fix(sync): enforce fork len when changing head #5244
Merged
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Similar to #5182 this PR is theoretically protocol breaking but the only case of interest is when we are working with forks close to finality threshold (which is orders of magnitude away from normal forks observed in the chain).
Most of the syncing logic seems to assume the "heaviest" tipset it receives as input is actually the head of our synced chain even if the lock is released. This is mostly innocuous and will be handled in follow-up PRs (to make this more explicit), but there are cases when we actually want to hold the lock while doing certain checks that are not related to the validity/integrity of a chain (either heaviest or not), namely how far away it is from the actual head we will be swapping it for (there may be other cases we should look into).