mpi integration test

examples/integration/mpi4py/_child.py

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+import os
+
+
+async def child_fn() -> str:
+    return f"child OK  pid={os.getpid()}"

examples/integration/mpi4py/inherit_parent_main.py

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+"""
+Integration test: spawning tractor actors from an MPI process.
+
+When a parent is launched via ``mpirun``, Open MPI sets ``OMPI_*`` env
+vars that bind ``MPI_Init`` to the ``orted`` daemon.  Tractor children
+inherit those env vars, so if ``inherit_parent_main=True`` (the default)
+the child re-executes ``__main__``, re-imports ``mpi4py``, and
+``MPI_Init_thread`` fails because the child was never spawned by
+``orted``::
+
+    getting local rank failed
+      --> Returned value No permission (-17) instead of ORTE_SUCCESS
+
+Passing ``inherit_parent_main=False`` and placing RPC functions in a
+separate importable module (``_child``) avoids the re-import entirely.
+
+Usage::
+
+    mpirun --allow-run-as-root -np 1 python -m \
+        examples.integration.mpi4py.inherit_parent_main
+"""
+
+from mpi4py import MPI
+
+import os
+import trio
+import tractor
+
+from ._child import child_fn
+
+
+async def main() -> None:
+    rank = MPI.COMM_WORLD.Get_rank()
+    print(f"[parent] rank={rank}  pid={os.getpid()}", flush=True)
+
+    async with tractor.open_nursery(start_method='trio') as an:
+        portal = await an.start_actor(
+            'mpi-child',
+            enable_modules=[child_fn.__module__],
+            # Without this the child replays __main__, which
+            # re-imports mpi4py and crashes on MPI_Init.
+            inherit_parent_main=False,
+        )
+        result = await portal.run(child_fn)
+        print(f"[parent] got: {result}", flush=True)
+        await portal.cancel_actor()
+
+
+if __name__ == "__main__":
+    trio.run(main)