ly_path_parse OPTIMIZE fail faster on invalid path

`ly_path_parse()` is called from various functions like
`lyplg_type_store_node_instanceid()` and `lyplg_type_lypath_new()` with
the `LY_PATH_BEGIN_ABSOLUTE` flag set.

This means that the path is expected to start with a '/' character.
If it doesn't, this could mean that the value is not a valid path.

This can happen while finding a union type, where it is not optimal to
call `lyxp_expr_parse()`, which is much heavier to compute.

Author: irfanHaslanded

src/path.c

@@ -328,6 +328,8 @@ ly_path_parse(const struct ly_ctx *ctx, const struct lysc_node *ctx_node, const
     struct lyxp_expr *exp = NULL;
     uint32_t tok_idx, cur_len;
     const char *cur_node, *prev_prefix = NULL, *ptr;
+    char path_begin_str[2] = {0};
+
     ly_bool is_abs;
 
     assert((begin == LY_PATH_BEGIN_ABSOLUTE) || (begin == LY_PATH_BEGIN_EITHER));
@@ -339,6 +341,14 @@ ly_path_parse(const struct ly_ctx *ctx, const struct lysc_node *ctx_node, const
         LOG_LOCSET(ctx_node, NULL);
     }
 
+    /* check if path begins with '/' if expected to and fail early if not */
+    if ((begin == LY_PATH_BEGIN_ABSOLUTE) && str_path && (str_path[0] != '/')) {
+        path_begin_str[0] = str_path[0];
+        LOGVAL(ctx, LY_VCODE_XP_INTOK2, path_begin_str, str_path, "Operator(Path)");
+        ret = LY_EVALID;
+        goto error;
+    }
+
     /* parse as a generic XPath expression, reparse is performed manually */
     LY_CHECK_GOTO(ret = lyxp_expr_parse(ctx, str_path, path_len, 0, &exp), error);
     tok_idx = 0;

tests/utests/types/instanceid.c

@@ -200,7 +200,7 @@ test_data_xml(void **state)
 
     TEST_ERROR_XML2("<cont xmlns=\"urn:tests:mod\"/>",
             "defs", "xmlns:m=\"urn:tests:mod\"", "l1", "[1]", LY_EVALID);
-    CHECK_LOG_CTX("Invalid instance-identifier \"[1]\" value - syntax error: Unexpected XPath token \"[\" (\"[1]\"), expected \"Operator(Path)\".",
+    CHECK_LOG_CTX("Invalid instance-identifier \"[1]\" value - syntax error: Unexpected XPath token \"[\" (\"[1]</l1>\"), expected \"Operator(Path)\".",
             "/defs:l1", 1);
 
     TEST_ERROR_XML2("<cont xmlns=\"urn:tests:mod\"><l2/></cont>",