mempool: recalculate stale BIP68 lockpoints with mempool parents in removeForReorg #35026

1. 
   javierpmateos commented at 8:13 pm on April 7, 2026: none

   When a transaction with nSequence=0 (BIP68 relative locktime 0) enters the mempool with an unconfirmed parent, CalculatePrevHeights assigns nCoinHeight=tip+1 and the resulting LockPoints are cached with maxInputBlock=genesis. After invalidateblock lowers the tip, TestLockPointValidity returns true for genesis (always on chain), so the stale cached lockpoints are reused. CheckSequenceLocksAtTip then incorrectly determines the lock is not satisfied and removes the transaction along with all its descendants.

   The fix detects the genesis sentinel (maxInputBlock->nHeight==0 with lp.height>0) in filter_final_and_mature, which indicates lockpoints computed with mempool parent inputs, and forces fresh recalculation via CalculateLockPointsAtTip instead of using the stale cache.

   Added mempool_reorg_bip68_stale_lockpoints.py which verifies that both BIP68-disabled and BIP68-enabled children with mempool parents survive a multi-block reorg via invalidateblock.

   Reproducer for unpatched nodes: https://gist.github.com/javierpmateos/c55d365973adbf488a852dc5e0b77dec

   Same class of issue fixed for TRUC in #33504.

   Closes #35007

2. DrahtBot added the label Mempool on Apr 7, 2026
3. 
   DrahtBot commented at 8:13 pm on April 7, 2026: contributor

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   LLM Linter (✨ experimental)

   Possible places where comparison-specific test macros should replace generic comparisons:

   • [test/functional/mempool_reorg_bip68_stale_lockpoints.py] assert node.getblockcount() == H - 1 -> Replace with assert_equal(node.getblockcount(), H - 1)

   2026-04-08 10:17:50

4. javierpmateos force-pushed on Apr 7, 2026
5. javierpmateos force-pushed on Apr 7, 2026
6. javierpmateos force-pushed on Apr 7, 2026
7. javierpmateos force-pushed on Apr 8, 2026
8. javierpmateos force-pushed on Apr 8, 2026
9. mempool: recalculate stale BIP68 lockpoints with mempool parents in removeForReorg

   When a transaction with nSequence=0 (BIP68 relative locktime 0) enters
   the mempool with an unconfirmed parent, CalculatePrevHeights assigns
   nCoinHeight=tip+1 and the resulting LockPoints are cached with
   maxInputBlock=genesis. After invalidateblock lowers the tip,
   TestLockPointValidity returns true for genesis (always on chain),
   so the stale cached lockpoints are reused. CheckSequenceLocksAtTip
   then incorrectly determines the lock is not satisfied and removes
   the transaction along with all its descendants.
   
   Fix: in filter_final_and_mature, detect the genesis sentinel
   (maxInputBlock->nHeight==0 with lp.height>0) which indicates
   lockpoints computed with mempool parent inputs, and force fresh
   recalculation via CalculateLockPointsAtTip.
   
   This is the same class of issue fixed for TRUC in #33504.
   
   Closes #35007

   9dc0642ac5
10. javierpmateos force-pushed on Apr 8, 2026
11. 
    instagibbs commented at 1:15 pm on April 8, 2026: member

    thanks for the PR. From my recollection this code is pretty complicated sadly. Will take a look soon hopefully to see if I can grasp it
12. DrahtBot added the label CI failed on Apr 9, 2026
13. DrahtBot removed the label CI failed on Apr 9, 2026

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