config: add default values to 'NM --print-config' output

There isn't an easy way to determine the effective value of some
configuration options as their default value can be set at build time;
the user has to search in logs or look at the manual page when
available.

This adds those default values that can be changed at build time to
the output of 'NetworkManager --print-config':

 [main]
 # plugins=ifcfg-rh,ifupdown,ifnet,ibft
 # rc-manager=symlink
 # auth-polkit=true
 dns=dnsmasq
 ...

 [logging]
 # backend=journal
 ...

1 files changed, 21 insertions, 1 deletions

diff --git a/src/nm-config-data.c b/src/nm-config-data.c
index 6f83e66566..73427d2a86 100644
--- a/src/nm-config-data.c
+++ b/src/nm-config-data.c
@@ -535,6 +535,17 @@ _nm_config_data_log_sort (const char **pa, const char **pb, gpointer dummy)
 	return 0;
 }
 
+static struct {
+	const char *group;
+	const char *key;
+	const char *value;
+} default_values[] = {
+	{ NM_CONFIG_KEYFILE_GROUP_MAIN,     "plugins",      NM_CONFIG_PLUGINS_DEFAULT },
+	{ NM_CONFIG_KEYFILE_GROUP_MAIN,     "rc-manager",   NM_CONFIG_DEFAULT_DNS_RC_MANAGER },
+	{ NM_CONFIG_KEYFILE_GROUP_MAIN,     "auth-polkit",  NM_CONFIG_DEFAULT_AUTH_POLKIT },
+	{ NM_CONFIG_KEYFILE_GROUP_LOGGING,  "backend",      NM_CONFIG_LOGGING_BACKEND_DEFAULT },
+};
+
 void
 nm_config_data_log (const NMConfigData *self,
                     const char *prefix,
@@ -544,7 +555,7 @@ nm_config_data_log (const NMConfigData *self,
 	const NMConfigDataPrivate *priv;
 	gs_strfreev char **groups = NULL;
 	gsize ngroups;
-	guint g, k;
+	guint g, k, i;
 	FILE *stream = print_stream;
 
 	g_return_if_fail (NM_IS_CONFIG_DATA (self));
@@ -591,6 +602,15 @@ nm_config_data_log (const NMConfigData *self,
 		_LOG (stream, prefix, "");
 		_LOG (stream, prefix, "[%s]%s", group, is_atomic && !stream ? " # atomic section" : "");
 
+		/* Print default values as comments */
+		for (i = 0; i < G_N_ELEMENTS (default_values); i++) {
+			if (   nm_streq (default_values[i].group, group)
+			    && !g_key_file_has_key (priv->keyfile, group, default_values[i].key, NULL)) {
+				_LOG (stream, prefix, "%s# %s=%s", key_prefix, default_values[i].key,
+				      default_values[i].value);
+			}
+		}
+
 		keys = g_key_file_get_keys (priv->keyfile, group, NULL, NULL);
 		for (k = 0; keys && keys[k]; k++) {
 			const char *key = keys[k];