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Prune monitors with zero claimable balances upon persisting #2236
Description
Currently, users can decide to prune their ChannelMonitors if a call to get_claimable_balances returns empty. This is only safe to do so in certain scenarios though, as we can also return empty if the channel is either still unconfirmed or it has confirmed but we don't have a to_self balance (like in the case of an inbound channel) and no HTLCs are present in our latest commitment transaction.
Activity
TheBlueMatt commentedon Apr 27, 2023
Hmm, so I think its safe as long as (a) the channel is closed (ie channelmanager doesnt know about it) and (b) the balances are empty, at least what's what we've been telling users.
We should, however, actually make use of this in our
Persisterstuff.TheBlueMatt commentedon Apr 29, 2023
I guess actually with the exception of 0confs where the funding may not be confirmed but the channel closed? Is there a bug there?
wpaulino commentedon May 1, 2023
Actually, it looks like we always push a
ClaimableOnChannelClosebalance (even if it's actually zero, which it can be on an inbound channel we've never received a payment on) when we haven't seen a spend onchain, so it should be safe to prune once there aren't any balances left (even 0-confs), assuming that behavior doesn't change (though it probably should?).rust-lightning/lightning/src/chain/channelmonitor.rs
Line 1837 in 0e8da58
TheBlueMatt commentedon May 1, 2023
Nice okay, good to know. Yea, I don't think we need to change that, I mean its dumb that we'll hang around to a monitor until our peer claims their money, but its easier than running the risk of removing a monitor too early before the channel is closed?
So I think this issue should just be re-titled to refer to actually using this in the persister?
wpaulino commentedon May 1, 2023
Yeah. We'll still need a way to clean up timed out funding attempts, otherwise we'll always hold on to their
ChannelMonitors. Though I guess that's a separate issue.[-]Expose check to safely prune ChannelMonitors[/-][+]Prune monitors with zero claimable balances upon persisting[/+]TheBlueMatt commentedon May 10, 2023
Note that async monitor updates will also throw a wrinkle into this - #2167 (comment)
wpaulino commentedon May 10, 2023
There's also another case pointed out here: when a revoked commitment confirms and the counterparty claims all of the outputs, we still keep the claimable balances around preventing us from pruning.
TheBlueMatt commentedon Dec 12, 2023
To summarize, aside from the one async monitor update issue linked above (ie only an issue for those returning
ChannelMonitorUpdateStatus::InProgressfrom a persist method), the only known issues are cases where we may fail to have an empty balance set even though we have no claimable balances. ie it is otherwise considered safe to remove monitors afterget_claimable_balacesreturns an empty set. For safety, I'd recommend waiting until two weeks or a month afterget_claimable_balancesis returning an empty set before pruning.TheBlueMatt commentedon Oct 13, 2024
Closing as completed in #2964. Tracking the one async issue mentioned in #3052