validation: Leave pruned blocks in `m_blocks_unlinked`, fix another `nSequenceId` issue #36149

pull mzumsande wants to merge 4 commits into bitcoin:master from mzumsande:202609_fix_cbi_pruning_unlinked changing 5 files +69 −37

mzumsande commented at 2:33 PM on September 2, 2026: contributor
m_blocks_unlinked serves two purposes:
1. finding blocks to setBlockIndexCandidates once their parents are received
2. setting m_chain_tx_count once the parents are received
In case the parent block of a previously pruned block is received, 2. is currently not being done, because pruning removed the entries from m_blocks_unlinked. This can result in CheckBlockIndex failures (#31512, #36021) and incorrect rpc results, plus we are in a temporarily inconsistent state (m_chain_tx_count is currently not set, but would get set if we restarted).

This scenario is unlikely to happen during normal node operation unless there are huge reorgs, but it can happen, for example, in getblockfrompeer scenarios. Fix this by leaving pruned blocks in m_chain_tx_count, and also adding them on restart.

This was one of two reasons, the fuzz target block_index_tree couldn't cover pruning of blocks not in the main chain.

The other one is similar to #34521 and #35070 - if we receive a block a second time (after pruning), we could change nSequenceId while the block is in setBlockIndexCandidates - fix this by attempting to remove it from the set before re-adding it.

After the two issues are fixed, the restriction from block_index_tree fuzz target is removed.

Fixes #36021 Fixes #31512
DrahtBot added the label Validation on Sep 2, 2026
DrahtBot commented at 2:33 PM on September 2, 2026: contributor



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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36149.



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validation: keep pruned blocks in m_blocks_unlinked

m_blocks_unlinked serves two purposes:
1. finding blocks to SetBlockIndexCandidates once their parents are
   received
2. Setting m_chain_tx_count once the parents are received

In case that a parent of a pruned block is received,
2. is currently not being done, because pruning removed the entries.
This can result in CheckBlockIndex failures, and an inconsistent state
(because in case of a restart, m_chain_tx would be set).
This is unlikely to happen during normal node operation unless
there are huge reorgs, but it can happen, for example, in getblockfrompeer scenarios.

Fix this by keeping pruned blocks in m_blocks_unlinked and also removing
the BLOCK_HAVE_DATA requirement during startup.

7c7ccc9cc0

test: add functional test for parents of pruned blocks

This test would lead to a CheckBlockIndex assertion fail without
the changes of the previous commit.

5d5af40fcb

validation: don't change nSequenceId of a block that is a chain candidate

nSequenceId is one of the sort keys of setBlockIndexCandidates, so changing it
for a block that currently is in one of these sets corrupts their ordering,
after which lookups for other entries can fail.

This is also necessary for the block_index_tree fuzz test extension in the
following commit, which would otherwise trip on this.

80477ba978

fuzz: allow to prune out-of-chain blocks in block_index_tree

After the issues with m_blocks_unlinked and
setBlockIndexCandidates have been resolved in previous commits,
the previous restriction to the fuzz test does no longer apply,
and we can also prune out-of-chain blocks.

40a31d97ab

mzumsande force-pushed on Sep 2, 2026
DrahtBot added the label CI failed on Sep 2, 2026
DrahtBot removed the label CI failed on Sep 2, 2026
l0rinc commented at 5:29 PM on September 2, 2026: contributor

Concept ACK, thanks for fixing it. I will try to review it a bit later.
mzumsande marked this as ready for review on Sep 2, 2026

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#31512 validation: CheckBlockIndex can fail in combination with pruning #34521 validation: fix UB in `LoadChainTip`#35070 validation: prevent FindMostWorkChain from causing UB #36021 validation: `Assertion failed: ((pindexFirstNeverProcessed == nullptr || pindex == snap_base) == pindex->HaveNumChainTxs())`

validation: Leave pruned blocks in `m_blocks_unlinked`, fix another `nSequenceId` issue #36149

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