Also see T227817

I dug a bit, and it seems action=info does not perform the usual normalisations and follow redirects that usually happen in MW (perhaps because this exception happens too early?). In this instance, there is one rewrite and one redirect: User:Brad_ -> User:Brad -> User:Bradley0110 (the first is a rewrite, i.e. the trailing _ is simply dropped, while the latter is a MW-level redirect).

The same problem occurs on: https://en.wikipedia.org/w/index.php?title=Audi&dir=prev&action=history

It seems that the first revision of that page is recorded to have been created by a user with the name "Brad_", which is invalid these days due to the trailing underscore:

https://en.wikipedia.org/wiki/Special:ApiSandbox#action=query&format=json&prop=revisions&titles=Audi&rvprop=ids%7Ctimestamp%7Cflags%7Ccomment%7Cuser%7Cuserid&rvlimit=1&rvdir=newer

{
    "continue": {
        "rvcontinue": "20020225154311|52561",
        "continue": "||"
    },
    "query": {
        "pages": {
            "848": {
                "pageid": 848,
                "ns": 0,
                "title": "Audi",
                "revisions": [
                    {
                        "revid": 234233,
                        "parentid": 0,
                        "minor": "",
                        "user": "Brad_",
                        "userid": 2594078,
                        "timestamp": "2001-09-26T12:34:19Z",
                        "comment": "Added English translation of Vorsprung durch Technik"
                    }
                ]
            }
        }
    }
}

But querying for an user with that ID, we get a "Brad":

https://en.wikipedia.org/wiki/Special:ApiSandbox#action=query&format=json&list=users&ususerids=2594078

{
    "batchcomplete": "",
    "query": {
        "users": [
            {
                "userid": 2594078,
                "name": "Brad"
            }
        ]
    }
}

Is this an incomplete rename (from 2008-ish…) or something else?

It seems that due to the actor migration the api now says it is anon

{
    "continue": {
        "rvcontinue": "20020225154311|52561",
        "continue": "||"
    },
    "query": {
        "pages": {
            "848": {
                "pageid": 848,
                "ns": 0,
                "title": "Audi",
                "revisions": [
                    {
                        "revid": 234233,
                        "parentid": 0,
                        "minor": "",
                        "user": "Brad_",
                        "anon": "",
                        "userid": 0,
                        "timestamp": "2001-09-26T12:34:19Z",
                        "comment": "Added English translation of Vorsprung durch Technik"
                    }
                ]
            }
        }
    }
}

So actor migration has identify this as a new user/actor and created a row for it.

In my opinion the database must be clean up and nothing to do on the gui side, because it also affect history or a diff to this version. Every place where the user name is shown.

Maybe we need another task to clean up the actor table to not hold invalid user names or ip addresses (Calling User::isValidUserName on each row)

I have started a query to see how many users ends with underscore on enwiki - https://quarry.wmflabs.org/query/40392

In T232451#5702158, @Umherirrender wrote:

I have started a query to see how many users ends with underscore on enwiki - https://quarry.wmflabs.org/query/40392

This query says it's still "executing" and it's five months later??

I just tried the page info, and its still an issue [Xo8@NQpAIDAAAE898msAAADG] 2020-04-09 15:24:37: Fatal exception of type "Wikimedia\Assert\ParameterAssertionException"

In T232451#6043506, @DannyS712 wrote:

I just tried the page info, and its still an issue [Xo8@NQpAIDAAAE898msAAADG] 2020-04-09 15:24:37: Fatal exception of type "Wikimedia\Assert\ParameterAssertionException"

The task is open and the username is still there. Why do you think it will disappear?

In T232451#6043514, @Ammarpad wrote:

In T232451#6043506, @DannyS712 wrote:

I just tried the page info, and its still an issue [Xo8@NQpAIDAAAE898msAAADG] 2020-04-09 15:24:37: Fatal exception of type "Wikimedia\Assert\ParameterAssertionException"

The task is open and the username is still there. Why do you think it will disappear?

Its been almost a full release cycle, so I was hoping

Change 611738 had a related patch set uploaded (by Umherirrender; owner: Umherirrender):
[mediawiki/core@master] Validate local user name in ExternalUserNames::getUserLinkTitle

https://gerrit.wikimedia.org/r/611738

Change 611738 abandoned by Umherirrender:
[mediawiki/core@master] Validate local user name in ExternalUserNames::getUserLinkTitle

Reason:
Needs another strategy

https://gerrit.wikimedia.org/r/611738

This was stuck in the hidden (backlock) column, needs attention.

Looks like this is triggered by invalid user names in the actor table:

> select rev_id, rev_timestamp, rev_actor, actor_user, actor_name from revision join actor on rev_actor = actor_id where rev_page = 8035619 and rev_timestamp <= "20100102120249" order by rev_timestamp desc  limit 50;

...

| 130477420 | 20091221015106 |  10630425 |       NULL | Alex
 Alex Lep            |

So actor 10630425 has a line break in the name.

We should probably be more defensive about creating user links from bad names. We should log such inceidents, but they shouldn't make the request fail.

Change 659329 had a related patch set uploaded (by Daniel Kinzler; owner: Daniel Kinzler):
[mediawiki/core@master] Linker: userLink() should not explode on malformed user names.

https://gerrit.wikimedia.org/r/659329

Change 659329 merged by jenkins-bot:
[mediawiki/core@master] Linker: userLink() should not explode on malformed user names.

https://gerrit.wikimedia.org/r/659329