How can I check whether a specific storage item was already accessed in the current block?

Question

Inside a FRAME runtime I want to check whether a specific storage item was already read in the current block. Reason is to determine the impact of storage accesses across the extrinsics of a block rather than only per extrinsic.

Basically, for every read I want to know if either one of those statements is true:

1. The storage item already been read in the current block.
2. The storage item has already been written in the current block.

In both cases the item wouldn't be included in the witness:

1. The witness does not contain duplicate keys.
2. If the key was written it can be pulled from the overlay and doesn't need to be in the witness.

Answer 1

You can not check whether a specific storage item was already read or written. You need to assume the worst case, aka always having a "fresh" read.