MultiFab: Fix Fixture Lifetime

pytest fixtures are cached by default. That's not ideal for us,
since we want to clean up and re-init AMReX and its memory
arenas between tests.

Thus, we now create a multifab fixture that is just a generator,
so that the returned/yielded object can be cached - but the
generated MultiFab is actually destroyed as we would expect once
the test finishes.

tests/conftest.py

@@ -68,10 +68,19 @@ def distmap(boxarr):
 
 
 @pytest.fixture(scope="function", params=list(itertools.product([1, 3], [0, 1])))
-def mfab(boxarr, distmap, request):
-    """MultiFab for tests"""
-    num_components = request.param[0]
-    num_ghost = request.param[1]
-    mfab = amrex.MultiFab(boxarr, distmap, num_components, num_ghost)
-    mfab.set_val(0.0, 0, num_components)
-    return mfab
+def make_mfab(boxarr, distmap, request):
+    """MultiFab for tests:
+    The MultiFab object itself is not a fixture because we want to avoid caching
+    it between amrex.initialize/finalize calls of various tests.
+    https://github.com/pytest-dev/pytest/discussions/10387
+    https://github.com/pytest-dev/pytest/issues/5642#issuecomment-1279612764
+    """
+
+    def create():
+        num_components = request.param[0]
+        num_ghost = request.param[1]
+        mfab = amrex.MultiFab(boxarr, distmap, num_components, num_ghost)
+        mfab.set_val(0.0, 0, num_components)
+        return mfab
+
+    return create

tests/test_multifab.py

@@ -6,8 +6,8 @@
 import amrex
 
 
-@pytest.mark.parametrize("nghost", [0, 1])
-def test_mfab_loop(mfab, nghost):
+def test_mfab_loop(make_mfab):
+    mfab = make_mfab()
     ngv = mfab.nGrowVect
     print(f"\n  mfab={mfab}, mfab.nGrowVect={ngv}")
 
@@ -77,7 +77,8 @@ def test_mfab_loop(mfab, nghost):
         #   TODO
 
 
-def test_mfab_simple(mfab):
+def test_mfab_simple(make_mfab):
+    mfab = make_mfab()
     assert mfab.is_all_cell_centered
     # assert(all(not mfab.is_nodal(i) for i in [-1, 0, 1, 2]))  # -1??
     assert all(not mfab.is_nodal(i) for i in [0, 1, 2])
@@ -142,8 +143,8 @@ def test_mfab_ops(boxarr, distmap, nghost):
     np.testing.assert_allclose(dst.max(0), 150.0)
 
 
-@pytest.mark.parametrize("nghost", [0, 1])
-def test_mfab_mfiter(mfab, nghost):
+def test_mfab_mfiter(make_mfab):
+    mfab = make_mfab()
     assert iter(mfab).is_valid
     assert iter(mfab).length == 8