interfaces: fix findAncestorByHash false positive #36473

pull Romansage6 wants to merge 1 commits into bitcoin:master from Romansage6:2026-10-fix-findancestorbyhash changing 2 files +3 −1

Romansage6 commented at 1:04 PM on October 8, 2026: none
findAncestorByHash looks up both block_hash and ancestor_hash, then was only supposed to clear the ancestor pointer when the lookups succeeded but the ancestor relationship didn't hold:
```
if (block && ancestor && block->GetAncestor(ancestor->nHeight) != ancestor) ancestor = nullptr;
```
Because of the && short circuit, an unknown block_hash makes the whole condition false without ever evaluating the relationship check, so a successful ancestor_hash lookup is passed through unmodified and the function reports true for a descendant that doesn't exist.

Before:
```
chain->findAncestorByHash(uint256{}, active[0]->GetBlockHash()) // returns true
```
After this change it correctly returns false when either hash is unknown, in addition to the existing relationship check.

Tested with:
```
cmake --build build --target test_bitcoin -j8
./build/bin/test_bitcoin --run_test=interfaces_tests
```
Full suite (./build/bin/test_bitcoin, 877 cases) also passes with no regressions. Added two new cases to interfaces_tests/findAncestorByHash covering unknown block_hash and unknown ancestor_hash, which fail on master and pass with this fix.

Fixes #36472

interfaces: fix findAncestorByHash false positive

findAncestorByHash looked up both block_hash and ancestor_hash, then
cleared the ancestor pointer only when both lookups succeeded but the
ancestor relationship did not hold:

  if (block && ancestor && block->GetAncestor(...) != ancestor)
      ancestor = nullptr;

Because of the && short circuit, an unknown block_hash made the whole
condition false without ever running the relationship check, so a
successful ancestor_hash lookup was passed through unmodified and the
function reported true for a descendant that does not exist.

Clear the ancestor whenever either lookup fails, not only when the
relationship check fails.

Add unit test cases for an unknown block_hash and an unknown
ancestor_hash, matching the reported regression.

Fixes #36472

ccea7c93f5

DrahtBot added the label IPC on Oct 8, 2026
DrahtBot commented at 1:05 PM on October 8, 2026: contributor


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External sites
- Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36473
- Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36473.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36473#author-ralph


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sedited commented at 1:10 PM on October 8, 2026: contributor

This seems to be a drive-by LLM contribution from an issue that was created not even an hour ago. Closing again.
sedited closed this on Oct 8, 2026

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#36472 interfaces: `findAncestorByHash` returns true for an unknown descendant

interfaces: fix findAncestorByHash false positive #36473

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