validation: remove unused code in FindMostWorkChain #34884

1. 
   stratospher commented at 3:57 pm on March 20, 2026: contributor

   recent PRs like #31405, #30666 mark all m_block_index descendants as invalid immediately whenever an invalid block is encountered in SetBlockFailureFlags. so by the time we reach FindMostWorkChain, the block in setBlockIndexCandidates already has BLOCK_FAILED_VALID set on it - not just on its ancestor. this means pindexTest = pindexFailed whenever fFailedChain fires, and the inner while (pindexTest != pindexFailed) loop body is never reached!

   I think we can remove it but I’ve just replaced it with Assume in this PR for safety + good to document this invariant in case the code changes in future. (noticed by @ stickies-v in #32950 (review))

   the second commit is unrelated and adds a unit test for the situation in https://github.com/bitcoin/bitcoin/issues/32173

2. validation: remove unused code in FindMostWorkChain

   the newer validation commits mark all m_block_index descendants
   as invalid immediately whenever an invalid block is encountered.
   (in SetBlockFailureFlags). there's also a CheckBlockIndex assert
   for this in ed764ea.
   
   so by the time we reach FindMostWorkChain, they're already
   marked as invalid and no need to repeat the work here.
   also nice logically for FindMostWorkChain to not deal with
   changing block validity status inside it.

   5690917fcb
3. test: add invalidateblock, reconsiderblock unit test

   invalidateblock (iterates throguh all block indices) and
   reconsiderblock (iterates through ancestors/descendants)
   are not symmetrical to each other and can lead to edge case
   block constellations like in issue #32173 (fixed in 37bc207)
   being formed.
   
   add unit test for this scenario to safe guard against
   future regressions.

   a122fbd4c5
4. DrahtBot added the label Validation on Mar 20, 2026
5. 
   DrahtBot commented at 3:58 pm on March 20, 2026: contributor

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   Possible typos and grammar issues:

   • we have a chain of 100 blocks: ... -> We have a chain of 100 blocks: ... [Sentence starts with lowercase “we”, which is a capitalization typo.]
   • by temporarily invalidating block99. the chain tip now falls to block98, -> by temporarily invalidating block99. The chain tip now falls to block98, [Lowercase “the” after a period is a capitalization typo.]

   2026-03-20 15:58:14

6. 
   stickies-v commented at 4:44 pm on March 20, 2026: contributor

   Concept ACK

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stratospher DrahtBot stickies-v

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