[python] Make sure we use the serial multithreading backend

The current implementatino of the Pythons where C++ calls Python
code without making sure to have the GIL is not thread-safe.

python/dumux/assembly/__init__.py

+# pylint: skip-file
+# until that decorator bit can be removed again
+
 """Classes and function related to the assembly of linear systems"""
 
 from dune.generator.generator import SimpleGenerator
@@ -5,6 +8,25 @@ from dune.common.hashit import hashIt
 from dumux.wrapping import cppWrapperCreator, cppWrapperClassAlias
 
 
+def decoratePre(pre):
+    def wrappedPre(*args, **kwargs):
+        preamble = pre(*args, **kwargs)
+        newPreamble = ""
+        for line in preamble.split("\n"):
+            newPreamble += line + "\n"
+            if line.startswith("#include <config.h>"):
+                newPreamble += "#undef DUMUX_MULTITHREADING_BACKEND\n"
+                newPreamble += "#define DUMUX_MULTITHREADING_BACKEND Serial\n"
+        return newPreamble
+
+    return wrappedPre
+
+
+myAttributes = vars(SimpleGenerator).copy()
+myAttributes["pre"] = decoratePre(myAttributes["pre"])
+MySimpleGenerator = type("MySimpleGenerator", (object,), myAttributes)
+
+
 @cppWrapperCreator
 def _createFVAssembler(*, problem, gridVariables, model, diffMethod="numeric", isImplicit=True):
     """
@@ -40,13 +62,20 @@ def _createFVAssembler(*, problem, gridVariables, model, diffMethod="numeric", i
     includes += ["dumux/python/assembly/fvassembler.hh"]
 
     moduleName = "fvassembler_" + hashIt(assemblerType)
-    generator = SimpleGenerator("FVAssembler", "Dumux::Python")
+    # remark: use SimpleGenerator again starting with dune 2.9
+    generator = MySimpleGenerator("FVAssembler", "Dumux::Python")
     module = generator.load(
         includes,
         assemblerType,
         moduleName,
         holder="std::shared_ptr",
         preamble=model.cppHeader,
+        # make sure the assembler is compiled with the Serial backend
+        # as currently the assembly in combination with Python is not thread-safe
+        # the following is nicer but only works with dune > 2.8
+        # extraCMake=[
+        #     "target_compile_definitions(TARGET PUBLIC DUMUX_MULTITHREADING_BACKEND=Serial)"
+        # ],
     )
     return module.FVAssembler(problem, problem.gridGeometry(), gridVariables)
 

python/dumux/common/CMakeLists.txt

@@ -2,7 +2,14 @@ add_python_targets(common
   __init__
   properties
 )
-dune_add_pybind11_module(NAME _common)
+
+# currently the Python bindings only work with
+# serial dumux code
+dune_add_pybind11_module(
+  NAME _common
+  COMPILE_DEFINITIONS DUMUX_MULTITHREADING_BACKEND=Serial
+)
+
 set_property(TARGET _common PROPERTY LINK_LIBRARIES dunecommon dunegrid APPEND)
 
 if(SKBUILD)