Looking at the code, it's probably https://github.com/wikimedia/mediawiki/commit/21d6f1cd6a0504089c5a13c2be0620c9cde45afd#diff-048e872833a49129ace85eee8a5dd6d21e5c0f5948265e634a5d0c9d242f9110L504 ??

I note that a little later this happened. Still on testwiki. Req-Id X-2QEApAMMoAAYEFLk0AAAAU

PHP Notice: Undefined index: Page4243

Stack trace:

#0 /srv/mediawiki/php-1.36.0-wmf.26/includes/api/ApiQueryInfo.php(504): MWExceptionHandler::handleError(integer, string, string, integer, array)
#1 /srv/mediawiki/php-1.36.0-wmf.26/includes/api/ApiQueryInfo.php(426): ApiQueryInfo->extractPageInfo(integer, Title)
#2 /srv/mediawiki/php-1.36.0-wmf.26/includes/api/ApiQuery.php(269): ApiQueryInfo->execute()
#3 /srv/mediawiki/php-1.36.0-wmf.26/includes/api/ApiMain.php(1612): ApiQuery->execute()
#4 /srv/mediawiki/php-1.36.0-wmf.26/includes/api/ApiMain.php(592): ApiMain->executeAction()
#5 /srv/mediawiki/php-1.36.0-wmf.26/includes/api/ApiMain.php(563): ApiMain->executeActionWithErrorHandling()
#6 /srv/mediawiki/php-1.36.0-wmf.26/api.php(90): ApiMain->execute()
#7 /srv/mediawiki/php-1.36.0-wmf.26/api.php(45): wfApiMain()
#8 /srv/mediawiki/w/api.php(3): require(string)
#9 {main}

From the original error:

URL: /w/api.php
Server: test.wikipedia.org
referrer: https://test.wikipedia.org/wiki/Special:ApiSandbox
message:
[X-2QEApAMMoAAYEFLk0AAAAU] /w/api.php ErrorException from line 504 of /srv/mediawiki/php-1.36.0-wmf.26/includes/api/ApiQueryInfo.php: PHP Notice: Undefined index: Alsjdakjsdh
reqId: X-2QEApAMMoAAYEFLk0AAAAU

To clarify, it's a POST request. There are no URL parameters, and logstash doesn't log the request body.

In T271804#6739408, @RhinosF1 wrote:

Looking at the code, it's probably https://github.com/wikimedia/mediawiki/commit/21d6f1cd6a0504089c5a13c2be0620c9cde45afd#diff-048e872833a49129ace85eee8a5dd6d21e5c0f5948265e634a5d0c9d242f9110L504 ??

Yes. This conditional is missing an isset() call:

$this->fld_watched && $this->watched && $this->watched[$ns][$dbkey]

should be

$this->fld_watched && isset( $this->watched[$ns][$dbkey] )

Change 655665 had a related patch set uploaded (by Daniel Kinzler; owner: Daniel Kinzler):
[mediawiki/core@master] Fix undefined index error in ApiQueryInfo

https://gerrit.wikimedia.org/r/655665

Triggered by requesting page info of non-existing pages. Should be fixed by the patch I just uploaded.

In T271804#6739764, @daniel wrote:

Triggered by requesting page info of non-existing pages. Should be fixed by the patch I just uploaded.

Yes, exactly right. This was me testing/comparing T268834#6739312, and keyboard mashing to get a page that I knew didn't exist (and thus wouldn't be watched and would be missing).

Change 655665 merged by jenkins-bot:
[mediawiki/core@master] Fix undefined index error in ApiQueryInfo

https://gerrit.wikimedia.org/r/655665

Change 655671 had a related patch set uploaded (by ArielGlenn; owner: Daniel Kinzler):
[mediawiki/core@wmf/1.36.0-wmf.26] Fix undefined index error in ApiQueryInfo

https://gerrit.wikimedia.org/r/655671

In T271804#6739652, @daniel wrote:

In T271804#6739408, RhinosF1 wrote:

Looking at the code, it's probably https://github.com/wikimedia/mediawiki/commit/21d6f1cd6a0504089c5a13c2be0620c9cde45afd#diff-048e872833a49129ace85eee8a5dd6d21e5c0f5948265e634a5d0c9d242f9110L504 ??

Yes. This conditional is missing an isset() call:

$this->fld_watched && $this->watched && $this->watched[$ns][$dbkey]

should be

$this->fld_watched && isset( $this->watched[$ns][$dbkey] )

Sorry about that, and thanks for the quick fix!

Change 655671 merged by jenkins-bot:
[mediawiki/core@wmf/1.36.0-wmf.26] Fix undefined index error in ApiQueryInfo

https://gerrit.wikimedia.org/r/655671

The above patch was deployed and seems to fix the issue; would the reporter like to test?

Agreed that this seems fixed to me.

@Jdforrester-WMF So I can close the task?