Fix the closing minimum fee. #6547
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```
$ l1-cli close 445x1x1
# Sending closing fee offer 183sat, with range 0sat-183sat
# Received closing fee offer 183sat, with range 0sat-1000000sat
{
"tx": "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",
"txid": "216ed9adf8df623770e219f52b31350de60cd1b236d6f043664d9eb5bc462ad4",
"type": "mutual"
}
rusty@rusty-Framework:~/devel/cvs/lightning (master)$ bt-cli getrawmempool
[
]
```
Logs show:
```
2023-08-11T02:20:13.947Z DEBUG plugin-bcli: sendrawtx exit 26 (bitcoin-cli -regtest -datadir=/home/rusty/.bitcoin sendrawtransaction 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) error code: -26\nerror message:\nmin relay fee not met, 183 < 192
Indeed, tx weight is 192!
```
Reported-by: @ShahanaFarooqui
Presumably we still want to insist that this have *some* chance to propagate. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
…l close. In spec commit 498f104fd399488c77f449d05cb21c0b604636a2 (August 2021), Bastien Teinturier removed the requirement that the mutual close fee be less than or equal the final commitment tx. We adopted that change in v0.10.2, but we made sure to never offer a fee under the final commitment tx's fee, so we didn't break older nodes. However, the closing tx can actually be larger than the final commitment tx! The final commit tx has a 22-byte P2WKH output and a 34-byte P2WSH output; the closing can have two 34-byte outputs, making it 4*8 = 32 Sipa heavier. Previously this would only happen if both sides asked for P2WSH outputs, but now it happens with P2TR, which we now do. The result is that we create a tx which is below the finally commitment tx fee, and may be below minrelayfee (as it was in regtest). So it's time to remove that backwards-compatibility hack. Changelog-Fixed: Protocol: We may propose mutual close transaction which has a slightly higher fee than the final commitment tx (depending on the outputs, e.g. two taproot outputs). Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Fixes: ElementsProject#6545
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Triggered by taproot, our mutual closing tx is slightly larger than our final commitment tx. This could happen in theory before (if both specified P2WSH outputs) but nobody ever did that.
We capped the fee at the final commitment_tx fee, for compatibility reasons which have now passed (the spec was changed to remove this 2 years ago by @t-bast, and we removed our enforcement of it in 0.10.2). Removing that simply fixes this problem: that if fees were really low (such as #6545 shows on regtest) we could create a mutual close which didn't even relay!