From 45ec61f419d8c79b8fcb047b20e740c48c4e4478 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Mon, 14 Jun 2021 14:32:42 -0700
Subject: [PATCH] Adding
 shared_ptr_reset_and_rescue_pointee_model_proof_of_concept.

---
 tests/test_class_sh_shared_from_this.cpp | 59 ++++++++++++++++++++++++
 tests/test_class_sh_shared_from_this.py  |  9 ++++
 2 files changed, 68 insertions(+)

diff --git a/tests/test_class_sh_shared_from_this.cpp b/tests/test_class_sh_shared_from_this.cpp
index d7673f0c7..09fdbff12 100644
--- a/tests/test_class_sh_shared_from_this.cpp
+++ b/tests/test_class_sh_shared_from_this.cpp
@@ -95,6 +95,62 @@ void obj_sft_reset() {
 
 } // namespace shared_from_this_custom_deleters
 
+namespace shared_ptr_reset_and_rescue_pointee_model {
+
+struct ToBeWrapped : std::enable_shared_from_this<ToBeWrapped> {};
+
+struct RescuingDeleter;
+
+struct PyWrapper {
+    std::unique_ptr<RescuingDeleter> rdel;
+    std::shared_ptr<ToBeWrapped> wobj;
+    std::shared_ptr<PyWrapper> self;
+};
+
+struct RescuingDeleter {
+    PyWrapper *pyw;
+    explicit RescuingDeleter(PyWrapper *pyw) : pyw{pyw} {}
+    void operator()(ToBeWrapped *raw_ptr) {
+        if (pyw->self.get() != nullptr) {
+            assert(raw_ptr->weak_from_this().expired()); // CRITICAL
+            pyw->wobj = std::shared_ptr<ToBeWrapped>(raw_ptr, *this);
+            pyw->self.reset();
+        } else {
+            delete raw_ptr;
+        }
+    }
+};
+
+std::shared_ptr<ToBeWrapped> release_to_cpp(const std::shared_ptr<PyWrapper> &pyw) {
+    std::shared_ptr<ToBeWrapped> return_value = pyw->wobj;
+    pyw->wobj.reset();
+    pyw->self = pyw;
+    return return_value;
+}
+
+void proof_of_concept() {
+    std::shared_ptr<PyWrapper> pyw(new PyWrapper);
+    pyw->rdel = std::unique_ptr<RescuingDeleter>(new RescuingDeleter(pyw.get()));
+    pyw->wobj = std::shared_ptr<ToBeWrapped>(new ToBeWrapped, *pyw->rdel);
+    std::shared_ptr<ToBeWrapped> cpp_owner = release_to_cpp(pyw);
+    assert(pyw->wobj.get() == nullptr);
+    assert(cpp_owner.use_count() == 1);
+    {
+        std::shared_ptr<ToBeWrapped> sft = cpp_owner->shared_from_this();
+        assert(cpp_owner.use_count() == 2);
+    }
+    assert(cpp_owner.use_count() == 1);
+    cpp_owner.reset();
+    assert(pyw->wobj.get() != nullptr);
+    assert(pyw->wobj.use_count() == 1);
+    {
+        std::shared_ptr<ToBeWrapped> sft = pyw->wobj->shared_from_this();
+        assert(pyw->wobj.use_count() == 2);
+    }
+}
+
+} // namespace shared_ptr_reset_and_rescue_pointee_model
+
 namespace test_class_sh_shared_from_this {
 
 // clang-format off
@@ -175,4 +231,7 @@ TEST_SUBMODULE(class_sh_shared_from_this, m) {
     m.def("obj1_owns", shared_from_this_custom_deleters::obj1_owns);
     m.def("obj2_owns", shared_from_this_custom_deleters::obj2_owns);
     m.def("obj_sft_reset", shared_from_this_custom_deleters::obj_sft_reset);
+
+    m.def("shared_ptr_reset_and_rescue_pointee_model_proof_of_concept",
+          shared_ptr_reset_and_rescue_pointee_model::proof_of_concept);
 }
diff --git a/tests/test_class_sh_shared_from_this.py b/tests/test_class_sh_shared_from_this.py
index 26501bf68..6319029f7 100644
--- a/tests/test_class_sh_shared_from_this.py
+++ b/tests/test_class_sh_shared_from_this.py
@@ -121,3 +121,12 @@ def test_shared_from_this_virt():
 )
 def test_shared_from_this_custom_deleters(test_func):
     test_func()
+
+
+def test_shared_ptr_reset_and_rescue_pointee_model(loop_count_max=10 ** 6):
+    loop_count = 0
+    while True:
+        m.shared_ptr_reset_and_rescue_pointee_model_proof_of_concept()
+        loop_count += 1
+        if loop_count == loop_count_max:
+            break