fix(sync): enforce ForkLengthThreshold for synced chain #5182
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magik6k
approved these changes
Dec 14, 2020
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In
syncFork, we currently only applyForkLengthThresholdto the tipsets being fetched for the forked chain (by means ofGetBlocks, if we walk back the fork chain without finding a common ancestorsyncForkwill fail). We do not apply the threshold for our synced chain, that is, the tipsets that would be "walked back" if the fork is selected as our new head.This fix introduces a variable (
forkLengthInHead) to track the tipsets walked back from our synced head to find the common ancestor with the fork. Before we allowed to freely walk back our chain (nts.Parents()) to match the next tipset height in the fork chain being traversed (to perform the common ancestor check).We left the previous check in place for the fork chain (in the
GetBlockscall) to minimize changes at this time, so we will now enforce the threshold on both sides of the fork. It should be later revisited what do we mean exactly by fork length (a term presently missing in the spec).In theory this is a protocol breaking change but in practice forks in the chain are orders of magnitude smaller than the current limit of 900 enforced here.