eth/downloader: only roll back light sync if not fully validating #21537
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holiman
reviewed
Sep 9, 2020
| @@ -1501,18 +1501,20 @@ func (d *Downloader) processHeaders(origin uint64, pivot uint64, td *big.Int) er | |||
| rollbackErr = err | |||
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| // If some headers were inserted, track them as uncertain | |||
| if n > 0 && rollback == 0 { | |||
| if (mode == FastSync || frequency > 1) && n > 0 && rollback == 0 { | |||
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Shouldn't it be frequency==1 ? Why do we add rollbacks only when the header check is every N:th block, but not when it's every block?
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Well, if it checked every block, there's nothing to roll back. We should only roll back if we checked every 24th block for PoW, since we don't know if the previous ones were valid or not
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If we pass the pivot point, the frequency is 1. In this case, we don't need to rollback?
what about this:
if frequency > 1 && n > 0 && rollback == 0 {
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This was referenced Jun 15, 2021
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Fixes #21505.
There is a notion of rollbacks during fast sync, where recent blocks that might not be clean are deleted from the database on a sync failure. This is needed to avoid attacks on fast sync since we're not running transactions.
For light sync however, rollbacks are pointless because there's no batch of full blocks imported on top to validate them. As long as the PoW checks out, light clients will take the node's word for it.
This PR disables the rollback mechanism for light syncs, which was causing crashes in the light txpool due to deleting headers that it imported before. Interestingly, the issue was that the rollback mechanism wasn't disabled after initial sync (opposed to fast sync switching to full sync).