lotus区块同步简易解析

1、TipSet同步流程

 2、块同步保存到本地流程

比较关键的代码：

        1、以下函数是将外部接收到的TipSet与本地做比对得到所有需要同步的TipSet,例如本地存储TipSet高度是10000，而外部接收到的TipSet是11000，则需要同步的是[10001,11000]

func (syncer *Syncer) collectHeaders(ctx context.Context, incoming *types.TipSet, known *types.TipSet, ignoreCheckpoint bool) ([]*types.TipSet, error) {
	ctx, span := trace.StartSpan(ctx, "collectHeaders")
	defer span.End()
	ss := extractSyncState(ctx)

	span.AddAttributes(
		trace.Int64Attribute("incomingHeight", int64(incoming.Height())),
		trace.Int64Attribute("knownHeight", int64(known.Height())),
	)

	// Check if the parents of the from block are in the denylist.
	// i.e. if a fork of the chain has been requested that we know to be bad.
	for _, pcid := range incoming.Parents().Cids() {
		if reason, ok := syncer.bad.Has(pcid); ok {
			newReason := reason.Linked("linked to %s", pcid)
			for _, b := range incoming.Cids() {
				syncer.bad.Add(b, newReason)
			}
			return nil, xerrors.Errorf("chain linked to block marked previously as bad (%s, %s) (reason: %s)", incoming.Cids(), pcid, reason)
		}
	}

	{
		// ensure consistency of beacon entires
		targetBE := incoming.Blocks()[0].BeaconEntries
		sorted := sort.SliceIsSorted(targetBE, func(i, j int) bool {
			return targetBE[i].Round < targetBE[j].Round
		})
		if !sorted {
			syncer.bad.Add(incoming.Cids()[0], NewBadBlockReason(incoming.Cids(), "wrong order of beacon entires"))
			return nil, xerrors.Errorf("wrong order of beacon entires")
		}

		for _, bh := range incoming.Blocks()[1:] {