[lamestllama]

What

enum-value(), bit-is-set() and deref() now check that the first node of their node-set argument is an element with a schema node before reading its schema and value. Any other first node has no value, so:

• enum-value() returns NaN (RFC 7950, section 10.5.1: "If the first node in "nodes" is not of type "enumeration", it returns NaN"),
• bit-is-set() returns false (section 10.6.1),
• deref() returns an empty node-set.

derived-from() and derived-from-or-self() already skip such nodes (if (!sleaf || !(sleaf->nodetype & LYD_NODE_TERM)) in xpath_derived_()), and the three functions now behave the same way.

Why

lyd_validate_dummy_when() evaluates the when of a missing node on an opaque dummy node created by lyd_new_opaq(), whose schema is NULL. The three functions dereferenced that NULL schema, so validating a schema such as this one crashed, even on empty input:

leaf v {
  when "enum-value(.) = 1";
  type enumeration {
    enum x;
    enum y;
  }
  mandatory true;
}

RFC 7950, section 7.21.5, gives the dummy node no value: "replacing all instances of the data node for which the "when" statement is defined with a single dummy node with the same name, but with no value and no children. If no such instance exists, the dummy node is tentatively created." Treating it as a node without a value is therefore the specified result, not only a way to avoid the crash.

The same missing check also crashed on the root node, which is stored in the set with a NULL node: for example must "enum-value(/) = 1", bit-is-set(/, 'f') or deref(/). The type check covers this case too. A metadata node is no longer read as if it were a data node; like the root node and an opaque node, it now gives no value to these three functions. A text node (text()) is skipped as well, as xpath_derived_() already does; before, these three functions read the value of its parent element.

Fixes #2604

Test

test_no_value in tests/utests/basic/test_xpath.c:

• four presence containers each hold a missing mandatory leaf whose when is true only if the function finds no value (string(enum-value(.)) = 'NaN', not(bit-is-set(., 'f')), not(deref(.)), not(derived-from-or-self(., 'c:one'))); each container alone is refused with Mandatory node "v" instance does not exist., which shows that the dummy node is read as having no value;
• lyd_eval_xpath() on a small tree checks string(enum-value(/)) = 'NaN' (true), bit-is-set(/, 'f') (false) and deref(/) (false).

Before the fix, utest_xpath stops at test_no_value with a segmentation fault (16 of 17 tests pass). After the fix, all 17 pass, utest_validation passes (22 of 22), and the whole ctest run passes (69 of 69; Debug build, GCC 13.3, aarch64).

[michalvasko]

Looks good, thanks.