Fix lifetime issue for BaseMatrix derived in Python

Using workaround from here:
pybind/pybind11#1546

Author: mhochsteger

linalg/python_linalg.cpp

@@ -7,6 +7,57 @@ using namespace ngla;
 #include <myadt.hpp>
 
 
+// Workaround to ensure same lifetime for C++ and Python objects
+// see https://github.com/pybind/pybind11/issues/1546
+namespace pybind11::detail {
+
+  template<>
+  struct type_caster<std::shared_ptr<BaseMatrix>>
+  {
+    PYBIND11_TYPE_CASTER (std::shared_ptr<BaseMatrix>, _("BaseMatrix"));
+
+    using BaseCaster = copyable_holder_caster<BaseMatrix, std::shared_ptr<BaseMatrix>>;
+
+    bool load (pybind11::handle src, bool b)
+    {
+      BaseCaster bc;
+      bool success = bc.load (src, b);
+      if (!success)
+      {
+        return false;
+      }
+
+      auto py_obj = py::reinterpret_borrow<py::object> (src);
+      auto base_ptr = static_cast<std::shared_ptr<BaseMatrix>> (bc);
+
+      // Construct a shared_ptr to the py::object
+      auto py_obj_ptr = std::shared_ptr<object>{
+          new object{py_obj},
+          [](auto py_object_ptr) {
+              // It's possible that when the shared_ptr dies we won't have the
+              // gil (if the last holder is in a non-Python thread), so we
+              // make sure to acquire it in the deleter.
+              gil_scoped_acquire gil;
+              delete py_object_ptr;
+         }
+      };
+
+      value = std::shared_ptr<BaseMatrix> (py_obj_ptr, base_ptr.get ());
+      return true;
+    }
+
+    static handle cast (std::shared_ptr<BaseMatrix> base,
+                        return_value_policy rvp,
+                        handle h)
+    {
+      return BaseCaster::cast (base, rvp, h);
+    }
+  };
+
+  template <>
+  struct is_holder_type<BaseMatrix, std::shared_ptr<BaseMatrix>> : std::true_type {};
+}
+
 template<typename T>
 void ExportSparseMatrix(py::module m)
 {
@@ -570,7 +621,7 @@ void NGS_DLL_HEADER ExportNgla(py::module &m) {
         new (instance) BaseMatrixTrampoline(); }
         )
     */
-    .def(py::init<> ([]() { return new BaseMatrixTrampoline(); }))
+    .def(py::init<> ())
     .def("__str__", [](BaseMatrix &self) { return ToString<BaseMatrix>(self); } )
     .def_property_readonly("height", [] ( BaseMatrix & self)
                            { return self.Height(); }, "Height of the matrix" )

tests/pytest/test_basematrix.py

@@ -15,6 +15,9 @@ def MultTransAdd(self, s, x, y):
         for i in range(len(x)):
             y[i] = s*sum(x[i:])
 
+    def CreateColVector(self):
+        return CreateVVector(5)
+
 def test_derive_basematrix():
     m = MyMatrix()
 
@@ -36,3 +39,21 @@ def test_derive_basematrix():
 
 
 
+def test_derive_basematrix_lifetime():
+    m = MyMatrix()@MyMatrix()
+    m1 = MyMatrix()
+
+    x = CreateVVector(5)
+    y = CreateVVector(5)
+    y1 = CreateVVector(5)
+
+    for i in range(len(x)):
+        x[i]=1+i
+
+    y.data = m*x
+    y1.data = (m1@m1)*x
+    assert list(y) == list(y1)
+
+
+
+