Regression: Multiupload when uploading more than allowed - TypeError when saving node

[indigoxela]

Description of the bug

Validation in file_field_widget_multiple_count_validate() does not work properly.

It should prevent too many field items, if a restriction via field cardinality exists on node (entity) save. It doesn't. Instead it causes a fatal error.
Same problem when selecting more than allowed from image library.

Steps To Reproduce

1. Attach an image field to a node
2. Set a limit on image count (field cardinality > 1, but < unlimited)
3. Open the node form and upload images, more than allowed

Alternative way to trigger:

1. Attach an image field to a node
2. Set a limit on image count (field cardinality > 1, but < unlimited)
3. Create a node with that amount of images (adhere max)
4. Go to admin/structure/types/manage/NODE_TYPE/fields/field_SOMETHING and lower the limit
5. Open that node form again and save without any changes

Another way to trigger:

1. Attach an image field to a node
2. Set a limit on image count (field cardinality > 1, but < unlimited), make sure the image browser's enabled
3. Open the node form
4. Open the image library dialog and add limit - 1 images
5. Open the image lib again and add as many as possible (more than allowed in field, only until Follow-up: Selecting multiple images from library allows too many items #7122 is resolved)

Actual behavior

You can upload more images than field cardinality allows. They show up in the field list in that form.

Now try to save the node.

TypeError: array_slice(): Argument #1 ($array) must be of type array, string given in array_slice() (line 1049 of ...core/modules/file/file.field.inc). 

Expected behavior

I shouldn't even have come to that point (too many images), but that's a topic on its own.
Saving the node should definitely not end up with a TypeError. 😬

Additional information

• Backdrop CMS version: latest dev after Issue [UX] Support "multiple" HTML5 attribute on File elements #440: Support "Multiple" attribute on file uploads.

Initially it thought, it's related to the dimension limit, but the actual problem is how form_state values get handled.

[indigoxela]

I have no idea, how file_field_widget_multiple_count_validate() should even work. Variables are something entirely different than what they seem...

May the force be with someone else, this code makes no sense to me.

[quicksketch]

Thanks @indigoxela. This problem of allowing too many files definitely needs to be fixed for the 1.34.0 release, but I think we can proceed with it in the preview release.

[indigoxela]

This problem of allowing too many files definitely needs to be fixed for the 1.34.0 release

Yes, we absolutely need to fix that.

This validation did work at some point in the past, but that's a while back and lots of things changed in the meantime it seems.

[indigoxela]

Still scratching head over the original code - I ended up with a rewrite. 😉

Here we go: backdrop/backdrop#5262

And I wonder, why no functional test crashed because of that problem. Should have...