[ogrisel]

Fixes the FlexiBLAS CI jobs, which started failing on master after an upstream FlexiBLAS change. Found while investigating the failures in #294 (#294).

Root cause

• Upstream FlexiBLAS (commits 024e3c19c and 28cf5ba3e, Nov 3 2026) made the default backend a CMake option named FLEXIBLAS_DEFAULT_BLAS. continuous_integration/install_flexiblas.sh still passed -DFLEXIBLAS_DEFAULT=OPENBLAS_CONDA, which is now silently ignored. FlexiBLAS falls back to NETLIB, so test_flexiblas and test_flexiblas_switch fail on Linux.
• In tests/empirical_scope_observation.py, run() starts a non-daemon thread that counts threads and only stops it when the BLAS/OpenMP probe returns normally. If the probe raises (on macOS, numpy's import-time sanity check failed under the NETLIB default), the thread keeps the interpreter alive and the job hangs until the 30 minute timeout.

Changes

• Pass -DFLEXIBLAS_DEFAULT_BLAS="OPENBLAS_CONDA".
• Fail the FlexiBLAS install step if the generated config does not contain default = OPENBLAS_CONDA, so a future silent fallback breaks the build immediately.
• Stop the thread counter in a finally block so an exception in the probe cannot hang the process.

Evidence

• Before (fa70d83, and FIX recursive wrap, plus controlling thread cleanups and doc improvements #294 at b57a863): Linux FlexiBLAS reports Default BLAS: NETLIB and fails the two tests. macOS hangs. The nightly master schedule failed the same jobs on Nov 5.
• After (3224de8): all 26 checks pass. Both FlexiBLAS jobs report Default BLAS: OPENBLAS_CONDA and current_backend: OPENBLAS_CONDA.
• Locally, the old scope script hangs when the probe raises and the patched version exits with the error.

Open questions

• Why numpy's sanity check fails on macOS with the NETLIB backend is not investigated here. It does not affect the setup once OPENBLAS_CONDA is the default.
• FlexiBLAS is still built from its master branch. Pinning a release tag would avoid this kind of breakage, at the cost of catching upstream regressions later.

[ogrisel]

LGTM, I will merge.