[bug] query phase aborts deterministically after successful build (beam search returns 1 element, point id stable per dataset; single-threaded too)

[sagitrs-architect]

Summary

Both neighbors-vamana and neighbors-pipnn binaries build a healthy graph, then abort deterministically during the query phase:

Error: for point id 9996 beam search returned 1 elements, which is less than k = 10
terminate called after throwing an instance of 'std::logic_error'
  what():  Error in beam search return value
Aborted (core dumped)   # exit 134

Key facts established:

• Single-threaded reproduces (PARLAY_NUM_THREADS=1: build takes 12.0s, then same abort) — so this is not a data race;
• Stable point id per dataset: always 9996 on SIFT-100K (ids 0-9999) across T=1/8/32; on a different (synthetic) 100K dataset it aborted at id 0 instead — data-dependent, thread-count-independent → looks like a data-dependent edge case in the query path (or UB with a data-dependent manifestation), not concurrency;
• The built graph is fine: we exported the graph before the abort and evaluated it with an independent beam-search implementation — recall@10 at L=100: vamana 1-pass 0.9993 (deg 40.7), vamana 2-pass 0.9998 (deg 38.3), pipnn 0.9993 (deg 28.8). The defect is isolated to the query phase inside the binaries; build-phase beam search is healthy.

Environment

• Ubuntu, GCC 15.2.0 (/usr/bin/g++ — only gcc-15/16 available on this box, so we could not A/B an older toolchain; happy to test a patch)
• CMake Release, parlaylib via your FetchContent pin, no custom flags
• Data: first 100K points of ann-benchmarks sift-128-euclidean (float32, 10K queries, exact GT)

Reproduce

PARLAY_NUM_THREADS=32 ./vamana/neighbors-vamana_FLOAT_T_EUCLIDEAN \
  -base_path sift100k_base.fbin -query_path sift100k_query.fbin -gt_path sift100k_gt.ibin \
  -k 10 -R 64 -L 100 -alpha 1.2 -num_passes 1 -graph_outfile g.graph -res_path r.res
# same abort with PARLAY_NUM_THREADS=1, and with the pipnn binary

(Apologies for cross-posting: an earlier copy of this report was mistakenly filed on a different repo — thustorage/PipeANN#19 — please disregard that one.)

Happy to share the exact binaries, graph files, or data slices if useful. Thanks for the library!