N-02 Relay Tokens From L1 Emits Empty Transaction Hash When Relaying USDC through CCTP

[bmzig]

The relayTokens functions [1] [2] are used to bridge tokens from L1 to ZK Stack. Both instances define a txHash variable [1] [2] which will later be assigned to the transaction hash value returned by the BRIDGE_HUB when initiating a bridging transaction. However, in both cases [1] [2] when CCTP is enabled, the _transferUsdc function is used, bypassing the BRIDGE_HUB. This function does not return a transaction hash, however, causing the relayTokens functions to emit an empty ZkStackMessageRelayed event [1] [2]. This behavior can be confusing for users, especially when trying to index the event based on the emitted hash.

In case no off-chain components rely on the emitted event, consider removing the emitted event to avoid confusion. Otherwise, consider adding thorough documentation for the ZkStackMessageRelayed event, outlining its expected behavior.

I don't believe we use ZkStackMessageRelayed anywhere off-chain, so I agree with the suggestion to remove it if it is unnecessary.

[pxrl]

+1 no instances of ZkStackMessageRelayed are found in the relayer repository.