Discard child block with parent_hash not matching hash of imported block #1684
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Pull Request Test Coverage Report for Build 2364a84d-8e6d-423f-8ee2-cbbe12f6f4a6
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As a result of #1657 out-of-place parent blocks are marked as non-consensus more reliably, i.e. they will eventually be fetched even if first attempt is unsuccessful. The `ConsensusEnsurer` module, on the contrary, only tried refetching block once, and left it as-is if the refetch failed. Now it is not needed anymore.
Consider a root block `A`, old chain `-- B -- C` and new one `-- B' -- C'`. Due to asynchronous nature of realtime fetcher it is possible for block `B` to be imported _later_ than the entire chain `-- B' -- C'`. In this case we should discard block `C'` and let it be refetched. When that happens, block `B` gets discarded to make sure block `B'` gets eventually fetched and the chain becomes consistent.
Consider a root block `A`, old chain `-- B -- C` and new one `-- B' -- C'`. Due to asynchronous nature of realtime fetcher it is possible for block `B` to be imported _later_ than the entire chain `-- B' -- C'`. In this case we should discard block `C'` and let it be refetched. When that happens, block `B` gets discarded to make sure block `B'` gets eventually fetched and the chain becomes consistent.
This worker only handled one of invalid consensus cases, i.e. when a parent block lost consensus, but the child one didn't. Even then it worked only when all blocks are reliably and sequentially imported (certainly not our case), and only once on indexer launch. Now this particular case is covered by previous commit, and we don't need this worker at all.
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Motivation
Consider a root block
A, old chain-- B -- Cand new one-- B' -- C'. Due to asynchronous nature of realtime fetcher it is possible for blockBto be imported later than the entire chain-- B' -- C'.In this case we should discard block
C'and let it be refetched. When that happens, blockBgets discarded to make sure blockB'gets eventually fetched and the chain becomes consistent.Even though it may cause multiple refetches of the same blocks, it eventually forces the chain to be consistent despite asynchronous or failing imports.
Changelog
ConsensusEnsurermodule from indexer, as forcing consensus loss is more reliable and doesn't leave us with inconsistent chain if refetch fails.InvalidConsensusworker from indexer, as it only handled one corner case in ideal conditions, and only ran once on launch.Upgrading
As new inconsistent blocks could have appeared due to the possibility of race conditions described above, running
priv/repo/migrations/scripts/20190326202921_lose_consensus_for_invalid_blocks.sqlis highly recommended after deploying this fix.