PathOverflow: Path Overflow from: '**********ZZZZ'

Issue Summary

Creating a new page from the administration area raises a PathOverflow error.

PathOverflow: Path Overflow from: '000100020003ZZZZ'
  File "django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 126, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "django/core/handlers/base.py", line 124, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/cache.py", line 44, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "wagtail/admin/urls/__init__.py", line 102, in wrapper
    return view_func(request, *args, **kwargs)
  File "wagtail/admin/decorators.py", line 34, in decorated_view
    return view_func(request, *args, **kwargs)
  File "wagtail/admin/views/pages.py", line 224, in create
    parent_page.add_child(instance=page)
  File "treebeard/mp_tree.py", line 1013, in add_child
    return MP_AddChildHandler(self, **kwargs).process()
  File "treebeard/mp_tree.py", line 387, in process
    newobj.path = self.node.get_last_child()._inc_path()
  File "treebeard/mp_tree.py", line 1114, in _inc_path
    raise PathOverflow(_("Path Overflow from: '%s'" % (self.path, )))

The only information I could find potentially regarding the issue was on SO at the following link: https://stackoverflow.com/questions/54166821/wagtail-pathoverflow-on-adding-new-child-page

There are only a handful of pages, so I’m not sure that particular issue applies though.

Steps to Reproduce

I haven’t been able to reproduce this myself, however a client has been seeing this behavior, which at the time I’ve just resolved by modifying the ZZZZ at the end to be numeric. I’m in the process of gathering some steps they may have taken to replicate it from this end (where they tried to add the page from)

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: no

Technical details

• Python version: 3.6.8
• Django version: 2.1.4
• Wagtail version: 2.4
• Browser version: N/A

[tomdyson]

I would still insist that a more fundamental approach would be to eliminate preview w/o save function altogether

FWIW I would miss the ability to preview without saving. This is something I do quite often, e.g. when I’m demonstrating Wagtail, or testing new StreamField blocks. Without this feature, I’d have to remember to delete draft pages.

[gasman]

Thanks for investigating this further, @dany-nonstop and @chosak!

Even if this does turn out to be a result of custom code, I think setting the path to ZZZZ is probably enough of a “foot gun” that it’s worth avoiding that setup in Wagtail’s code. On further digging, it looks like the older implementation of preview (prior to #3383) actually had some code for setting the temporary path to current-maximum-plus-one instead:

https://github.com/wagtail/wagtail/blob/0d1c277e85c1ba3b73742811b9dd5d75a2493633/wagtail/wagtailadmin/views/pages.py#L609-L621

…however, I evidently failed to notice that this was being overwritten a few lines further down, and consequently this code was dropped as redundant in #3383. I think we should reinstate that version.