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eth/downloader: remove header rollback mechanism #668
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This pull request removes the rollback mechanism in downloader.
Originally, this mechanism is tailored for Proof-of-Work consensus engine, which does PoW verification randomly. Therefore, whenever sync process exits abnormally, a part of tip headers will be removed for safety consideration.
In terms of Proof-of-Stake(also for Proof-of-Authority), each header is verified one by one, there is no reason to rewind the written headers which are certainly valid.
And another big reason for removing this additional mechanism is: inside of Rollback, the genesis state may be committed. It results in a weird scenario that persistent state is both modified by the Rollback function and also the state syncer. The concurrent state write will result in a corrupted state eventually, should be avoided in the first place.
This is partially cherry-pick: ethereum/go-ethereum#28147