core: compare the DNS configurations before updating DNS

DNS manager always sets `priv->config_changed = TRUE` and overwrites
the "resolv.conf" file. To fix it, compare the new configuration with
the old configuration, if there is no change, skipping the update.

Fixes-test: @ipv4_ignore_resolveconf_with_ignore_auto_dns
Fixes-test: @ipv4_ignore_resolveconf_with_ignore_auto_dns_var1

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1023

src/core/dns/nm-dns-manager.c

@@ -35,6 +35,7 @@
 #include "nm-dns-plugin.h"
 #include "nm-dns-systemd-resolved.h"
 #include "nm-dns-unbound.h"
+#include "nm-ip-config.h"
 #include "nm-l3-config-data.h"
 #include "nm-manager.h"
 #include "nm-utils.h"
@@ -99,6 +100,9 @@ typedef struct {
     char *hostname;
     guint updates_queue;
 
+    guint8 hash[HASH_LEN];      /* SHA1 hash of current DNS config */
+    guint8 prev_hash[HASH_LEN]; /* Hash when begin_updates() was called */
+
     NMDnsManagerResolvConfManager rc_manager;
     char *                        mode;
     NMDnsPlugin *                 sd_resolve_plugin;
@@ -1097,6 +1101,30 @@ update_resolv_conf(NMDnsManager *                self,
     return write_file_result;
 }
 
+static void
+compute_hash(NMDnsManager *self, const NMGlobalDnsConfig *global, guint8 buffer[static HASH_LEN])
+{
+    nm_auto_free_checksum GChecksum *sum = NULL;
+    NMDnsConfigIPData *              ip_data;
+
+    sum = g_checksum_new(G_CHECKSUM_SHA1);
+    nm_assert(HASH_LEN == g_checksum_type_get_length(G_CHECKSUM_SHA1));
+
+    if (global)
+        nm_global_dns_config_update_checksum(global, sum);
+    else {
+        const CList *head;
+
+        /* FIXME(ip-config-checksum): this relies on the fact that an IP
+         * configuration without DNS parameters gives a zero checksum. */
+        head = _mgr_get_ip_data_lst_head(self);
+        c_list_for_each_entry (ip_data, head, ip_data_lst)
+            nm_ip_config_dns_hash(ip_data->l3cd, sum, ip_data->addr_family);
+    }
+
+    nm_utils_checksum_get_digest_len(sum, buffer, HASH_LEN);
+}
+
 static gboolean
 merge_global_dns_config(NMResolvConfData *rc, NMGlobalDnsConfig *global_conf)
 {
@@ -1649,6 +1677,9 @@ update_dns(NMDnsManager *self, gboolean no_caching, gboolean force_emit, GError
     data          = nm_config_get_data(priv->config);
     global_config = nm_config_data_get_global_dns_config(data);
 
+    /* Update hash with config we're applying */
+    compute_hash(self, global_config, priv->hash);
+
     _collect_resolv_conf_data(self,
                               global_config,
                               &searches,
@@ -2006,6 +2037,10 @@ nm_dns_manager_begin_updates(NMDnsManager *self, const char *func)
 
     priv = NM_DNS_MANAGER_GET_PRIVATE(self);
 
+    /* Save current hash when starting a new batch */
+    if (priv->updates_queue == 0)
+        memcpy(priv->prev_hash, priv->hash, sizeof(priv->hash));
+
     priv->updates_queue++;
 
     _LOGD("(%s): queueing DNS updates (%d)", func, priv->updates_queue);
@@ -2016,12 +2051,15 @@ nm_dns_manager_end_updates(NMDnsManager *self, const char *func)
 {
     NMDnsManagerPrivate *priv;
     gs_free_error GError *error = NULL;
+    guint8 new[HASH_LEN];
 
     g_return_if_fail(self != NULL);
 
     priv = NM_DNS_MANAGER_GET_PRIVATE(self);
     g_return_if_fail(priv->updates_queue > 0);
 
+    compute_hash(self, nm_config_data_get_global_dns_config(nm_config_get_data(priv->config)), new);
+    priv->config_changed = (memcmp(new, priv->prev_hash, sizeof(new)) != 0) ? TRUE : FALSE;
     _LOGD("(%s): DNS configuration %s", func, priv->config_changed ? "changed" : "did not change");
 
     priv->updates_queue--;
@@ -2574,6 +2612,7 @@ nm_dns_manager_init(NMDnsManager *self)
                                                (GDestroyNotify) _dns_config_data_free,
                                                NULL);
 
+    compute_hash(self, NULL, NM_DNS_MANAGER_GET_PRIVATE(self)->hash);
     g_signal_connect(G_OBJECT(priv->config),
                      NM_CONFIG_SIGNAL_CONFIG_CHANGED,
                      G_CALLBACK(config_changed_cb),

src/core/nm-config-data.c

@@ -982,6 +982,61 @@ nm_global_dns_config_cmp(const NMGlobalDnsConfig *a,
     return 0;
 }
 
+void
+nm_global_dns_config_update_checksum(const NMGlobalDnsConfig *dns_config, GChecksum *sum)
+{
+    NMGlobalDnsDomain *domain;
+    guint              i, j;
+    guint8             v8;
+
+    g_return_if_fail(dns_config);
+    g_return_if_fail(sum);
+
+    v8 = NM_HASH_COMBINE_BOOLS(guint8,
+                               !dns_config->searches,
+                               !dns_config->options,
+                               !dns_config->domain_list);
+    g_checksum_update(sum, (guchar *) &v8, 1);
+
+    if (dns_config->searches) {
+        for (i = 0; dns_config->searches[i]; i++)
+            g_checksum_update(sum,
+                              (guchar *) dns_config->searches[i],
+                              strlen(dns_config->searches[i]) + 1);
+    }
+    if (dns_config->options) {
+        for (i = 0; dns_config->options[i]; i++)
+            g_checksum_update(sum,
+                              (guchar *) dns_config->options[i],
+                              strlen(dns_config->options[i]) + 1);
+    }
+
+    if (dns_config->domain_list) {
+        for (i = 0; dns_config->domain_list[i]; i++) {
+            domain = g_hash_table_lookup(dns_config->domains, dns_config->domain_list[i]);
+            nm_assert(domain);
+
+            v8 = NM_HASH_COMBINE_BOOLS(guint8, !domain->servers, !domain->options);
+            g_checksum_update(sum, (guchar *) &v8, 1);
+
+            g_checksum_update(sum, (guchar *) domain->name, strlen(domain->name) + 1);
+
+            if (domain->servers) {
+                for (j = 0; domain->servers[j]; j++)
+                    g_checksum_update(sum,
+                                      (guchar *) domain->servers[j],
+                                      strlen(domain->servers[j]) + 1);
+            }
+            if (domain->options) {
+                for (j = 0; domain->options[j]; j++)
+                    g_checksum_update(sum,
+                                      (guchar *) domain->options[j],
+                                      strlen(domain->options[j]) + 1);
+            }
+        }
+    }
+}
+
 static void
 global_dns_domain_free(NMGlobalDnsDomain *domain)
 {

src/core/nm-config-data.h

@@ -267,7 +267,8 @@ gboolean           nm_global_dns_config_is_empty(const NMGlobalDnsConfig *dns_co
 int                nm_global_dns_config_cmp(const NMGlobalDnsConfig *a,
                                             const NMGlobalDnsConfig *b,
                                             gboolean                 check_internal);
-void               nm_global_dns_config_free(NMGlobalDnsConfig *dns_config);
+void nm_global_dns_config_update_checksum(const NMGlobalDnsConfig *dns_config, GChecksum *sum);
+void nm_global_dns_config_free(NMGlobalDnsConfig *dns_config);
 
 NMGlobalDnsConfig *nm_global_dns_config_from_dbus(const GValue *value, GError **error);
 void               nm_global_dns_config_to_dbus(const NMGlobalDnsConfig *dns_config, GValue *value);

src/core/nm-ip-config.c

@@ -498,6 +498,77 @@ nm_ip4_config_class_init(NMIP4ConfigClass *klass)
     g_object_class_install_properties(object_class, _PROPERTY_ENUMS_LAST_ip4, obj_properties_ip4);
 }
 
+void
+nm_ip_config_dns_hash(const NML3ConfigData *l3cd, GChecksum *sum, int addr_family)
+{
+    guint              i;
+    int                val;
+    const char *const *nameservers;
+    const in_addr_t *  wins;
+    const char *const *domains;
+    const char *const *searches;
+    const char *const *options;
+    guint              num_nameservers;
+    guint              num_wins;
+    guint              num_domains;
+    guint              num_searches;
+    guint              num_options;
+
+    g_return_if_fail(l3cd);
+    g_return_if_fail(sum);
+
+    nameservers = nm_l3_config_data_get_nameservers(l3cd, addr_family, &num_nameservers);
+    for (i = 0; i < num_nameservers; i++) {
+        g_checksum_update(sum,
+                          nm_ip_addr_from_packed_array(addr_family, nameservers, i),
+                          nm_utils_addr_family_to_size(addr_family));
+    }
+
+    if (addr_family == AF_INET) {
+        wins = nm_l3_config_data_get_wins(l3cd, &num_wins);
+        for (i = 0; i < num_wins; i++)
+            g_checksum_update(sum, (guint8 *) &wins[i], 4);
+    }
+
+    domains = nm_l3_config_data_get_domains(l3cd, addr_family, &num_domains);
+    for (i = 0; i < num_domains; i++) {
+        g_checksum_update(sum, (const guint8 *) domains[i], strlen(domains[i]));
+    }
+
+    searches = nm_l3_config_data_get_searches(l3cd, addr_family, &num_searches);
+    for (i = 0; i < num_searches; i++) {
+        g_checksum_update(sum, (const guint8 *) searches[i], strlen(searches[i]));
+    }
+
+    options = nm_l3_config_data_get_dns_options(l3cd, addr_family, &num_options);
+    for (i = 0; i < num_options; i++) {
+        g_checksum_update(sum, (const guint8 *) options[i], strlen(options[i]));
+    }
+
+    val = nm_l3_config_data_get_mdns(l3cd);
+    if (val != NM_SETTING_CONNECTION_MDNS_DEFAULT)
+        g_checksum_update(sum, (const guint8 *) &val, sizeof(val));
+
+    val = nm_l3_config_data_get_llmnr(l3cd);
+    if (val != NM_SETTING_CONNECTION_LLMNR_DEFAULT)
+        g_checksum_update(sum, (const guint8 *) &val, sizeof(val));
+
+    val = nm_l3_config_data_get_dns_over_tls(l3cd);
+    if (val != NM_SETTING_CONNECTION_DNS_OVER_TLS_DEFAULT)
+        g_checksum_update(sum, (const guint8 *) &val, sizeof(val));
+
+    /* FIXME(ip-config-checksum): the DNS priority should be considered relevant
+     * and added into the checksum as well, but this can't be done right now
+     * because in the DNS manager we rely on the fact that an empty
+     * configuration (i.e. just created) has a zero checksum. This is needed to
+     * avoid rewriting resolv.conf when there is no change.
+     *
+     * The DNS priority initial value depends on the connection type (VPN or
+     * not), so it's a bit difficult to add it to checksum maintaining the
+     * assumption of checksum(empty)=0
+     */
+}
+
 /*****************************************************************************/
 
 /* public */

src/core/nm-ip-config.h

@@ -51,6 +51,8 @@ NMIPConfig *nm_ip_config_new(int addr_family, NML3Cfg *l3cfg);
 
 void nm_ip_config_take_and_unexport_on_idle(NMIPConfig *self_take);
 
+void nm_ip_config_dns_hash(const NML3ConfigData *l3cd, GChecksum *sum, int addr_family);
+
 /*****************************************************************************/
 
 static inline NML3Cfg *