Re: [bitcoin-dev] [Lightning-dev] Take 2: Removing the Dust Limit

[Jeremy <jlrubin@mit.edu>]

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IMO this is not a big problem. The problem is not if a 0 value ever enters
the mempool, it's if it is never spent. And even if C2/P1 goes in, C1 still
can be spent. In fact, it increases it's feerate with P1's confirmation so
it's somewhat likely it would go in. C2 further has to be pretty expensive
compared to C1 in order to be mined when C2 would not be, so the user
trying to do this has to pay for it.

If we're worried it might never be spent again since no incentive, it's
rational for miners *and users who care about bloat* to save to disk the
transaction spending it to resurrect it. The way this can be broken is if
the txn has two inputs and that input gets spent separately.

That said, I think if we can say that taking advantage of keeping the 0
value output will cost you more than if you just made it above dust
threshold, it shouldn't be economically rational to not just do a dust
threshold value output instead.

So I'm not sure the extent to which we should bend backwards to make 0
value outputs impossible v.s. making them inconvenient enough to not be
popular.



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Consensus changes below:
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Another possibility is to have a utxo with drop semantics; if UTXO X with
some flag on it is not spent in the block it is created, it expires and can
never be spent. This is essentially an inverse timelock, but severely
limited to one block and mempool evictions can be handled as if a conflict
were mined.

These types of 0 value outputs could be present just for attaching fee in
the mempool but be treated like an op_return otherwise. We could add two
cases for this: one bare segwit version (just the number, no data) and one
that's equivalent to taproot. This covers OP_TRUE anchors very efficiently
and ones that require a signature as well.

This is relatively similar to how Transaction Sponsors works, but without
full tx graph de-linkage... obviously I think if we'll entertain a
consensus change, sponsors makes more sense, but expiring utxos doesn't
change as many properties of the tx-graph validation so might be simpler.

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