From 43816c2528697d0428bab1b9432d681684d6ed6f Mon Sep 17 00:00:00 2001
From: Pablo Brubeck <brubeck@protonmail.com>
Date: Thu, 27 Aug 2026 11:12:38 +0100
Subject: [PATCH] Leave PETSc initialisable after importing petsctools

`options.py` needs `PETSc.Options` at import time, to subclass it, but must
not initialise PETSc to get it, so it calls `petsc4py.lib.ImportPETSc`.  That
registers the extension module under the name of the `petsc4py/PETSc.py`
shim it shadows, both in `sys.modules` and as an attribute of the `petsc4py`
package.  The shim is the only thing that ever calls `PETSc._initialize`, so
once it is shadowed it never runs again: every later `from petsc4py import
PETSc` in the process hands back a PETSc that nothing has initialised.

Anything that then reaches into PETSc dereferences a null pointer.  In
Firedrake's `tests/tsfc` session, which imports `petsctools` through
`finat.citations` and never calls `petsctools.init`, every worker segfaults
at its first form compilation:

    petsctools/citation.py:58 in cite -> PETSc.Sys.registerCitation
    libpetsc.so.3.025 PetscSegBufferGet+0x1c   <- PetscCitationsList is NULL

Put the import machinery back as it was found, so that the shim still runs
for whoever imports `petsc4py.PETSc` next.  The module object is unchanged:
CPython keeps its own cache of single-phase extension modules, so the later
import returns the same object and only adds the initialisation.

This was found with the assistance of Claude Code.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
---
 petsctools/options.py | 34 +++++++++++++++++++++++++++++++++-
 tests/test_options.py | 20 ++++++++++++++++++++
 2 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/petsctools/options.py b/petsctools/options.py
index 5e382eb..f4a073d 100644
--- a/petsctools/options.py
+++ b/petsctools/options.py
@@ -4,6 +4,7 @@
 import functools
 import itertools
 import numbers
+import sys
 import types
 import warnings
 import weakref
@@ -20,9 +21,40 @@
     PetscToolsWarning,
 )
 
+
+def _import_petsc_without_initialising():
+    """Import ``petsc4py.PETSc`` without initialising PETSc.
+
+    Returns
+    -------
+    types.ModuleType
+        The ``petsc4py.PETSc`` extension module.
+
+    Notes
+    -----
+    This is the dance `petsctools.init` does, except that it puts the import
+    machinery back as it found it.  `petsc4py.lib.ImportPETSc` registers the
+    extension module as ``petsc4py.PETSc``, which is the name of the shim it
+    shadows, and that shim is the only thing that ever calls
+    ``PETSc._initialize``.  Left registered, it stops the shim running for the
+    rest of the process, so a later ``from petsc4py import PETSc`` hands back
+    a PETSc that nothing has initialised, and the first call into it
+    dereferences a null pointer.
+
+    """
+    in_modules = "petsc4py.PETSc" in sys.modules
+    is_attribute = hasattr(petsc4py, "PETSc")
+    PETSc = petsc4py.lib.ImportPETSc()
+    if not in_modules:
+        del sys.modules["petsc4py.PETSc"]
+    if not is_attribute:
+        delattr(petsc4py, "PETSc")
+    return PETSc
+
+
 # Do this instead of 'from petsc4py import PETSc' to make sure we don't import
 # (and hence initialise) PETSc.
-PETSc = petsc4py.lib.ImportPETSc()
+PETSc = _import_petsc_without_initialising()
 
 _commandline_options = None
 
diff --git a/tests/test_options.py b/tests/test_options.py
index 1fa3b0a..db334b0 100644
--- a/tests/test_options.py
+++ b/tests/test_options.py
@@ -1,3 +1,5 @@
+import subprocess
+import sys
 import warnings
 
 import pytest
@@ -376,3 +378,21 @@ def test_inserted_options_dict():
     with petsctools.inserted_options(parameters=params, options_prefix=prefix):
         assert PETSc.Options().getInt("prefix_opt_int") == 3
         assert PETSc.Options().getBool("prefix_opt_flag")
+
+
+@pytest.mark.skipnopetsc4py
+def test_importing_petsctools_leaves_petsc_initialisable():
+    """Importing petsctools must not stop the petsc4py shim from running.
+
+    The shim is the only thing that calls ``PETSc._initialize``, so for any
+    code that does not call `petsctools.init` itself it is what initialises
+    PETSc.  Run in a subprocess, since PETSc is already initialised here.
+    """
+    script = (
+        "import sys\n"
+        "import petsctools\n"
+        "assert 'petsc4py.PETSc' not in sys.modules\n"
+        "from petsc4py import PETSc\n"
+        "assert PETSc.Sys.isInitialized()\n"
+    )
+    subprocess.run([sys.executable, "-c", script], check=True)