libi3: validate UTF8 strings

Will validate container / window titles.

Fixes #3156.

src/load_layout.c

@@ -611,9 +611,16 @@ void tree_append_json(Con *con, const char *buf, const size_t len, char **errorm
     yajl_config(hand, yajl_allow_comments, true);
     /* Allow multiple values, i.e. multiple nodes to attach */
     yajl_config(hand, yajl_allow_multiple_values, true);
-    /* Allow strings that are not valid UTF8. Could be possible if a container
-     * name contains such characters. If yajl stops parsing because of this, an
-     * in-place restart could fail: see #3156. */
+    /* We don't need to validate that the input is valid UTF8 here.
+     * tree_append_json is called in two cases:
+     * 1. With the append_layout command. json_validate is called first and will
+     *    fail on invalid UTF8 characters so we don't need to recheck.
+     * 2. With an in-place restart. The rest of the codebase should be
+     *    responsible for producing valid UTF8 JSON output. If not,
+     *    tree_append_json will just preserve invalid UTF8 strings in the tree
+     *    instead of failing to parse the layout file which could lead to
+     *    problems like in #3156.
+     * Either way, disabling UTF8 validation slightly speeds up yajl. */
     yajl_config(hand, yajl_dont_validate_strings, true);
     json_node = con;
     to_focus = NULL;