`Print` / `PrintToFile` ignore constructor `MPI_Comm` when deciding emitting rank

[WeiqunZhang]

• Type: Correctness / Rank-space mismatch
• Severity: Medium
• Component: Print utilities
• Location:
  • Src/Base/AMReX_Print.H:64
  • Src/Base/AMReX_Print.H:70
  • Src/Base/AMReX_Print.H:145
  • Src/Base/AMReX_Print.H:151
  • Src/Base/AMReX_Print.H:184

Problem

Both classes accept a user-provided MPI_Comm and store it in comm, but rank checks use ParallelContext::MyProcSub() unconditionally:

if (rank == AllProcs || rank == ParallelContext::MyProcSub())

This ignores the supplied communicator and compares rank in the wrong rank space.

Impact

• Printing can happen on the wrong rank (or not happen) when using non-default communicators.
• Behavior contradicts the API contract documented in constructors taking (rank, comm).

Suggested patch

Use communicator-relative rank for filtering, and only use sub-context file stream plumbing when communicator matches sub-communicator.

--- a/Src/Base/AMReX_Print.H
+++ b/Src/Base/AMReX_Print.H
@@
-            if (rank == AllProcs || rank == ParallelContext::MyProcSub()) {
+            int const my_rank = ParallelDescriptor::MyProc(comm);
+            if (rank == AllProcs || rank == my_rank) {
@@
-            if (rank == AllProcs || rank == ParallelContext::MyProcSub()) {
+            int const my_rank = ParallelDescriptor::MyProc(comm);
+            if (rank == AllProcs || rank == my_rank) {
@@
-            int my_proc = ParallelContext::MyProcSub();
+            int my_proc = ParallelDescriptor::MyProc(comm);

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