LLVM sanitizers (ASan / UBSan / TSan / MSan) are incompatible with Open MPI 5+'s default UCX transport. mpi detects this combination and automatically works around it.

Symptom

With a sanitizer-enabled build linked against Open MPI 5+, every MPI process crashes early in MPI_Init with a SEGV inside libucs.so (ucs_handle_error / ucs_debug_print_backtrace near the top of the stack). The same code on Open MPI 4.x runs cleanly.

Cause

Open MPI 5+ uses the pml/ucx layer by default. UCX's libucm installs malloc/mmap relocation hooks to track RDMA buffers; LLVM sanitizers install their own interceptors on the same symbols. The double-patched malloc table desynchronizes ASan's shadow map, and the next allocation routed through the wrong path lands on an unmapped page.

This is a known upstream issue with no fix at the time of writing:

open-mpi/ompi#13069 — Segmentation fault when using address sanitizer (Open MPI 5.0.6, clang and gcc, both crash around MPI_Init).
openucx/ucx#5030 — upstream UCX issue tracing the crash to ucm/malloc_hook.cc under ASan.

Automatic fix

When c++/mpi/CMakeLists.txt sees Open MPI 5+ and any of ASAN/UBSAN/MSAN/TSAN set in the cache, it appends

--mca pml ob1 --mca btl self,vader --mca osc ^ucx

to the MPIEXEC_PREFLAGS cache variable. These flags route Open MPI through the ob1 PML and shared-memory BTLs, so libucm is never loaded. Because every TRIQS-ecosystem add_test already threads ${MPIEXEC_PREFLAGS} to mpiexec, the workaround reaches mpi, nda, triqs, and any downstream project with no per-project change.

Confirm it's active by looking for this configure-time line:

-- Sanitizer + Open MPI 5.x detected: appending '--mca pml ob1 --mca btl self,vader --mca osc ^ucx' to MPIEXEC_PREFLAGS to avoid UCX/sanitizer SEGV

Manual workaround

If you launch MPI by hand (raw mpiexec, Slurm scripts, CI without ctest), pass the same flags:

$ mpiexec --mca pml ob1 --mca btl self,vader --mca osc ^ucx -n 2 ./my_test

Alternatives

Preferred long-term: rebuild Open MPI without UCX (configure --without-ucx) in your sanitizer image.
Use Open MPI 4.x for sanitizer CI — it defaults to ob1/vader and is unaffected.
Do not suppress the crash with ASAN_OPTIONS=handle_segv=0; that hides the corrupted shadow map and may mask real bugs.

The detection is gated on MPI_CXX_LIBRARY_VERSION_STRING matching Open MPI v<N>, so Intel MPI, MPICH, and other implementations are unaffected.