BugFix: Don't INCREF a Py section just allocated

src/nrnpython/nrnpy_nrn.cpp

@@ -1881,20 +1881,9 @@ static PyObject* mech_of_segment_iter_safe(NPySegObj* self) {
 static Object* seg_from_sec_x(Section* sec, double x) {
     auto pyseg = nb::steal((PyObject*) PyObject_New(NPySegObj, psegment_type));
     auto* pseg = (NPySegObj*) pyseg.ptr();
-    auto* pysec = static_cast<NPySecObj*>(sec->prop->dparam[PROP_PY_INDEX].get<void*>());
-    if (pysec) {
-        pseg->pysec_ = pysec;
-        Py_INCREF(pysec);
-    } else {
-        pysec = (NPySecObj*) psection_type->tp_alloc(psection_type, 0);
-        pysec->sec_ = sec;
-        pysec->name_ = 0;
-        pysec->cell_weakref_ = 0;
-        Py_INCREF(pysec);
-        pseg->pysec_ = pysec;
-    }
+    pseg->pysec_ = newpysechelp(sec);  // newpysechelp() already increfs
     pseg->x_ = x;
-    return nrnpy_pyobject_in_obj(pyseg.ptr());
+    return nrnpy_pyobject_in_obj((PyObject*) pseg);
 }
 
 static Object** pp_get_segment(void* vptr) {

test/unit_tests/hoc_python/test_refcounts.py

@@ -0,0 +1,55 @@
+from neuron import h, nrn
+import gc
+import sys
+
+
+def test_seg_from_sec_x_ref_python():
+    """
+    A reproducer of calling `seg_from_sec_x` with a Python Section
+    """
+    soma1 = nrn.Section("soma1")
+    nc1 = h.NetCon(soma1(0.5)._ref_v, None)
+    seg1 = nc1.preseg()
+    del soma1
+
+    assert sys.getrefcount(seg1) == 2
+    # It is unclear why we get 3 ref-counts when we use plain python. With nrniv we get 2.
+    # At least it is consistent with the test below and we assert that the section
+    # really gets deleted (in hoc)
+    assert sys.getrefcount(seg1.sec) == 3  # seg->pysec_ + 1
+
+    del seg1  # NOTE: section fully destroyed together with segment
+    gc.collect()
+
+    h("n_sections=0")
+    h("forall { n_sections+=1 }")
+    h('soma_is_valid=section_exists("soma1")')
+    assert h.n_sections == 0
+    assert not h.soma_is_valid
+
+
+def test_seg_from_sec_x_ref_hocpy():
+    """A reproducer of calling `seg_from_sec_x` without a Python Section
+
+    Without the fix we would get one extra ref to sec and possibly (upon interpreter exit):
+      "Fatal Python error: bool_dealloc: "deallocating True or False: bug likely caused by a
+       refcount error in a C extension"
+    """
+    h("create soma")
+    h("objref nc")
+    h("objref nil")
+    h("soma nc = new NetCon(&v(0.5), nil)")
+    nc = h.nc
+    seg = nc.preseg()  # uses `seg_from_sec_x`
+
+    assert sys.getrefcount(seg) == 2
+    assert sys.getrefcount(seg.sec) == 3  # seg->pysec_ + 1
+
+    del seg  # NOTE: section py wrapper destroyed with segment, hoc-level section kept
+    gc.collect()
+
+    h("n_sections=0")
+    h("forall { n_sections+=1 }")
+    h('soma_is_valid=section_exists("soma")')
+    assert h.n_sections == 1
+    assert h.soma_is_valid