Pickling of a form fails after a form was used in a template

[Torsten Bronger]

The following view function fails from its second invocation on:

def main(request):
    import django.forms as forms
    import pickle
    pickle.dump(forms.Form(), open("/dev/null", "wb"))  # This fails, but not at the very first invocation.
    return render(request, "main.html", {"example_form": ExampleForm()})

Steps to reproduce:

1. django-admin startproject pickle_problem
2. cd pickle_problem
3. Apply the attached patch with -p1.
4. Apply migrations, start testserver.
5. Open the URL “/” on the testserver. You should see a bogus form with one boolean field.
6. Reload the page.

What happens:

The pickling of form.Form() fails. See the attached traceback.

I observe this problem since Django 1.11. Up to Django 1.10, it has worked fine for me.

[Torsten Bronger]

Output of Django's testserver, including the traceback.

[Torsten Bronger]

Patch to create a minimal example.

[Torsten Bronger]

With the following view function, the error can be reproduced without reloading the page, which may be easier for automatic reproducing:

def main(request):
    pickle.dump(forms.Form(), open("/dev/null", "wb"))
    result = render(request, "main.html", {"example_form": ExampleForm()})
    pickle.dump(forms.Form(), open("/dev/null", "wb"))
    return result

[Carlton Gibson]

This looks like a duplicate of #28223. (The traceback error is the same as reported there.)