[khromenokroman]

@olofhagsand Hello

Description:
Related to #669

Problem

leafref_opt_search_detect() enables binary search when all nodes in xvec
have the same YANG spec and share the same parent (leaf-list) or grandparent
(list key). leafref_opt_search() then searches among all siblings under
that parent, but xvec may be only a subset of them if the leafref path
contains a predicate on the list, e.g.:

  leaf ref {
    type leafref { path "../iface[enabled='true']/name"; }
  } 

A reference to an entry that the predicate excluded is then wrongly accepted
instead of failing with instance-required.

This already happened when exactly one entry matched the predicate. Since
ff7dafe (#669), which allows list entries with different parents, it happens
for any number of matches.

Example with LEAFREF_OPTIMIZE (default):

  <top xmlns="urn:t">
    <iface><name>a</name><enabled>true</enabled></iface>
    <iface><name>b</name><enabled>false</enabled></iface>
    <iface><name>c</name><enabled>true</enabled></iface>
    <ref>b</ref>   <!-- validates OK, but must fail -->
  </top>

With LEAFREF_OPTIMIZE undefined (linear search) the same data correctly
fails validation.

Fix

Use binary search only if xvec contains all siblings with the same YANG
spec, i.e. the number of such children of the common parent equals xlen.
If the xpath filtered anything out, fall back to the linear search.

Predicates above the common parent (e.g. selecting a context entry with
/contexts/context[name=current()/...]/...) do not filter the siblings and
keep the optimization. The extra check is a single pass over the parent's
children, the same order as the existing detection loop.

Testing

• test/test_leafref.sh: added sender/active-template leafref
  /sender[enabled='true']/name; a reference to a disabled sender must fail
  validation, a reference to an enabled one must pass.

• Verified with a standalone validation program (xml_yang_validate_all_top):

  case	linear	master	this PR
  ref to disabled entry, 1 match	fail	pass	fail
  ref to disabled entry, 2 matches	fail	pass	fail
  ref to enabled entry	pass	pass	pass

• Performance unchanged for paths without predicates (the List binary search is broken #669 case):
  20000 list entries + 20000 leafrefs validate in 0.29s, vs 75s with linear search.