PHP 8 fatal error during migration in `_field_info_prepare_extra_fields()` caused by incorrect variable type

[richardlampitt]

Description of the bug

During a D7 → B1 migration, I encountered the following error, breaking the migration import:

An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: http://thensmc.local/core/update.php?op=selection&token=aeynOFF00trJeEvucZj5bu07xcPClxYNAuX3Jqb5YdU&action=update&id=53219&op=do_nojs&op=do
StatusText: Service unavailable (with message)
ResponseText: TypeError: array_keys(): Argument #1 ($array) must be of type array, null given in array_keys() (line 453 of [...]\httpdocs\core\modules\field\field.info.inc).

Steps To Reproduce

1. Have / build a Drupal 7 site with the modules below shown
2. Attempt to run a complete migration to BackdropCMS 1.
3. Run database updates as part of the migration process.
4. Fatal error.

Actual behavior

Fatal error, caused by something passing array_keys a null rather than a [].

Expected behavior

No fatal error, migration database update continues uninterrupted.

Additional information

Add any other information that could help, such as:

• Backdrop CMS version: 1.31.2
• Web server and its version: apache 2.4.54
• PHP version: 8.0+ (tested with 8.1)
• Database sever (MySQL or MariaDB?) and its version: MariaDB Version: 10.11.11
• Operating System and its version: Windows 10 22H2

The issue is this line in the _field_info_prepare_extra_fields() function:

    $view_modes = array_merge(array('default'), array_keys($entity_type_info['view modes']));

There's nothing to guard or check that $entity_type_info['view modes'] both exists and is valid.

This was during an import of an existing D7 site that's quite old (10 years 1 month at the time of writing since the 'administrator' account was created). Here are the modules that are active in the Drupal 7 database:

Category	Name (machine_name)	Type	Status	Version
Administration	Administration menu (admin_menu)	Module	Enabled	7.x-3.0-rc7
Administration	Administration menu Toolbar style (admin_menu_toolbar)	Module	Enabled	7.x-3.0-rc7
Chaos tool suite	Chaos tools (ctools)	Module	Enabled	7.x-1.21
Charts	Charts (charts)	Module	Enabled	7.x-2.1
Charts	Highcharts (charts_highcharts)	Module	Enabled	7.x-2.1
Core	Announcements (announcements_feed)	Module	Enabled	7.102
Core	Block (block)	Module	Enabled	7.102
Core	Blog (blog)	Module	Enabled	7.102
Core	Color (color)	Module	Enabled	7.102
Core	Comment (comment)	Module	Enabled	7.102
Core	Contextual links (contextual)	Module	Enabled	7.102
Core	Dashboard (dashboard)	Module	Enabled	7.102
Core	Database logging (dblog)	Module	Enabled	7.102
Core	Field (field)	Module	Enabled	7.102
Core	Field SQL storage (field_sql_storage)	Module	Enabled	7.102
Core	Field UI (field_ui)	Module	Enabled	7.102
Core	File (file)	Module	Enabled	7.102
Core	Filter (filter)	Module	Enabled	7.102
Core	Help (help)	Module	Enabled	7.102
Core	Image (image)	Module	Enabled	7.102
Core	List (list)	Module	Enabled	7.102
Core	Menu (menu)	Module	Enabled	7.102
Core	Node (node)	Module	Enabled	7.102
Core	Number (number)	Module	Enabled	7.102
Core	Options (options)	Module	Enabled	7.102
Core	Path (path)	Module	Enabled	7.102
Core	PHP filter (php)	Module	Enabled	7.102
Core	RDF (rdf)	Module	Enabled	7.102
Core	Search (search)	Module	Enabled	7.102
Core	Shortcut (shortcut)	Module	Enabled	7.102
Core	System (system)	Module	Enabled	7.102
Core	Taxonomy (taxonomy)	Module	Enabled	7.102
Core	Text (text)	Module	Enabled	7.102
Core	Toolbar (toolbar)	Module	Enabled	7.102
Core	Update manager (update)	Module	Enabled	7.102
Core	User (user)	Module	Enabled	7.102
Date/Time	Date (date)	Module	Enabled	7.x-2.14
Date/Time	Date API (date_api)	Module	Enabled	7.x-2.14
Date/Time	Date Popup (date_popup)	Module	Enabled	7.x-2.14
Date/Time	Date Views (date_views)	Module	Enabled	7.x-2.14
Features	Features (features)	Module	Enabled	7.x-2.15
Fields	Entity Reference (entityreference)	Module	Enabled	7.x-1.10
Fields	Field Collection (field_collection)	Module	Enabled	7.x-1.2
Fields	Field Group (field_group)	Module	Enabled	7.x-1.8
Fields	Insert (insert)	Module	Enabled	7.x-1.5
Fields	Link (link)	Module	Enabled	7.x-1.13
Location	Location (location)	Module	Enabled	7.x-3.7
MailChimp	MailChimp (mailchimp)	Module	Enabled	7.x-5.7
Media	File Entity (file_entity)	Module	Enabled	7.x-2.40
Media	IMCE (imce)	Module	Enabled	7.x-1.11
Media	Media (media)	Module	Enabled	7.x-2.30
Media	Media Field (mediafield)	Module	Enabled	7.x-2.30
Media	Media Internet Sources (media_internet)	Module	Enabled	7.x-2.30
Media	Media WYSIWYG (media_wysiwyg)	Module	Enabled	7.x-2.30
Migration	Migrate (migrate)	Module	Enabled	7.x-2.12
Other	Backdrop upgrade status (backdrop_upgrade_status)	Module	Enabled	7.x-1.3
Other	Backup and Migrate (backup_migrate)	Module	Enabled	7.x-3.10
Other	Block Class (block_class)	Module	Enabled	7.x-2.4
Other	Breakpoints (breakpoints)	Module	Enabled	7.x-1.6
Other	Diff (diff)	Module	Enabled	7.x-3.5
Other	Disqus (disqus)	Module	Enabled	7.x-1.12
Other	Entity API (entity)	Module	Enabled	7.x-1.12
Other	Entity tokens (entity_token)	Module	Enabled	7.x-1.12
Other	EU Cookie Compliance (eu_cookie_compliance)	Module	Enabled	7.x-1.43
Other	Job Scheduler (job_scheduler)	Module	Enabled	7.x-2.0
Other	Libraries (libraries)	Module	Enabled	7.x-2.5
Other	Link checker (linkchecker)	Module	Enabled	7.x-1.6
Other	Menu Block (menu_block)	Module	Enabled	7.x-2.9
Other	Pathauto (pathauto)	Module	Enabled	7.x-1.4
Other	Redirect (redirect)	Module	Enabled	7.x-1.0-rc4
Other	Site map (site_map)	Module	Enabled	7.x-1.3
Other	Taxonomy menu (taxonomy_menu)	Module	Enabled	7.x-1.7
Other	Token (token)	Module	Enabled	7.x-1.9
Other - OrigamiUSA	Entity Plus [stub] (entity_plus)	Module	Enabled	7.x-1.0
Other - OrigamiUSA	Entity UI [stub] (entity_ui)	Module	Enabled	7.x-1.0
Picture	Picture (picture)	Module	Enabled	7.x-2.14
Spam control	CAPTCHA (captcha)	Module	Enabled	7.x-1.7
Statistics	Google Analytics (googleanalytics)	Module	Enabled	7.x-2.8
User interface	jQuery Update (jquery_update)	Module	Enabled	7.x-4.1
Views	Better Exposed Filters (better_exposed_filters)	Module	Enabled	7.x-3.6
Views	Views (views)	Module	Enabled	7.x-3.30
Views	Views block area (views_block_area)	Module	Enabled	7.x-1.2
Views	Views Bootstrap (views_bootstrap)	Module	Enabled	7.x-3.5
Views	Views Bulk Operations (views_bulk_operations)	Module	Enabled	7.x-3.7
Views	Views TimelineJS (views_timelinejs)	Module	Enabled	7.x-1.1
Views	Views UI (views_ui)	Module	Enabled	7.x-3.30
Webform	Webform (webform)	Module	Enabled	7.x-4.27
Webform	Webform AJAX (webform_ajax)	Module	Enabled	7.x-2.0
Webform	Webform Import (webform_import)	Module	Enabled	7.x-1.3
XML sitemap	XML sitemap (xmlsitemap)	Module	Enabled	7.x-2.7
XML sitemap	XML sitemap node (xmlsitemap_node)	Module	Enabled	7.x-2.7
Core	Bartik (bartik)	Theme	Enabled	7.102
Core	Seven (seven)	Theme	Enabled	7.102

And here's a list of the entities on the site:

0. comment
1. field_collection_item
2. node
3. redirect
4. file
5. taxonomy_term
6. taxonomy_vocabulary
7. user

Workaround

Here's the change that I made to get the migration to finish:

    $array_keys = ( $entity_type_info['view modes'] ?? false ) ?: [];
    $view_modes = array_merge([ 'default' ], array_keys($array_keys));

However, I'm not familiar with working with entities, so I don't know if any of these are supposed to have no view modes.