Cleaner handling of HTLC settlement during force-close #3090
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When an HTLC settles downstream while the upstream channel is closing, this allows us to either publish HTLC-success transactions for which we were missing the preimage (if the downstream HTLC was fulfilled) or stop watching the HTLC output (if the downstream HTLC was failed). We previously re-computed every closing transaction in that case, which was confusing and useless. We now explicitly handle those two cases and only republish the HTLC-success transactions that become available, if any. We also change the default feerate used for `claim-htlc-txs`: we used a high feerate in the channel actor, which meant we would skip small HTLCs that weren't economical to spend at that high feerate. But the feerate is actually set inside the tx-publisher actor based on the HTLC expiry, which may happen many blocks after the beginning of the force-close, in which case the feerate may have changed a lot. We now use the minimum feerate in the channel actor to ensure we don't skip HTLCs and let the tx-publisher actor handle RBF.
This is something we should have added a long time ago to help the closing tests be more readable. Better late than never!
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sstone
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Jun 2, 2025
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When an HTLC settles downstream while the upstream channel is closing, this allows us to either publish HTLC-success transactions for which we were missing the preimage (if the downstream HTLC was fulfilled) or stop watching the HTLC output (if the downstream HTLC was failed).
We previously re-computed every closing transaction in that case, which was confusing and useless (and also made the tests very verbose, because we had to ignore all the transactions and watches that were duplicates). We now explicitly handle those two cases and only republish the HTLC-success transactions that become available, if any.
We also change the default feerate used for
claim-htlc-txs: we used a high feerate in the channel actor, which meant we would skip small HTLCs that weren't economical to spend at that high feerate. But the feerate is actually set inside the tx-publisher actor based on the HTLC expiry, which may happen many blocks after the beginning of the force-close, in which case the feerate may have changed a lot. We now use the minimum feerate in the channel actor to ensure we don't skip HTLCs and let the tx-publisher actor handle RBF.