TipSetCache's bug

[cxytz01]

Describe the bug:

I think there is a bug here.
In Watcher.init, warm function inserts the current tipset’s parents to ring buffer except for current tipset. https://github.com/filecoin-project/lily/blob/master/chain/watch/watcher.go#L112
And in watcher.index, function SetCurrent just replaces the tail of the buffer’s tipset to current tipset, the tipset that has been replaced is deleted. The tipset has been deleted has no opportunity to be indexing.

lily/chain/cache/tipset.go

Line 116 in 85faa23

c.buffer[c.idxHead] = ts

There are the tss variable data when warm function has been executed:
tss height: 1409742
tss height: 1409741
tss height: 1409740
tss height: 1409739
tss height: 1409738
tss height: 1409737
tss height: 1409734
tss height: 1409733
tss height: 1409732
tss height: 1409731
and the current height is: 1409744
when SetCurrent function has been executed, the buffer is:
i: 0, height: 1409731
i: 1, height: 1409732
i: 2, height: 1409733
i: 3, height: 1409734
i: 4, height: 1409737
i: 5, height: 1409738
i: 6, height: 1409739
i: 7, height: 1409740
i: 8, height: 1409741
i: 9, height: 1409744
height: 1409742 was deleted.

Steps to Reproduce:

Lily Version: vx.x.x

[frrist]

@cxytz01 I suspect you are correct. Would love a PR that fixes this if you're up for it.