ITS#7771 fix cursor tracking on fake pages

node_del shifts nodes around, cursors pointing at fake pages
need to have their mc_pg[0] corrected.

libraries/liblmdb/mdb.c

@@ -6783,8 +6783,14 @@ put_sub:
 				for (m2 = mc->mc_txn->mt_cursors[mc->mc_dbi]; m2; m2=m2->mc_next) {
 					if (m2 == mc || m2->mc_snum < mc->mc_snum) continue;
 					if (!(m2->mc_flags & C_INITIALIZED)) continue;
-					if (m2->mc_pg[i] == mp && m2->mc_ki[i] == mc->mc_ki[i]) {
+					if (m2->mc_pg[i] == mp) {
+						if (m2->mc_ki[i] == mc->mc_ki[i]) {
 							mdb_xcursor_init2(m2, mx, new_dupdata);
+						} else if (!insert_key) {
+							MDB_node *n2 = NODEPTR(mp, m2->mc_ki[i]);
+							if (!(n2->mn_flags & F_SUBDATA))
+								m2->mc_xcursor->mx_cursor.mc_pg[0] = NODEDATA(n2);
+						}
 					}
 				}
 			}
@@ -6882,12 +6888,18 @@ mdb_cursor_del(MDB_cursor *mc, unsigned int flags)
 					mdb_node_shrink(mp, mc->mc_ki[mc->mc_top]);
 					leaf = NODEPTR(mp, mc->mc_ki[mc->mc_top]);
 					mc->mc_xcursor->mx_cursor.mc_pg[0] = NODEDATA(leaf);
-					/* fix other sub-DB cursors pointed at this fake page */
+					/* fix other sub-DB cursors pointed at fake pages on this page */
 					for (m2 = mc->mc_txn->mt_cursors[mc->mc_dbi]; m2; m2=m2->mc_next) {
 						if (m2 == mc || m2->mc_snum < mc->mc_snum) continue;
-						if (m2->mc_pg[mc->mc_top] == mp &&
-							m2->mc_ki[mc->mc_top] == mc->mc_ki[mc->mc_top])
+						if (m2->mc_pg[mc->mc_top] == mp) {
+							if (m2->mc_ki[mc->mc_top] == mc->mc_ki[mc->mc_top]) {
 								m2->mc_xcursor->mx_cursor.mc_pg[0] = NODEDATA(leaf);
+							} else {
+								MDB_node *n2 = NODEPTR(mp, m2->mc_ki[mc->mc_top]);
+								if (!(n2->mn_flags & F_SUBDATA))
+									m2->mc_xcursor->mx_cursor.mc_pg[0] = NODEDATA(n2);
+							}
+						}
 					}
 				}
 				mc->mc_db->md_entries--;