Form produces server errors when adding fields

[eloquence]

Steps to reproduce

1. Create a new form page
2. Add the first field to the form

Expected behavior

Form renders with field

Actual behavior

Internal server error in preview. If I save the draft, the error goes away.

Notes

It looks like form functionality has continued to degrade with recent Wagtail/Django upgrades; see #925 for a similar issue from last year.

[chigby]

Error occurs in this code:

securedrop.org/forms/templatetags/form_tags.py

Lines 21 to 28 in 8a2240d

	try:
	field_obj = filter(
	lambda mfield: form_field.name == mfield.clean_name,
	context['page'].form_fields.all()
	).__next__()
	except StopIteration:
	raise ValueError('Form field {} not found in form'.format(form_field.name))
	return field_obj

Which is triggered by the call to field_extras in the form field template. In this function, the form has not yet been saved (because a new page is being created), then the DB query in line 24 will return no records (because none have been saved yet), causing a StopIteration and then an uncaught ValueError.

The simplest way to fix this is to return None instead of raising an error, but I'm wondering if there's a more elegant way to add data to form fields that could be done in a way that wouldn't break the ClusterableModel-based preview feature.

[chigby]

Full traceback from my local system:

Traceback (most recent call last):
  File "/django/forms/templatetags/form_tags.py", line 22, in field_extras
    field_obj = filter(
StopIteration

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.9/site-packages/wagtail/models/__init__.py", line 622, in _get_response
    response = response.render()
  File "/usr/local/lib/python3.9/site-packages/django/template/response.py", line 105, in render
    self.content = self.rendered_content
  File "/usr/local/lib/python3.9/site-packages/django/template/response.py", line 83, in rendered_content
    return template.render(context, self._request)
  File "/usr/local/lib/python3.9/site-packages/django/template/backends/django.py", line 61, in render
    return self.template.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 170, in render
    return self._render(context)
  File "/usr/local/lib/python3.9/site-packages/django/test/utils.py", line 100, in instrumented_test_render
    return self.nodelist.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 938, in render
    bit = node.render_annotated(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/loader_tags.py", line 150, in render
    return compiled_parent._render(context)
  File "/usr/local/lib/python3.9/site-packages/django/test/utils.py", line 100, in instrumented_test_render
    return self.nodelist.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 938, in render
    bit = node.render_annotated(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/loader_tags.py", line 150, in render
    return compiled_parent._render(context)
  File "/usr/local/lib/python3.9/site-packages/django/test/utils.py", line 100, in instrumented_test_render
    return self.nodelist.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 938, in render
    bit = node.render_annotated(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/loader_tags.py", line 150, in render
    return compiled_parent._render(context)
  File "/usr/local/lib/python3.9/site-packages/django/test/utils.py", line 100, in instrumented_test_render
    return self.nodelist.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 938, in render
    bit = node.render_annotated(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/loader_tags.py", line 62, in render
    result = block.nodelist.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 938, in render
    bit = node.render_annotated(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/loader_tags.py", line 62, in render
    result = block.nodelist.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 938, in render
    bit = node.render_annotated(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/loader_tags.py", line 62, in render
    result = block.nodelist.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 938, in render
    bit = node.render_annotated(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/defaulttags.py", line 214, in render
    nodelist.append(node.render_annotated(context))
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/loader_tags.py", line 195, in render
    return template.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 172, in render
    return self._render(context)
  File "/usr/local/lib/python3.9/site-packages/django/test/utils.py", line 100, in instrumented_test_render
    return self.nodelist.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 938, in render
    bit = node.render_annotated(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/base.py", line 905, in render_annotated
    return self.render(context)
  File "/usr/local/lib/python3.9/site-packages/django/template/library.py", line 192, in render
    output = self.func(*resolved_args, **resolved_kwargs)
  File "/django/forms/templatetags/form_tags.py", line 27, in field_extras
    raise ValueError('Form field {} not found in form'.format(form_field.name))
ValueError: Form field favorite_color not found in form

[harrislapiroff]

I'm open to whoever takes this ticket on doing some timeboxed research (say 30m) to determine if there's an easy way to access the draft form field data. If it helps, request.is_preview allows you to determine whether the current request is a preview. (I believe request should be present in context.)

If no approach can be identified, returning None or a fake object seems appropriate. We might want to add a note to the rendered page explaining that the preview output may be different from the final page.