Fixes for updated PETSc (#3853)

* Add petsc_raises fixture to catch PETSc callback exceptions

tests/conftest.py

@@ -1,7 +1,7 @@
 """Global test configuration."""
 
 import pytest
-from firedrake.petsc import get_external_packages
+from firedrake.petsc import PETSc, get_external_packages
 
 
 def pytest_configure(config):
@@ -122,3 +122,34 @@ def fin():
             assert len(tape.get_blocks()) == 0
 
     request.addfinalizer(fin)
+
+
+class _petsc_raises:
+    """Context manager for catching PETSc-raised exceptions.
+
+    The usual `pytest.raises` exception handler is not suitable for errors
+    raised inside a callback to PETSc because the error is wrapped inside a
+    `PETSc.Error` object and so this context manager unpacks this to access
+    the actual internal error.
+
+    Parameters
+    ----------
+    exc_type :
+        The exception type that is expected to be raised inside a PETSc callback.
+
+    """
+    def __init__(self, exc_type):
+        self.exc_type = exc_type
+
+    def __enter__(self):
+        pass
+
+    def __exit__(self, exc_type, exc_val, traceback):
+        if exc_type is PETSc.Error and isinstance(exc_val.__cause__, self.exc_type):
+            return True
+
+
+@pytest.fixture
+def petsc_raises():
+    # This function is needed because pytest does not support classes as fixtures.
+    return _petsc_raises

tests/macro/test_macro_multigrid.py

@@ -140,7 +140,7 @@ def test_macro_grid_transfer(hierarchy, space, degrees, variant, transfer_type):
 
 
 @pytest.mark.parametrize("degree", (1,))
-def test_macro_multigrid_poisson(hierarchy, degree, variant):
+def test_macro_multigrid_poisson(hierarchy, degree, variant, petsc_raises):
     mesh = hierarchy[-1]
     V = FunctionSpace(mesh, "CG", degree, variant=variant)
     u = TrialFunction(V)
@@ -153,7 +153,7 @@ def test_macro_multigrid_poisson(hierarchy, degree, variant):
     problem = LinearVariationalProblem(a, L, uh, bcs=bcs)
     solver = LinearVariationalSolver(problem, solver_parameters=mg_params)
     if complex_mode and variant == "alfeld":
-        with pytest.raises(NotImplementedError):
+        with petsc_raises(NotImplementedError):
             solver.solve()
     else:
         solver.solve()
@@ -172,7 +172,7 @@ def square_hierarchy():
 
 
 @pytest.mark.parametrize("family", ("HCT-red", "HCT"))
-def test_macro_multigrid_biharmonic(square_hierarchy, family):
+def test_macro_multigrid_biharmonic(square_hierarchy, family, petsc_raises):
     mesh = square_hierarchy[-1]
     V = FunctionSpace(mesh, family, 3)
     u = TrialFunction(V)
@@ -185,7 +185,7 @@ def test_macro_multigrid_biharmonic(square_hierarchy, family):
     problem = LinearVariationalProblem(a, L, uh, bcs=bcs)
     solver = LinearVariationalSolver(problem, solver_parameters=mg_params)
     if complex_mode:
-        with pytest.raises(NotImplementedError):
+        with petsc_raises(NotImplementedError):
             solver.solve()
     else:
         solver.solve()

tests/slate/test_slate_hybridization.py

@@ -130,7 +130,7 @@ def test_slate_hybridization(degree, hdiv_family, quadrilateral):
     assert u_err < 1e-11
 
 
-def test_slate_hybridization_wrong_option(setup_poisson):
+def test_slate_hybridization_wrong_option(setup_poisson, petsc_raises):
     a, L, W = setup_poisson
 
     w = Function(W)
@@ -145,18 +145,9 @@ def test_slate_hybridization_wrong_option(setup_poisson):
                                                'pc_fieldsplit_type': 'frog'}}}
     problem = LinearVariationalProblem(a, L, w)
     solver = LinearVariationalSolver(problem, solver_parameters=params)
-    with pytest.raises(ValueError):
-        # HybridizationPC isn't called directly from the Python interpreter,
-        # it's a callback that PETSc calls. This means that the call stack from pytest
-        # down to HybridizationPC goes via PETSc C code, which interferes with the exception
-        # before it is observed outside. Hence removing PETSc's error handler
-        # makes the problem go away, because PETSc stops interfering.
-        # We need to repush the error handler because popErrorHandler globally changes
-        # the system state for all future tests.
-        from firedrake.petsc import PETSc
-        PETSc.Sys.pushErrorHandler("ignore")
+
+    with petsc_raises(ValueError):
         solver.solve()
-        PETSc.Sys.popErrorHandler("ignore")
 
 
 def test_slate_hybridization_nested_schur(setup_poisson):