validation BUGFIX when-false review fixes

Address review findings on the LYD_WHEN_FALSE handling.

- Initialize the return code before skipping a when-false node in
  lyd_validate_final_r so a when-false first sibling no longer reads an
  uninitialized value.
- Clear a stale LYD_WHEN_FALSE mark when a when later resolves to true so
  XPath no longer treats a now-valid node as non-existent on revalidation.
- Skip container default handling for when-false nodes in addition to
  their child validation.
- Add a nested when-in-when regression test confirming the node_when
  descendant cleanup suppresses the spurious child when error.

Author: szabolcs-szekely

src/validation.c

@@ -479,7 +479,9 @@ lyd_validate_unres_when(struct lyd_node **tree, const struct lys_module *mod, st
                     LY_VAL_ERR_GOTO(r, rc = r, val_opts, cleanup);
                 }
             } else {
-                /* when true */
+                /* when true; clear any stale when-false mark from a previous validation run
+                 * so XPath no longer treats the node as non-existent */
+                node->flags &= ~LYD_WHEN_FALSE;
                 node->flags |= LYD_WHEN_TRUE;
             }
 
@@ -1814,6 +1816,7 @@ lyd_validate_final_r(struct lyd_node *first, const struct lyd_node *parent, cons
         /* skip further validation for when-false nodes: the node is logically non-existent,
          * so its own must constraints and obsolete checks must not run */
         if (node->flags & LYD_WHEN_FALSE) {
+            r = LY_SUCCESS;
             goto next_iter;
         }
 
@@ -1846,20 +1849,17 @@ lyd_validate_final_r(struct lyd_node *first, const struct lyd_node *parent, cons
             break;
         }
 
-        /* skip when-false nodes: they are logically non-existent, so their children must not be validated */
-        if (node->flags & LYD_WHEN_FALSE) {
-            goto skip_children;
-        }
-
-        /* validate all children recursively */
-        r = lyd_validate_final_r(lyd_child(node), node, node->schema, NULL, ext, val_opts,
-                int_opts & ~LYD_INTOPT_SKIP_SIBLINGS, must_xp_opts, getnext_ht);
-        LY_VAL_ERR_GOTO(r, rc = r, val_opts, cleanup);
-
-skip_children:
+        /* skip when-false nodes: they are logically non-existent, so neither their children
+         * nor their container-default handling must run */
+        if (!(node->flags & LYD_WHEN_FALSE)) {
+            /* validate all children recursively */
+            r = lyd_validate_final_r(lyd_child(node), node, node->schema, NULL, ext, val_opts,
+                    int_opts & ~LYD_INTOPT_SKIP_SIBLINGS, must_xp_opts, getnext_ht);
+            LY_VAL_ERR_GOTO(r, rc = r, val_opts, cleanup);
 
-        /* set default for containers */
-        lyd_np_cont_dflt_set(node);
+            /* set default for containers */
+            lyd_np_cont_dflt_set(node);
+        }
 
         if (int_opts & LYD_INTOPT_SKIP_SIBLINGS) {
             break;

tests/utests/data/test_validation.c

@@ -1832,6 +1832,46 @@ test_when_must_cross_ref(void **state)
             LYD_VALIDATE_PRESENT | LYD_VALIDATE_MULTI_ERROR, LY_EVALID, tree);
     CHECK_LOG_CTX_APPTAG("checker requires item[a]/value=expected", "/twm4:top/checker", 0, "checker-bad");
     CHECK_LOG_CTX("When condition \"../flag = 'true'\" not satisfied.", "/twm4:top/item[name='a']", 0);
+
+    /* Regression: a when-false node with a descendant that also carries a when
+     * must NOT emit a spurious when error for the descendant. The descendant's
+     * when references its when-gated ancestor, so its evaluation returns
+     * LY_EINCOMPLETE and it stays queued in node_when until the ancestor is
+     * resolved to false. The node_when descendant cleanup in
+     * lyd_validate_unres_when then removes it before it can be (re-)evaluated.
+     * Without that cleanup, the descendant's own when is evaluated against the
+     * logically non-existent subtree and a second, spurious when error is
+     * reported. Only the ancestor's when error is expected. */
+    const char *schema5 =
+            "module twm5 {\n"
+            "    namespace urn:tests:twm5;\n"
+            "    prefix twm5;\n"
+            "    yang-version 1.1;\n"
+            "\n"
+            "    container top {\n"
+            "        leaf flag {\n"
+            "            type boolean;\n"
+            "        }\n"
+            "        container outer {\n"
+            "            when \"../flag = 'true'\";\n"
+            "            leaf inner {\n"
+            "                when \"../../flag = 'true'\";\n"
+            "                type string;\n"
+            "            }\n"
+            "        }\n"
+            "    }\n"
+            "}";
+    const char *data5 =
+            "<top xmlns=\"urn:tests:twm5\">\n"
+            "  <flag>false</flag>\n"
+            "  <outer><inner>x</inner></outer>\n"
+            "</top>\n";
+
+    UTEST_ADD_MODULE(schema5, LYS_IN_YANG, NULL, NULL);
+
+    CHECK_PARSE_LYD_PARAM(data5, LYD_XML, 0,
+            LYD_VALIDATE_PRESENT | LYD_VALIDATE_MULTI_ERROR, LY_EVALID, tree);
+    CHECK_LOG_CTX("When condition \"../flag = 'true'\" not satisfied.", "/twm5:top/outer", 0);
 }
 
 int