Ensure partial MPP claims continue to blocks channels on restart #3928
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In 9cc6e08, we started using the `RAAMonitorUpdateBlockingAction` logic to block RAAs which may remove a preimage from one `ChannelMonitor` if it isn't durably stored in another that is a part of the same MPP claim. At the time, when we claimed a payment, if we saw that the HTLC was already claimed in the channel, we'd simply drop the new RAA blocker. This can happen on reload when replaying MPP claims. However, just because an HTLC is no longer present in `ChannelManager`'s `Channel`, doesn't mean that the `ChannelMonitorUpdate` which stored the preimage actually made it durably into the `ChannelMonitor` on disk. We could begin an MPP payment, have one channel get the preimage durably into its `ChannelMonitor`, then step forward another update with the peer. Then, we could reload, causing the MPP claims to be replayed across all chanels, leading to the RAA blocker(s) being dropped and all channels being unlocked. Finally, if the first channel managed to step forward a further update with its peer before the (now-replayed) `ChannelMonitorUpdate`s for all MPP parts make it to disk we could theoretically lose the preimage. This is, of course, a somewhat comically unlikely scenario, but I had an old note to expand the test and it turned up the issue, so we might as well fix it.
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In 9cc6e08, we started using the
RAAMonitorUpdateBlockingActionlogic to block RAAs which may remove a preimage from oneChannelMonitorif it isn't durably stored in another that is a part of the same MPP claim.At the time, when we claimed a payment, if we saw that the HTLC was already claimed in the channel, we'd simply drop the new RAA blocker. This can happen on reload when replaying MPP claims.
However, just because an HTLC is no longer present in
ChannelManager'sChannel, doesn't mean that theChannelMonitorUpdatewhich stored the preimage actually made it durably into theChannelMonitoron disk.We could begin an MPP payment, have one channel get the preimage durably into its
ChannelMonitor, then step forward another update with the peer. Then, we could reload, causing the MPP claims to be replayed across all chanels, leading to the RAA blocker(s) being dropped and all channels being unlocked. Finally, if the first channel managed to step forward a further update with its peer before the (now-replayed)ChannelMonitorUpdates for all MPP parts make it to disk we could theoretically lose the preimage.This is, of course, a somewhat comically unlikely scenario, but I had an old note to expand the test and it turned up the issue, so we might as well fix it.