Change 657798 had a related patch set uploaded (by ProcrastinatingReader; owner: ProcrastinatingReader):
[mediawiki/core@master] Validate page title for partial blocks

https://gerrit.wikimedia.org/r/657798

Change 676290 had a related patch set uploaded (by STran; author: STran):

[mediawiki/core@master] Don't allow blocking on pages that don't exist

https://gerrit.wikimedia.org/r/676290

If an invalid title name is given to the api it fails also: Exception caught: "Call to a member function getArticleID() on null" on PageRestriction.php(114)
Invalid title is "In<valid"

Try:
https://en.wikipedia.beta.wmflabs.org/wiki/Special:ApiSandbox#action=block&format=json&user=Drwpb&allowusertalk=1&partial=1&pagerestrictions=nonexistentpage%7Cin%3Cvalid

Change 676290 merged by jenkins-bot:

[mediawiki/core@master] Don't allow blocking on pages that don't exist

https://gerrit.wikimedia.org/r/676290

Some warnings appeared on later changes, I found it related to this task.

23:19:51 Running "banana:core" (banana) task
23:19:57 >> 1 message lacks documentation in qqq.json.
23:19:57 >> Message "cant-block-nonexistent-page" lacks documentation in qqq.json.
23:19:57 Warning: Task "banana:core" failed. Use --force to continue.

In T269341#6987805, @Func wrote:

Some warnings appeared on later changes, I found it related to this task.

23:19:51 Running "banana:core" (banana) task
23:19:57 >> 1 message lacks documentation in qqq.json.
23:19:57 >> Message "cant-block-nonexistent-page" lacks documentation in qqq.json.
23:19:57 Warning: Task "banana:core" failed. Use --force to continue.

https://gerrit.wikimedia.org/r/c/mediawiki/core/+/678099

I cannot reproduce the bug in the description. When I try to submit an API block with a nonexistent or deleted page, the API returns:

{
    "error": {
        "code": "cant-block-nonexistent-page",
        "info": "You cannot block a user from Nonexistentpage because the page does not exist",
        "*": "See https://en.wikipedia.beta.wmflabs.org/w/api.php for API usage. Subscribe to the mediawiki-api-announce mailing list at <https://lists.wikimedia.org/mailman/listinfo/mediawiki-api-announce> for notice of API deprecations and breaking changes."
    },
    "servedby": "deployment-mediawiki11"
}

This is a fatal error, and blocks completion of the request. This is in contrast to trying to block a nonexistent namespace, which just returns a warning but otherwise completes the request (ignoring the nonexistent namespace).

I tested that otherwise I could still perform some of the actions I could before, such as blocking a single page, blocking a specific page in a custom namespace, partial block without page restrictions, etc.

The code change modifies a function used by both API:Block and Special:Block. We do not appear to see any difference with Special:Block, however, as OOUI was already checking that the page restrictions existed (even without JS).

That being said, I did do some regression testing of Special:Block (with and without JS) just in case.

Test Environment: https://en.wikipedia.beta.wmflabs.org MediaWiki 1.37.0-alpha (f61d37d) 18:48, 13 April 2021.

This is a fatal error, and blocks completion of the request. This is in contrast to trying to block a nonexistent namespace, which just returns a warning but otherwise completes the request (ignoring the nonexistent namespace).

From the Slack discussion, @Tchanders reckons the risk for disruption is quite low. We should start a task to fix this in the future for the sake of consistency, however. @dom_walden could you start a task for that? Thank you!

In T269341#6998282, @Niharika wrote:

This is a fatal error, and blocks completion of the request. This is in contrast to trying to block a nonexistent namespace, which just returns a warning but otherwise completes the request (ignoring the nonexistent namespace).

From the Slack discussion, @Tchanders reckons the risk for disruption is quite low. We should start a task to fix this in the future for the sake of consistency, however. @dom_walden could you start a task for that? Thank you!

Raised T282574.