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

Is there an existing issue for this?

• I have searched the existing issues

Current behaviour

While testing an external python capnp ipc client on regtest, I found that findAncestorByHash returns success for an unknown descendant hash when the ancestor is known. I also reproduced this directly on master.

Expected behaviour

findAncestorByHash should return false when the descendant hash is unknown

Steps to reproduce

Add this to the existing findAncestorByHash test in src/test/interfaces_tests.cpp to reproduce the issue

BOOST_CHECK(!chain->findAncestorByHash(uint256{}, active[0]->GetBlockHash()));

and then running

cmake --build build --target test_bitcoin -j8 && ./build/bin/test_bitcoin --run_test=interfaces_tests/findAncestorByHash

Relevant log output

test/interfaces_tests.cpp:97: error: in "interfaces_tests/findAncestorByHash": check !chain->findAncestorByHash(uint256{}, active[0]->GetBlockHash()) has failed

How did you obtain Bitcoin Core

Compiled from source

What version of Bitcoin Core are you using?

v31.1

Operating system and version

MacOS 27.0.1 arm

Machine specifications

No response

cc @Sjors @ryanofsky

i followed this steps above to reproduce this issue:

• added the failing findAncestorByHash test to src/test/interfaces_tests.cpp
• rebuilt Core v32.99.0-d4b0e1e4b078 on my machine (macOS 15.8.01)
• ran that specific test and got this result:

<img width="799" height="122" alt="Image" src="https://github.com/user-attachments/assets/d7bbaf57-cc50-45d3-94ba-4a9b8ce1fc22" />