libnm: support direct string properties as NMRefString

Several properties like "connection.type" are enum-like and only take a few
known values. We can use a NMRefString to share their instances.

Currently nm_setting_duplicate() does not yet explicitly handle direct properties.
But it should, because it can handle them more efficiently. If it would do that, it
would be very cheap to "copy" a NMRefString. But even with the current implementation
will the result be deduplicated.

src/libnm-core-impl/nm-setting.c

@@ -9,6 +9,7 @@
 #include "nm-setting.h"
 
 #include "libnm-core-intern/nm-core-internal.h"
+#include "libnm-glib-aux/nm-ref-string.h"
 #include "libnm-glib-aux/nm-secret-utils.h"
 #include "nm-property-compare.h"
 #include "nm-setting-private.h"
@@ -640,6 +641,7 @@ _property_direct_set_string(const NMSettInfoSetting  *sett_info,
     nm_assert(property_info->property_type->direct_type == NM_VALUE_TYPE_STRING);
     nm_assert(((!!property_info->direct_set_string_ascii_strdown)
                + (!!property_info->direct_set_string_strip)
+               + (!!property_info->direct_string_is_refstr)
                + (property_info->direct_set_string_mac_address_len > 0)
                + (property_info->direct_set_string_ip_address_addr_family != 0))
               <= (property_info->direct_hook.set_string_fcn ? 0 : 1));
@@ -650,6 +652,12 @@ _property_direct_set_string(const NMSettInfoSetting  *sett_info,
 
     dst = _nm_setting_get_private(setting, sett_info, property_info->direct_offset);
 
+    if (property_info->direct_string_is_refstr) {
+        nm_assert(property_info->param_spec);
+        nm_assert(!NM_FLAGS_HAS(property_info->param_spec->flags, NM_SETTING_PARAM_SECRET));
+        return nm_ref_string_reset_str_upcast((const char **) dst, src);
+    }
+
     if (property_info->direct_set_string_ascii_strdown) {
         s = src ? g_ascii_strdown(src, -1) : NULL;
         goto out_take;
@@ -1074,7 +1082,9 @@ _finalize_direct(NMSetting *setting)
             char **p_val =
                 _nm_setting_get_private(setting, sett_info, property_info->direct_offset);
 
-            if (NM_FLAGS_HAS(property_info->param_spec->flags, NM_SETTING_PARAM_SECRET))
+            if (property_info->direct_string_is_refstr)
+                nm_clear_pointer(p_val, nm_ref_string_unref_upcast);
+            else if (NM_FLAGS_HAS(property_info->param_spec->flags, NM_SETTING_PARAM_SECRET))
                 nm_clear_pointer(p_val, nm_free_secret);
             else
                 nm_clear_g_free(p_val);

src/libnm-core-impl/tests/test-setting.c

@@ -4590,6 +4590,11 @@ test_setting_metadata(void)
                 g_assert(sip->property_type->direct_type == NM_VALUE_TYPE_STRING);
             if (sip->direct_set_string_strip)
                 g_assert(sip->property_type->direct_type == NM_VALUE_TYPE_STRING);
+            if (sip->direct_string_is_refstr) {
+                g_assert(sip->property_type->direct_type == NM_VALUE_TYPE_STRING);
+                g_assert(sip->param_spec);
+                g_assert(!NM_FLAGS_HAS(sip->param_spec->flags, NM_SETTING_PARAM_SECRET));
+            }
 
             if (sip->direct_set_string_mac_address_len != 0) {
                 g_assert(NM_IN_SET(sip->property_type,
@@ -4604,7 +4609,8 @@ test_setting_metadata(void)
             n_special_options = (sip->direct_set_string_mac_address_len != 0)
                                 + (!!sip->direct_set_string_strip)
                                 + (!!sip->direct_set_string_ascii_strdown)
-                                + (sip->direct_set_string_ip_address_addr_family != 0);
+                                + (sip->direct_set_string_ip_address_addr_family != 0)
+                                + (!!sip->direct_string_is_refstr);
 
             G_STATIC_ASSERT_EXPR(AF_UNSPEC + 1 != 0);
             g_assert(NM_IN_SET((int) sip->direct_set_string_ip_address_addr_family,

src/libnm-core-intern/nm-core-internal.h

@@ -807,6 +807,9 @@ struct _NMSettInfoProperty {
      * is passed to _nm_utils_ipaddr_canonical_or_invalid(). */
     bool direct_set_string_ip_address_addr_family_map_zero_to_null : 1;
 
+    /* Whether the string property is implemented as a (downcast) NMRefString. */
+    bool direct_string_is_refstr : 1;
+
     /* Usually, properties that are set to the default value for the GParamSpec
      * are not serialized to GVariant (and NULL is returned by to_dbus_data().
      * Set this flag to force always converting the property even if the value