core: add nm_match_spec_join() function

We have a special implemenation nm_match_spec_split() to split
a string. We also need the reverse operation to be able to convert
a list of specs to string without loss.

src/NetworkManagerUtils.c

@@ -1419,6 +1419,85 @@ nm_match_spec_split (const char *value)
 	return g_slist_reverse (pieces);
 }
 
+/**
+ * nm_match_spec_join:
+ * @specs: the device specs to join
+ *
+ * This is based on g_key_file_parse_string_as_value(), analog to
+ * nm_match_spec_split() which is based on g_key_file_parse_value_as_string().
+ *
+ * Returns: (transfer-full): a joined list of device specs that can be
+ *   split again with nm_match_spec_split(). Note that
+ *   nm_match_spec_split (nm_match_spec_join (specs)) yields the original
+ *   result (which is not true the other way around because there are multiple
+ *   ways to encode the same joined specs string).
+ */
+char *
+nm_match_spec_join (GSList *specs)
+{
+	const char *p;
+	GString *str;
+
+	str = g_string_new ("");
+
+	for (; specs; specs = specs->next) {
+		p = specs->data;
+
+		if (!p || !*p)
+			continue;
+
+		if (str->len > 0)
+			g_string_append_c (str, ',');
+
+		/* escape leading whitespace */
+		switch (*p) {
+		case ' ':
+			g_string_append (str, "\\s");
+			p++;
+			break;
+		case '\t':
+			g_string_append (str, "\\t");
+			p++;
+			break;
+		}
+
+		for (; *p; p++) {
+			switch (*p) {
+			case '\n':
+				g_string_append (str, "\\n");
+				break;
+			case '\r':
+				g_string_append (str, "\\r");
+				break;
+			case '\\':
+				g_string_append (str, "\\\\");
+				break;
+			case ',':
+				g_string_append (str, "\\,");
+				break;
+			case ';':
+				g_string_append (str, "\\;");
+				break;
+			default:
+				g_string_append_c (str, *p);
+				break;
+			}
+		}
+
+		/* escape trailing whitespaces */
+		switch (str->str[str->len - 1]) {
+		case ' ':
+			g_string_overwrite (str, str->len - 1, "\\s");
+			break;
+		case '\t':
+			g_string_overwrite (str, str->len - 1, "\\t");
+			break;
+		}
+	}
+
+	return g_string_free (str, FALSE);
+}
+
 const char *
 nm_utils_get_shared_wifi_permission (NMConnection *connection)
 {