Partially re-enable a real mpi lib in run_tests.py

This restores some of the mpi lib selection functionality that was
removed in #4397. However, I am *not*
fully re-enabling mpi support, because I'm not sure what's needed at
this point to actually run pFUnit via MPI. So this just puts in place an
ability to build the code using an MPI-based software stack rather than
an mpi-serial-based software stack.

I'm hoping this will help resolve some issues with doing unit testing of
the CESM_share code via GitHub actions, where we have an mpi-enabled
ESMF rather than an mpi-serial-based ESMF.

Author: billsacks

scripts/fortran_unit_testing/run_tests.py

@@ -108,6 +108,16 @@ def parse_command_line(args):
         help="""Number of processes to use for build.""",
     )
 
+    parser.add_argument(
+        "--mpilib",
+        default="mpi-serial",
+        help="""MPI Library to use in build.
+                        If not specified, use mpi-serial.
+                        Note that we don't fully support running parallel tests,
+                        but this option allows use of an mpi-based software stack
+                        rather than requiring an mpi-serial-based software stack."""
+    )
+
     parser.add_argument(
         "--test-spec-dir",
         default=".",
@@ -161,6 +171,7 @@ def parse_command_line(args):
         args.machine,
         args.machines_dir,
         args.make_j,
+        args.mpilib,
         args.test_spec_dir,
         args.ctest_args,
         args.use_openmp,
@@ -300,6 +311,7 @@ def _main():
         machine,
         machines_dir,
         make_j,
+        mpilib,
         test_spec_dir,
         ctest_args,
         use_openmp,
@@ -359,12 +371,10 @@ def _main():
     # Functions to perform various stages of build.
     # =================================================
 
-    # In the switch from pFUnit3 to pFUnit4, we have dropped support for MPI for now
-    # because it seems like the way this is done differs for pFUnit4 and we weren't
-    # leveraging the parallel capabilities of pFUnit anyway. So we force mpilib =
-    # "mpi-serial" and use_mpiserial = True for now until we need to generalize this.
-    mpilib = "mpi-serial"
-    use_mpiserial = True
+    # Note that we currently only have partial support for a real MPI library: We allow
+    # use of something other than mpi-serial in order to find the appropriate software
+    # stack and do the build, but we don't currently support running parallel tests.
+    use_mpiserial = (mpilib == "mpi-serial")
 
     if compiler is None:
         compiler = machobj.get_default_compiler()