eth/downloader: fix light client cht binary search issue #18196
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| // If we're doing a light sync, ensure the floor doesn't go below the CHT, as | ||
| // all headers before that point will be missing. | ||
| if d.mode == LightSync { |
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Does this need to be conditional on LightSync? Other sync modes might set a non-zero genesis in the future.
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But other modes don't suffer from gaps in the ancestor list, right?
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Currently only light sync is broken. The reason I named it genesis was indeed so we may move the genesis block for full/fast sync too eventually, but that would be a lot more change and potentially breaking at it. I didn't want to screw around adding more stuff to a pre-release hotfix.
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And we currently don't support non-0 genesis blocks, so we can't even test it that it works. Better to be restrictive imho than to accidentally break something.
The binary search based ancestor lookup doesn't work for light clients. The reason is that a light client will inject a random header (the one stored in the CHT) and will want to sync from that point onward. A binary search will however try to find the common ancestor from
[HEAD-90K, REMOTE_HEAD]. Mapping that interval to our header chain would look something like[nil, nil, nil, nil, nil, CHT_HEADER, nil]. You can't binary search on this.The only proper solution is to start from the current header and set the floor of the binary search either to the
HEAD-90Klimit, or to the earliest ancestor that we do have in our database. Unfortunately there's no way to actually find that apart from traversing the chain backward, so that's what this PR does. It also caches the result so we don't do it on every sync cycle.PS: The reason this worked previously is because the ancestor lookup shortcut always retrieved the current header too. A previous polish of this code however resulted in the current head never being retrieved, only it's parent.