Circumstances where the chain flips regardless of TotalDifficultyCompare.

[riemannulus]

A slightly strange log was detected.

[15:52:38 DBG] 815992201/1598638844: Block #1745949 e394a727e9a78f42f4c3e81c1e68e5ed2de38c42befd22bb6facda8849d5d6dd was appended.
[15:52:38 DBG] Downloaded 1 block(s) from 0x4d92cd06Fc559520d038BDA3bBc63a9e51b76e72.Unspecified/3.22.224.238:50586.42.189.229.92.
[15:52:38 DBG] 815992201/1598638844: Got 1 blocks from Peer 4d92cd06Fc559520d038BDA3bBc63a9e51b76e72 (tip: #1745949 e394a727e9a78f42f4c3e81c1e68e5ed2de38c42befd22bb6facda8849d5d6dd)
[15:52:38 DBG] 815992201: FillBlocksAsync() completed (chain ID: 85b185bc-70e1-461e-aeb7-66a554b760a0, tip: #1745949 e394a727e9a78f42f4c3e81c1e68e5ed2de38c42befd22bb6facda8849d5d6dd).
[15:52:38 DBG] 815992201: Finished FillBlocksAsync().
[15:52:38 DBG] 815992201: Swap the chain 8780651b-241a-47f5-8af0-4015cfc10f1c for the chain 85b185bc-70e1-461e-aeb7-66a554b760a0...
[15:52:38 DBG] The blockchain was reorged from 8780651b-241a-47f5-8af0-4015cfc10f1c (#1745950 314ed0ee12c64df8aaa77946e61caf360ee9c56fdcf5d362fc5b3b9db39b7829) to 85b185bc-70e1-461e-aeb7-66a554b760a0 (#1745949 e394a727e9a78f42f4c3e81c1e68e5ed2de38c42befd22bb6facda8849d5d6dd).

Note the chain is swapped to the wrong chain.

And I found some strange code on Swarm.BlockSync.SyncPreviousBlockSync().
https://github.com/planetarium/libplanet/blob/2eaab5a8b1/Libplanet/Net/Swarm.BlockSync.cs#L175

May this code cause wrong logic if the chain tip is null.

PS. I attach some code snippets and chain data for the debug.
https://gist.github.com/riemannulus/8e24f9a99aaad16db41a0ee2fb8bc1f4
https://download.nine-chronicles.com/miner.tar.gz

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[longfin]

As there are no additional reports, this issue is closed for now. After that, please open it when there is a related report.