Implementing Chain Types

[dutterbutter]

Need to implement the basic chain structures as defined in the spec under Chain. This task has a dependency on having a Chain Store implemented (e.g. for retrieving head tipset).

1. Chain type
2. Chain methods
3. Checkpoints

[ 1 ] The chain type is defined in the spec as:

type Chain struct {
    HeadTipset Tipset
}

However, I cannot see this defined in the go-filecoin implementation or how its exactly used. The closest implementation to this and the chain methods are in the porcelain package here go-filecoin/internal/app/go-filecoin/porcelain/chain.go / .

[ 2 ] Much like the Chain type, the methods described in the spec are not clearly defined in the go implementation. As defined in the spec:

// Returns the tipset at or immediately prior to `epoch`.
func (chain *Chain_I) TipsetAtEpoch(epoch block.ChainEpoch) Tipset {
	current := chain.HeadTipset()
	for current.Epoch() > epoch {
		current = current.Parents()
	}

	return current
}

// Draws randomness from the tipset at or immediately prior to `epoch`.
func (chain *Chain_I) RandomnessAtEpoch(epoch block.ChainEpoch) util.Bytes {
	ts := chain.TipsetAtEpoch(epoch)
	return ts.MinTicket().DrawRandomness(epoch)
}

func (chain *Chain_I) LatestCheckpoint() epoch block.ChainEpoch {
}

For TipsetAtEpoch, I think this should be TipSetByKey, where you would pass in a TipSetKey as a param rather than epoch imo.

[ 3 ] Checkpoints x3, can be implemented and are defined as:

type Checkpoint &block.BlockHeader
type SoftCheckpoint Checkpoint
type TrustedCheckpoint Checkpoint

For more information refer to Chain. I was not able to find any reference to checkpoint in the go implementation except for the rust file scheduler.rs under sector-builder.

[dutterbutter]

Started implementing, when the TipIndex from #106 is completed will open for PR.

Although not really well defined in the spec, and the use of SoftCheckpoint and Checkpoint don't seem to be referenced anywhere only TrustedCheckpoint.

[amerameen]

This issue had #106 as a blocker? Is that not needed anymore? Was there a workaround?

[dutterbutter]

@amerameen this issue originally was dependant on the TipIndex due to the method TipsetAtEpoch which was included #109 as a subtask of #106. Although the checkpoints have not been completed I have closed this issue as everything else has either been implemented elsewhere.

As for the checkpoints, I believe a new issue can be created when we complete blocksync and other sync functionality, otherwise it will sit as dead code for a while.