DAOS-17148 common: btree delete may move target out of boundary - b26

During btr_node_del_rec(), if the record to be deleted from current
node is the last one, the logic will try to migrate one record from
its sibling node. If current node is at the tail position in parent
node, original logic may migrate an invalid record from a non-exist
sibling node on the right. That may cause kinds of corruption since
the content in such invalid record is undefined.

Signed-off-by: Fan Yong <[email protected]>

src/common/btree.c

@@ -1,5 +1,6 @@
 /**
  * (C) Copyright 2016-2024 Intel Corporation.
+ * (C) Copyright 2025 Hewlett Packard Enterprise Development LP
  *
  * SPDX-License-Identifier: BSD-2-Clause-Patent
  */
@@ -3152,12 +3153,13 @@ btr_node_del_rec(struct btr_context *tcx, struct btr_trace *par_tr,
 		D_DEBUG(DB_TRACE, "Parent trace at=%d, key_nr=%d\n",
 			par_tr->tr_at, par_nd->tn_keyn);
 
+		D_ASSERT(par_tr->tr_at < par_nd->tn_keyn);
+
 		if (par_tr->tr_at == 0) {
 			/* only has sibling on the right side */
 			sib_off = btr_node_child_at(tcx, par_tr->tr_node, 1);
 			sib_on_right = true;
-
-		} else if (par_tr->tr_at == par_nd->tn_keyn) {
+		} else if (par_tr->tr_at == par_nd->tn_keyn - 1) {
 			/* only has sibling on the left side */
 			sib_off = btr_node_child_at(tcx, par_tr->tr_node,
 						     par_tr->tr_at - 1);