Attempted fix for pythonx reference issue

c_src/pythonx.cpp

@@ -476,7 +476,9 @@ pythonx_handle_io_write = ctypes.CFUNCTYPE(
   None, ctypes.c_char_p, ctypes.c_char_p, ctypes.c_bool
 )(pythonx_handle_io_write_ptr)
 
-pythonx_handle_send_tagged_object = ctypes.CFUNCTYPE(
+# PYFUNCTYPE keeps the GIL held during the call, which the handler needs
+# to take a reference to the object.
+pythonx_handle_send_tagged_object = ctypes.PYFUNCTYPE(
   None, ctypes.c_char_p, ctypes.c_char_p, ctypes.py_object, ctypes.c_char_p
 )(pythonx_handle_send_tagged_object_ptr)
 
@@ -1638,6 +1640,11 @@ pythonx_handle_send_tagged_object(const char *pid_bytes, const char *tag,
   auto pid = ErlNifPid{};
   std::memcpy(&pid, pid_bytes, sizeof(ErlNifPid));
 
+  // The object is a borrowed reference, while the resource releases a
+  // reference once garbage collected, so we take one for it. The caller
+  // holds the GIL (see the PYFUNCTYPE prototype).
+  pythonx::python::Py_IncRef(py_object);
+
   auto msg = fine::encode(
       env, std::make_tuple(
                fine::Atom(tag),

test/pythonx_test.exs

@@ -475,6 +475,36 @@ defmodule PythonxTest do
       assert_receive {:message_from_python, %Pythonx.Object{} = object}
       assert repr(object) == "('hello', 1)"
     end
+
+    test "pythonx.send_tagged_object takes a reference for the sent object" do
+      # Like any %Pythonx.Object{}, the sent one holds a reference, which
+      # its resource releases once it's garbage collected. Without taking
+      # one, that release frees the object while Python still uses it.
+      #
+      # Both counts are taken here, while the message is in the mailbox.
+      # Sending leaves reference cycles behind (from inspect.stack()) that
+      # hold on to the object, so those are collected first.
+      {result, _globals} =
+        Pythonx.eval(
+          """
+          import gc
+          import sys
+          import pythonx
+
+          obj = [1, 2, 3]
+          before = sys.getrefcount(obj)
+          pythonx.send_tagged_object(pid, "message_from_python", obj)
+          gc.collect()
+          (before, sys.getrefcount(obj))
+          """,
+          %{"pid" => self()}
+        )
+
+      assert_receive {:message_from_python, %Pythonx.Object{}}
+
+      {before, after_send} = Pythonx.decode(result)
+      assert after_send == before + 1
+    end
   end
 
   describe "remote evaluation" do