mhnsw: simplify memory management of returned results

instead of pointers to FVectorRef's (which are stored elsewhere)
let's return one big array of all refs. Freeing this array will
free the complete result set.

sql/vector_mhnsw.cc

@@ -739,31 +739,30 @@ int mhnsw_first(TABLE *table, KEY *keyinfo, Item *dist, ulonglong limit)
                             ef_search, 0, &candidates))
     return err;
 
-  // 8. return results
-  FVectorRef **context= thd->alloc<FVectorRef*>(limit + 1);
+  size_t context_size=limit * h->ref_length + sizeof(ulonglong);
+  char *context= thd->alloc(context_size);
   graph->context= context;
 
-  FVectorRef **ptr= context;
+  *(ulonglong*)context= limit;
+  context+= context_size;
+
   while (limit--)
-    *ptr++= candidates.pop();
-  *ptr= nullptr;
+  {
+    context-= h->ref_length;
+    memcpy(context, candidates.pop()->get_ref(), h->ref_length);
+  }
+  DBUG_ASSERT(context - sizeof(ulonglong) == graph->context);
 
   return mhnsw_next(table);
 }
 
 int mhnsw_next(TABLE *table)
 {
-  FVectorRef ***context= (FVectorRef ***)&table->hlindex->context;
-  FVectorRef *cur_vec= **context;
-  if (cur_vec)
+  uchar *ref= (uchar*)(table->hlindex->context);
+  if (ulonglong *limit= (ulonglong*)ref)
   {
-    int err= table->file->ha_rnd_pos(table->record[0],
-                                     (uchar *)(cur_vec)->get_ref());
-    // release vectors
-    // delete cur_vec;
-
-    (*context)++;
-    return err;
+    ref+= sizeof(ulonglong) + (--*limit) * table->file->ref_length;
+    return table->file->ha_rnd_pos(table->record[0], ref);
   }
   return HA_ERR_END_OF_FILE;
 }