As i’m finalizing the BIP, @ajtowns suggested to me a neat idea.
In addition to mandating the nLockTime of coinbase transactions be set to the block height minus 1, we could also require their nSequence not be final. This would make it so the timelock is enforced.
Once (and if) the Consensus Cleanup is activated and its activation height buried, this would give us the following property: “BIP30 validation is never necessary after CC activation height”.
Note we don’t otherwise necessarily have this property. Technically now that we removed checkpoints[1], Bitcoin Core could validate a chain containing a coinbase transaction before BIP34 activation height such that it committed, according to both BIP34 and CC, to a block height post CC activation. In this case, it would be necessary to resume BIP30 validation or a duplicate coinbase could be let in.
But mandating the coinbase’s nSequence never be final, by leveraging that timelocks are also checked on coinbase transactions, makes it so it cannot be possible for a previous transaction to have the very same txid.
Of course it does not matter for any practical purpose. But it’s pretty neat. Thoughts?