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eth/downloader: retrieve pivot header from local chain if necessary #24610

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Merged
merged 5 commits into from
Apr 4, 2022
Merged

eth/downloader: retrieve pivot header from local chain if necessary #24610

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91ed02e
eth/downloader: retrieve pivot header from local chain if necessary
rjl493456442 Mar 30, 2022
8f8cba6
eth/downloader: improve readability
rjl493456442 Mar 31, 2022
b1b5690
eth/downloader: update fix
rjl493456442 Apr 1, 2022
8d651cd
eth/downloader: add beacon sync tests
rjl493456442 Apr 1, 2022
b59d235
eth/downloader: remove duplicated code
rjl493456442 Apr 1, 2022
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18 changes: 15 additions & 3 deletions eth/downloader/downloader.go
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Original file line number Diff line number Diff line change
Expand Up @@ -158,6 +158,9 @@ type LightChain interface {
// GetHeaderByHash retrieves a header from the local chain.
GetHeaderByHash(common.Hash) *types.Header

// GetHeaderByNumber retrieves a block header from the local chain by number.
GetHeaderByNumber(number uint64) *types.Header

// CurrentHeader retrieves the head header from the local chain.
CurrentHeader() *types.Header

Expand Down Expand Up @@ -477,15 +480,22 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td, ttd *
return err
}
if latest.Number.Uint64() > uint64(fsMinFullBlocks) {
pivot = d.skeleton.Header(latest.Number.Uint64() - uint64(fsMinFullBlocks))
number := latest.Number.Uint64() - uint64(fsMinFullBlocks)
pivot = d.skeleton.Header(number)
if pivot == nil {
// Retrieve the pivot point from the local chain if it's
// not in the range of skeleton.
pivot = d.lightchain.GetHeaderByNumber(number)
}
}
}
// If no pivot block was returned, the head is below the min full block
// threshold (i.e. new chain). In that case we won't really snap sync
// anyway, but still need a valid pivot block to avoid some code hitting
// nil panics on access.
var syncState = true // means whether state sync is required in this cycle
if mode == SnapSync && pivot == nil {
pivot = d.blockchain.CurrentBlock().Header()
pivot, syncState = d.blockchain.CurrentBlock().Header(), false
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@karalabe karalabe Mar 31, 2022

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This code path is interesting. Previously the pivot block was retrieved from the master peer, so we had some fairly strong guarantees that it either exists or if it's nil, the head is < 64 depth. In that case, the original code is correct.

With the beacon sync addition, a nil pivot would mean that there's a gap between the downloaded skeleton chain segment and the local chain. But the skeleton syncer will only stop when the two are linked, which would mean that a gap is invalid internal state. In that case, my 2c is not to fix it, rather to return a sync error.

I could imagine that happening if there's some rollback raicing with a sync trigger. Or maybe sethead or something. Either way, what I'd do is to add the nil check after

				// Retrieve the pivot point from the local chain if it's
				// not in the range of skeleton.
				pivot = d.lightchain.GetHeaderByNumber(number)

and log an error and return. That IMHO is fine because it will just cause the local node to wait for the next block from the beacon client and then restart sync, filling any gas produced by some weird mechanism. It also makes the remainder of the code more deterministic vs. trying to correctly sync on top of an invalid internal state.

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@rjl493456442 rjl493456442 Apr 1, 2022

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Good point. I updated it and also catch another issue.

}
height := latest.Number.Uint64()

Expand Down Expand Up @@ -516,7 +526,9 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td, ttd *
origin = 0
} else {
pivotNumber := pivot.Number.Uint64()
if pivotNumber <= origin {

// Cap the sync origin by pivot header when state sync is required.
if pivotNumber <= origin && syncState {
origin = pivotNumber - 1
}
// Write out the pivot into the database so a rollback beyond it will
Expand Down