FillBoundaryAux ignores `m_safe_guard_cells`

[WeiqunZhang]

Summary

• WarpX::FillBoundaryAux(int lev, amrex::IntVect) (Parallelization/WarpXComm.cpp:1121-1132) always forwards the caller-supplied ng to ablastr::utils::communication::FillBoundary.
• Every other FillBoundary helper first checks m_safe_guard_cells and upgrades the requested ng to the full MultiFab::nGrowVect() when safe-guard mode is active, ensuring all allocated guard cells stay synchronized during debugging runs.
• Because the aux helper skipped that copy-pasted guard, enabling warpx.safe_guard_cells=1 still leaves Efield_aux/Bfield_aux guard regions partially stale, defeating the point of the safety mode and making aux gathers behave differently from the main E/B grids.

Impact

• Developers rely on m_safe_guard_cells to detect missing guard exchanges; aux grids now silently skip that enforcement, so particle gathers can still read garbage on the aux MultiFabs even though the rest of the code is running in "safe" mode.
• Diagnostics and solvers that alias Efield_aux see different guard widths refreshed compared to Efield_fp, which is confusing and makes it harder to compare auxiliary grids against their sources when debugging guard overruns.

Suggested patch

--- a/Parallelization/WarpXComm.cpp
+++ b/Parallelization/WarpXComm.cpp
@@
 WarpX::FillBoundaryAux (int lev, IntVect ng)
 {
     ablastr::fields::MultiLevelVectorField Efield_aux = m_fields.get_mr_levels_alldirs(FieldType::Efield_aux, finest_level);
     ablastr::fields::MultiLevelVectorField Bfield_aux = m_fields.get_mr_levels_alldirs(FieldType::Bfield_aux, finest_level);
  
     const amrex::Periodicity& period = Geom(lev).periodicity();
-    ablastr::utils::communication::FillBoundary(*Efield_aux[lev][0], ng, WarpX::do_single_precision_comms, period);
-    ablastr::utils::communication::FillBoundary(*Efield_aux[lev][1], ng, WarpX::do_single_precision_comms, period);
-    ablastr::utils::communication::FillBoundary(*Efield_aux[lev][2], ng, WarpX::do_single_precision_comms, period);
-    ablastr::utils::communication::FillBoundary(*Bfield_aux[lev][0], ng, WarpX::do_single_precision_comms, period);
-    ablastr::utils::communication::FillBoundary(*Bfield_aux[lev][1], ng, WarpX::do_single_precision_comms, period);
-    ablastr::utils::communication::FillBoundary(*Bfield_aux[lev][2], ng, WarpX::do_single_precision_comms, period);
+    auto fill_aux = [this, &period, &ng](amrex::MultiFab* mf)
+    {
+        const amrex::IntVect& nghost = (m_safe_guard_cells) ? mf->nGrowVect() : ng;
+        ablastr::utils::communication::FillBoundary(*mf, nghost, WarpX::do_single_precision_comms, period);
+    };
+    fill_aux(Efield_aux[lev][0]);
+    fill_aux(Efield_aux[lev][1]);
+    fill_aux(Efield_aux[lev][2]);
+    fill_aux(Bfield_aux[lev][0]);
+    fill_aux(Bfield_aux[lev][1]);
+    fill_aux(Bfield_aux[lev][2]);
 }

Prepared by Codex