Create a set_storage_or_clear that deletes an storage item if 0u256 is passed as value

[athei]

On Ethereum writing zero to a key constitutes deleting the key. We should add a new host function that behaves the same as set_storage except that the value is deleted if zero is passed. This is useful because it allows refunding the storage deposit when a key is deleted.

Note that revive could do that without any new host function by calling clear_storage in case the key is zero. However, this would increase contract code by adding this additional check.

In the same vein we could also add a get_storage_or_zero that returns 0 of the key does not exist. It would remove some code from the contract that is needed to check the return value of get_storage.

cc @xermicus

[xermicus]

Good idea, I think we might just want to provide EVM tailored storage setters and getters but leave the more generic ones intact for other languages that are more flexible?

Regarding set_storage, other contract languages might not be limited to 32 byte storage values, and if the value is large the check might get costly and unwanted? Maybe for Solidity we should just add a set_storage_or_delete which accepts 32 bytes key and value and deletes the key if the value equals zero?

For the getter, how I implemented it in revive for now is contracts zero out the output buffer of get_storage and ignore the return value. Which results in the desired behavior. However it would be slightly more efficient if the pallet would do it. We could just change get_storage to resemble the EVM semantics, however other languages might have a use case for a fallible get_storage method, so adding an infallible get_storage_or_zero host function seems sensible to me.

[athei]

Yes, the idea is to add two new functions tailored towards EVM and leave the other ones intact.

[xermicus]

Sounds good. I can take care of this, i.e. implementing those pallet-revive plus a companion in revive. But I'll not assign it just yet as it's also a good first issue for anyone wanting to help out.