Remember pruned channels #706
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We won't revalidate them if we don't have a good reason to, that is until we receive a recent `channel_update` again. Fixes #624.
This was needed when we didn't remember pruned channels. But now we never have pruned channels in memory, they are cleaned up on `TickPruneStaleChannels` and never re-added unless they have a recent `channel_update`.
Same reason as b34ccad.
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sstone
previously approved these changes
Sep 17, 2018
sstone
approved these changes
Sep 17, 2018
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Currently we don't remember channels that we have pruned, so we will happily revalidate the same channels again if a node re-sends them to us, and prune them again, a.k.a. the "zombie churn".
Before channel queries, rejecting a stale channel without validating it wasn't trivial, because nodes sent us the
channel_announcementbeforechannel_updates, and only after receiving thechannel_updatecould we know if the channel was still stale. Since we had no way of requesting thechannel_announcementfor one particular channel, we would have to buffer it, which would lead to potential DOS issues.But now that we have channel queries, we can now be much more efficient. Process goes like this:
(1) channel x is detected as stale gets pruned, and its id is added to the pruned db
(2) later on we receive a
channel_announcementfrom Eve, we ignore it because the channel is in the pruned db(3) we also receive old
channel_updates from Eve nodes, just ignore them because we don't know the channel(4) then one day some other node George sends us the
channel_announcement, we still ignore it because the channel is still in the pruned db(5) but then George sends us recent
channel_updates, and we know that the channel is back from the dead. We ignore thosechannel_updates, but we aldo remove the channel id from the pruned db, and we request announcements for that node from George(6) George sends us the
channel_announcementagain, we validate it(7) George then sends us the
channel_updates again, we process them(8) done!
Needless to say that this leads to a huge reduction in CPU/bandwidth usage on well-connected nodes.
Fixes #624.