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taproot annex
From what I can tell, the annex is basically additional inputs to a script that might have additional constraints put on it. Is that right? I don't quite follow how moving the max height to the annex helps script caching here. I wasn't able to find much information on how the annex is envisioned to be used. Would you mind elaborating on how this would work?
Also, I think the proposal as it stands already addresses script caching (in the
Transaction Evaluation section). The result of the script can be cached as long as the cache item also contains information requiring just the OP_BBV to be re-evaluated (for the relevant block).
The normal approach for this problem would be a design that adds an "annex field" (where the details on how to delimit annex fields is not yet standardized) for a maxheight value, and add a consensus rule that transaction with one (or more?) maxheight fields are invalid in blocks whose height exceeds this (or any) maxheight value. Then you could/would add an OP code to push a copy of the (smallest) maxheight value from the annex onto the stack or maybe an opcode to compare a stack item with this (every) maxheight value from the annex. This indirection is how OP_CLTV and OP_CSV work and this indirection makes script validity cacheable because script remains a function of the transaction data only. Since transaction data doesn't change, neither does the outcome of script evaluation. The rule that invalidates late transactions looks only at the annex and is independent of any script evaluation considerations.
> this auto-double-spend wallet would send every payment with an annex value that limits the payment to being valid only up to the next block
One possible solution to that would be to require that the input to OP_BBV to be in the script itself and not originate from the witness.
Regardless, I think the ideal solution is to not have any of these such rules if we can simply change the definition for what counts as finalization to account for the fact that BBV transactions mined close to their expiration. Is there a reason this finalization-redefinition is not an adequate solution?
Generally speaking, you cannot solve security problems through optional and completely voluntary transaction relay policy. I'll just send my about-to-expire transactions directly to miners and they will probably mine them because they are, in fact, valid, and pay fees. Why wouldn't they mine it?
(Yes, I know this logic also applies to RBF flagged transactions. Indeed, you cannot rely on an RBF flag to prevent double spending, Yes I think the RBF flag ought to be removed from consideration and every transaction should be considered RBFable. Maybe that even happens to be my own node's relay policy.)
I apologize, but I don't think I have further time to engage in an idea that I don't consider likely to achieve broad community support.