https://github.com/bitcoin/bitcoin/pull/15402/commits/9ce9c37004440d6a329874dbf66b51666d497dcb
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This thing has been merged (D4803) and reverted (D4959) before. Seems to be working correctly now, with the lock to m_cs_chainstate in ActivateBestChain having a corresponding one in UnwindBlock.
Concludes backport of Core PR15402