Fix fluid_rho broadcast using MPI_LOGICAL instead of mpi_p

[sbryngelson]

Summary

Severity: CRITICAL — silently corrupts reference densities on non-root ranks.

File: src/pre_process/m_mpi_proxy.fpp, line 61

fluid_rho is a real(wp) array but is broadcast using MPI_LOGICAL instead of mpi_p. MPI doesn't type-check its arguments, so the bytes are reinterpreted as logicals on the receiving ranks, silently producing wrong density values.

Before

call MPI_BCAST(fluid_rho(1), num_fluids_max, MPI_LOGICAL, 0, MPI_COMM_WORLD, ierr)
!                                             ^^^^^^^^^^^ should be mpi_p

After

call MPI_BCAST(fluid_rho(1), num_fluids_max, mpi_p, 0, MPI_COMM_WORLD, ierr)

Why this went undetected

On most systems, MPI_LOGICAL and a real happen to be the same size (4 or 8 bytes), so the call doesn't segfault. It just reinterprets the bits, producing subtly wrong values that are hard to trace back to a broadcast type mismatch.

Test plan

• Run multi-rank simulation and verify fluid_rho is identical across all ranks

🤖 Generated with Claude Code

Fixes #1197

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[codeant-ai]

Nitpicks 🔍

🔒 No security issues identified

⚡ Recommended areas for review MPI datatype mapping The new broadcast uses mpi_p for fluid_rho. Confirm mpi_p exactly matches the Fortran kind of fluid_rho (real(wp)). If mpi_p is not the MPI datatype corresponding to real(wp) (e.g. MPI_REAL8 for double precision), values will still be corrupted across ranks. Validate the mapping across platforms and build configurations. Potential uninitialized data The call broadcasts num_fluids_max elements. If only num_fluids entries are valid on root, the extra entries may be uninitialized and lead to unpredictable values on other ranks. Consider broadcasting only the active count or ensuring the tail is initialized before broadcast.

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[Copilot]

Pull request overview

Fixes critical MPI broadcast type mismatch where fluid_rho, a real-valued array, was incorrectly broadcast using MPI_LOGICAL instead of mpi_p, causing data corruption on non-root MPI ranks through bit reinterpretation.

Changes:

• Corrected MPI datatype for fluid_rho broadcast from MPI_LOGICAL to mpi_p

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.05%. Comparing base (84c46e0) to head (27cad3b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1176   +/-   ##
=======================================
  Coverage   44.05%   44.05%           
=======================================
  Files          70       70           
  Lines       20498    20498           
  Branches     1990     1990           
=======================================
  Hits         9030     9030           
  Misses      10329    10329           
  Partials     1139     1139           

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