Contract trapped when implementing delegate cross-contract call

Question

I'm trying to implement a delegate cross-contract call but it fails during an e2e test. Deploying to a local node also shows that the contract is trapped when invoking this call.

My code is here: https://github.com/ical10/cross-contract-flipper/commit/dd52cbc28034184a6545a848f379e7e68c4a3be7

Anyone could point me to the right direction?

Answer 1

This is a great question. So delegating a call is often misunderstood. There isn't much documentation out there on this topic. This article does exist:

• https://medium.com/coinmonks/delegatecall-calling-another-contract-function-in-solidity-b579f804178c

However, the above article can be dense and hard to understand.

Basically when you are using ink! DelegateCall you are "delegating" the call (ink! message) to another contract's code (by using that contract's on-chain code hash).

However, there are caveats.

First of all, as demonstrated in the example, if the delegated code intends to mutate the caller's storage, a developer needs to be mindful. If the delegated code modifies layout-full storage (i.e. it contains at least non-Lazy, non-Mapping field), the CallFlags::TAIL_CALL flag needs to be specified and the storage layouts must match. This is due to the way ink! execution call stack is operated (see Stack Exchange Answer for more explanation).

• https://github.com/use-ink/ink-examples/tree/main/upgradeable-contracts#delegator

I created a working example here:

• https://github.com/brunopgalvao/cross-contract-flipper/pull/2

You could try using DelegateCall to use a message from another code hash that does a cross-contract call to another contract.

Another example can be found here:

• https://github.com/use-ink/ink-examples/blob/main/upgradeable-contracts/delegator/lib.rs

Hopefully, this sheds some light on delegate calls.