core: fall back to loading all known settings plugins

Currently it is possible to specify a list of default settings plugins
to be used when configuration doesn't contain the main.plugins key.

We want to add a mechanism that allows to automatically load any
plugin found in the plugins directory without needing
configuration. This mechanism is useful when plugins are shipped in a
different, optional subpackage, to automatically use them.

With such mechanism, the actual list of plugins will be determined
(in order of evaluation):

 1. via explicit user configuration in /etc, if any
 2. via distro configuration in /usr, if any
 3. using the build-time default, if any
 4. looking for known plugins in /usr/lib

5 files changed, 46 insertions, 27 deletions

diff --git a/NEWS b/NEWS
index 35f9be8c51..1712a35e4d 100644
--- a/NEWS
+++ b/NEWS
@@ -13,6 +13,11 @@ USE AT YOUR OWN RISK.  NOT RECOMMENDED FOR PRODUCTION USE!
 * libnm: add new dummy crypto backend "null" that does nothing.
 * Veth devices with name "eth*" are now managed by default via the
   udev rule. This is to support managing the network in LXD containers.
+* When the list of plugins is not specified via "main.plugins" in
+  NetworkManager.conf and no build-time default is set with
+  "--with-config-plugins-default" configure argument, now all known
+  plugins found in the plugin directory are loaded (and the built-in
+  "keyfile" plugin is preferred over others).
 
 =============================================
 NetworkManager-1.36
diff --git a/config.h.meson b/config.h.meson
index 192246e558..7d1feb53ad 100644
--- a/config.h.meson
+++ b/config.h.meson
@@ -82,6 +82,12 @@
 /* Path to netconfig */
 #mesondefine NETCONFIG_PATH
 
+/* Build with ifcfg-rh settings plugin */
+#mesondefine WITH_CONFIG_PLUGIN_IFCFG_RH
+
+/* Build with ifupdown settings plugin */
+#mesondefine WITH_CONFIG_PLUGIN_IFUPDOWN
+
 /* The default value of the logging.audit configuration option */
 #mesondefine NM_CONFIG_DEFAULT_LOGGING_AUDIT
 
diff --git a/configure.ac b/configure.ac
index cd373d4d34..46013af1d8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -185,12 +185,6 @@ AC_ARG_WITH(config-plugins-default,
             AS_HELP_STRING([--with-config-plugins-default=PLUGINS],
                            [Default configuration option for main.plugins setting, used as fallback if the configuration option is unset]),
             [config_plugins_default="$withval"], [config_plugins_default=""])
-if test -z "$config_plugins_default" -o "$config_plugins_default" = no; then
-	config_plugins_default=''
-	test "$enable_ifcfg_rh" = "yes"           && config_plugins_default="$config_plugins_default,ifcfg-rh"
-	test "$enable_ifupdown" = "yes"           && config_plugins_default="$config_plugins_default,ifupdown"
-	config_plugins_default="${config_plugins_default#,}"
-fi
 
 AC_DEFINE_UNQUOTED(NM_CONFIG_DEFAULT_MAIN_PLUGINS, "$config_plugins_default", [Default configuration option for main.plugins setting])
 
@@ -199,6 +193,18 @@ if test "$enable_ifcfg_rh" = "yes"; then
 fi
 AC_SUBST(DISTRO_NETWORK_SERVICE)
 
+if test "$enable_ifcfg_rh" = yes; then
+	AC_DEFINE(WITH_CONFIG_PLUGIN_IFCFG_RH, 1, [Build with ifcfg-rh settings plugin])
+else
+	AC_DEFINE(WITH_CONFIG_PLUGIN_IFCFG_RH, 0, [Build with ifcfg-rh settings plugin])
+fi
+
+if test "$enable_ifupdown" = yes; then
+	AC_DEFINE(WITH_CONFIG_PLUGIN_IFUPDOWN, 1, [Build with ifupdown settings plugin])
+else
+	AC_DEFINE(WITH_CONFIG_PLUGIN_IFUPDOWN, 0, [Build with ifupdown settings plugin])
+fi
+
 # Code coverage
 GNOME_CODE_COVERAGE
 
diff --git a/meson.build b/meson.build
index c1016e0068..b5c6944659 100644
--- a/meson.build
+++ b/meson.build
@@ -308,21 +308,11 @@ enable_ifcfg_rh = get_option('ifcfg_rh') or (distro == 'redhat')
 enable_ifupdown = get_option('ifupdown') or (distro == 'debian')
 
 config_plugins_default = get_option('config_plugins_default')
-if config_plugins_default == ''
-  config_plugins = []
-
-  if enable_ifcfg_rh
-    config_plugins += ['ifcfg-rh']
-  endif
-
-  if enable_ifupdown
-    config_plugins += ['ifupdown']
-  endif
-
-  config_plugins_default = ','.join(config_plugins)
-endif
 config_h.set_quoted('NM_CONFIG_DEFAULT_MAIN_PLUGINS', config_plugins_default)
 
+config_h.set10('WITH_CONFIG_PLUGIN_IFCFG_RH', enable_ifcfg_rh)
+config_h.set10('WITH_CONFIG_PLUGIN_IFUPDOWN', enable_ifupdown)
+
 config_h.set_quoted('NM_DIST_VERSION', dist_version)
 
 enable_wifi = get_option('wifi')
diff --git a/src/core/settings/nm-settings.c b/src/core/settings/nm-settings.c
index da09571b0d..6619d3e0a2 100644
--- a/src/core/settings/nm-settings.c
+++ b/src/core/settings/nm-settings.c
@@ -3266,7 +3266,7 @@ add_plugin(NMSettings *self, NMSettingsPlugin *plugin, const char *pname, const
 }
 
 static gboolean
-add_plugin_load_file(NMSettings *self, const char *pname, GError **error)
+add_plugin_load_file(NMSettings *self, const char *pname, gboolean ignore_not_found, GError **error)
 {
     gs_free char                     *full_name = NULL;
     gs_free char                     *path      = NULL;
@@ -3281,10 +3281,12 @@ add_plugin_load_file(NMSettings *self, const char *pname, GError **error)
 
     if (stat(path, &st) != 0) {
         errsv = errno;
-        _LOGW("could not load plugin '%s' from file '%s': %s",
-              pname,
-              path,
-              nm_strerror_native(errsv));
+        if (!ignore_not_found) {
+            _LOGW("could not load plugin '%s' from file '%s': %s",
+                  pname,
+                  path,
+                  nm_strerror_native(errsv));
+        }
         return TRUE;
     }
     if (!S_ISREG(st.st_mode)) {
@@ -3378,7 +3380,7 @@ load_plugins(NMSettings *self, const char *const *plugins, GError **error)
             continue;
         }
 
-        success = add_plugin_load_file(self, pname, error);
+        success = add_plugin_load_file(self, pname, FALSE, error);
         if (!success)
             break;
     }
@@ -3872,8 +3874,18 @@ nm_settings_start(NMSettings *self, GError **error)
     /* Load the plugins; fail if a plugin is not found. */
     plugins = nm_config_data_get_plugins(nm_config_get_data_orig(priv->config), TRUE);
 
-    if (!load_plugins(self, (const char *const *) plugins, error))
-        return FALSE;
+    if (plugins && plugins[0]) {
+        if (!load_plugins(self, (const char *const *) plugins, error))
+            return FALSE;
+    } else {
+        add_plugin_keyfile(self);
+#if WITH_CONFIG_PLUGIN_IFCFG_RH
+        add_plugin_load_file(self, "ifcfg-rh", TRUE, NULL);
+#endif
+#if WITH_CONFIG_PLUGIN_IFUPDOWN
+        add_plugin_load_file(self, "ifupdown", TRUE, NULL);
+#endif
+    }
 
     for (iter = priv->plugins; iter; iter = iter->next) {
         NMSettingsPlugin *plugin = NM_SETTINGS_PLUGIN(iter->data);