Is it safe to delete elements when iterating through a storage map in Substrate?

Question

In my pallet, I have a StorageDoubleMap called MyDoubleMap. I want to delete all the kv pairs under key1. So I may write the code like below:

for (key2, _value) in MyDoubleMap::<T>::iter_prefix(key1) {
    MyDoubleMap::<T>::remove(key1, key2);
}

Apparently the map was changed during the iteration. Will it be problematic?

Answer 1

As the docs are mentioning, you will get undefined results. If you want to remove all these elements, you should directly use drain_prefix as this will give you directly what you want.