resolv.conf" */
+EVENT2_EXPORT_SYMBOL
+void evutil_set_resolvconf_filename_(const char *filename);
+EVENT2_EXPORT_SYMBOL
+const char *evutil_resolvconf_filename_(void);
+
 EVENT2_EXPORT_SYMBOL
 struct evutil_addrinfo *evutil_new_addrinfo_(struct sockaddr *sa,
     ev_socklen_t socklen, const struct evutil_addrinfo *hints);