Keep track of submitted logs

[s1na]

Now

Any log can be submitted to network infinite times, and the subscriber would be invoked every time.

Goal

• Keep track of every submitted log.
• Check if log has been submitted before in submitLog.
• And let relayers know (by emitting events) which ones have been submitted.

Possible solutions

• Store the root of a sparse merkle tree on-chain.
  • Relayers would have to, in addition to submitting log proofs, provide proof of non-inclusion of log in the SMT.
  • Contracts wouldn't be able to query ethbase to see if a log has been emitted recently.
  • Each block, only one transaction would modify the merkle root, and the rest of txes will fail, as their proof becomes invalid. Discussion regarding this issue.

[HarryR]

Building any tree structure on-chain will be more expensive than a simple dictionary/mapping check of 'has log been submitted before'. Keep it simple, if you want to prevent replays, use a dictionary of 'events which have been relayed'. e.g. building a tree on-chain will cost at least O(log2(N)) storage operations per insert, whereas just keeping a simple mapping is O(1). The average and worst cases for on-chain SMT are much higher than the KISS solution.