WIP Emit a helpful error message when trying to run in forking mode

pytest_mpi/plugin.py

@@ -86,13 +86,28 @@ def pytest_generate_tests(metafunc):
 
 @pytest.hookimpl()
 def pytest_collection_modifyitems(config, items):
+    from mpi4py import MPI
+
     global _plugin_in_use
 
     _plugin_in_use = any(item.get_closest_marker("parallel") for item in items)
 
     if not _plugin_in_use:
         return
 
+    if MPI.COMM_WORLD.size == 1:
+        has_multi_rank_tests = False
+        for item in items:
+            if item.get_closest_marker("parallel") and _extract_nprocs_for_single_test(item) > 1:
+                has_multi_rank_tests = True
+                break
+        if has_multi_rank_tests and not _mpi_supports_multiple_inits():
+            raise pytest.UsageError(
+                "MPI library does not support multiple calls to MPI_Init so parallel "
+                "tests must be run with mpiexec on the 'outside'. For example:\n\t"
+                "mpiexec -n 2 pytest test_something.py -m parallel[2]"
+            )
+
     for item in items:
         if item.get_closest_marker("parallel"):
             # Add extra markers to each test to allow for querying specific levels of
@@ -191,6 +206,17 @@ def _xdist_active(session):
         return False
 
 
+def _mpi_supports_multiple_inits():
+    from mpi4py import MPI
+
+    assert MPI.Is_initialized()
+    try:
+        MPI.Init()
+        return True
+    except MPI.Exception:
+        return False
+
+
 def _set_parallel_callback(item):
     """Replace the callback for a test item with one that calls ``mpiexec``.