```[08a09c9f4968bff42dd6c232] [no req] MWContentSerializationException from line 299 of /srv/mediawiki/php-1.33.0-wmf.18/extensions/Wikibase/lib/includes/Store/EntityContentDataCodec.php: $entityId and $targetId can not be the same.
```
Reproduction case: https://www.wikidata.org/w/api.php?action=query&format=json&prop=revisions&formatversion=2&titles=Lexeme%3AL123&rvprop=content%7Cuser%7Cids&rvlimit=max&continue=%7C%7C&rvcontinue=20190401115845%7C899647120
https://logstash.wikimedia.org/app/discover#/doc/logstash-*/logstash-deploy-2021.06.28?id=UM85U3oB3UDnvh-QHmqq
Req ID: `AWk0u32gjSm8tOlx-BX_`
Stacktrace for that web request:
```lines=5
#0from /srv/mediawiki/php-1.337.0-wmf.19/extensions/Wikibase/repo/includes/Content/EntityHandler.php(384): Wikibase\Lib\Store\EntityContentDataCodec->decodeRedirect(string, NULL1/extensions/Wikibase/lib/includes/Store/EntityContentDataCodec.php(300)
#1#0 /srv/mediawiki/php-1.337.0-wmf.19/includes/Revision/RevisionStore11/extensions/Wikibase/repo/includes/Content/EntityHandler.php(1472371): Wikibase\Repo\ContentLib\Store\EntityHandler->unserializeContenContentDataCodec->decodeRedirect(string, NULL)
#2#1 /srv/mediawiki/php-1.337.0-wmf.19/includes/Revision/RevisionStore.php(1634): MediaWiki\Revision\RevisionStore->loadSlotContent(MediaWiki\Revision\SlotRecord, NULL, NULL, NULL1/includes/Revision/RevisionStore.php(1208): Wikibase\Repo\Content\EntityHandler->unserializeContent(string, integerNULL)
#3 [internal function]: Closure$MediaWiki\Revision\RevisionStore::loadSlotRecords(MediaWiki\Revision\SlotRecord2 /srv/mediawiki/php-1.37.0-wmf.11/includes/Revision/RevisionStore.php(1449): MediaWiki\Revision\RevisionStore->loadSlotContent(MediaWiki\Revision\SlotRecord, NULL, NULL, NULL, integer)
#4 /srv/mediawiki/php-1.33.0-wmf.19/includes/Revision/SlotRecord.php(307): call_user_func(Closure$MediaWiki\Revision\RevisionStore::loadSlotRecords;2106, 3 [internal function]: MediaWiki\Revision\RevisionStore->MediaWiki\Revision\{closure}(MediaWiki\Revision\SlotRecord)
#5#4 /srv/mediawiki/php-1.337.0-wmf.1911/includes/Revision/RevisionSlotRecord.php(175):324): call_user_func(Closure, MediaWiki\Revision\SlotRecord->getContent()
#6#5 /srv/mediawiki/php-1.337.0-wmf.1911/includes//api/ApiQueryRevisionsBase.php(923481): MediaWiki\Revision\RevisionSlotRecord->getContent(string, integer, User)
#7#6 /srv/mediawiki/php-1.337.0-wmf.1911/includes/page/Articlapi/ApiQueryRevisionsBase.php(494): 390): ApiQueryRevision->getContent(sBase->extractSlotInfo(MediaWiki\Revision\SlotRecord, integer, UserNULL)
#8#7 /srv/mediawiki/php-1.337.0-wmf.19/includes/page/Article.php(714): Article->fetchRevisionRecord(1/includes/api/ApiQueryRevisionsBase.php(314): ApiQueryRevisionsBase->extractAllSlotInfo(MediaWiki\Revision\RevisionStoreRecord, integer)
#9#8 /srv/mediawiki/php-1.337.0-wmf.19/extensions/Wikibase/repo/includes/Actions/ViewEntityAction.php(79): Article->view(1/includes/api/ApiQueryRevisions.php(438): ApiQueryRevisionsBase->extractRevisionInfo(MediaWiki\Revision\RevisionStoreRecord, stdClass)
#10#9 /srv/mediawiki/php-1.337.0-wmf.19/extensions/Wikibase/repo/includes/Actions/ViewEntityAction11/includes/api/ApiQueryRevisionsBase.php(54): Wikibase\ViewEntityAction->showEntityPage62): ApiQueryRevisions->run()
#11#10 /srv/mediawiki/php-1.337.0-wmf.1911/includes/MediaWikiapi/ApiQuery.php(501): Wikibase\ViewEntityAction->show333): ApiQueryRevisionsBase->execute()
#12#11 /srv/mediawiki/php-1.337.0-wmf.1911/includes/MediaWikiapi/ApiMain.php(294): MediaWiki->performAction(Article, Title1721): ApiQuery->execute()
#13#12 /srv/mediawiki/php-1.337.0-wmf.1911/includes/MediaWikiapi/ApiMain.php(867): MediaWiki->performRequest702): ApiMain->executeAction()
#14#13 /srv/mediawiki/php-1.337.0-wmf.1911/includes/MediaWikiapi/ApiMain.php(517): MediaWiki->main673): ApiMain->executeActionWithErrorHandling()
#15#14 /srv/mediawiki/php-1.337.0-wmf.19/index11/api.php(42): MediaWiki->run90): ApiMain->execute()
#16#15 /srv/mediawiki/w/indexphp-1.37.0-wmf.11/api.php(45): wfApiMain()
#16 /srv/mediawiki/w/api.php(3): includrequire(string)
#17 {main}
```
Note there is also a field `exception.previous.trace`:
```lines=5
from /srv/mediawiki/php-1.37.0-wmf.11/vendor/wikibase/data-model/src/Entity/EntityRedirect.php(41)
#0 /srv/mediawiki/php-1.3337.0-wmf.19/extensions/Wikibase/lib/includes/Store/EntityContentDataCodec.php(296): Wikibase\DataModel\Entity\EntityRedirect->__construct(Wikibase\DataModel\Entity\Item1/extensions/Wikibase/lib/includes/Store/EntityContentDataCodec.php(297): Wikibase\DataModel\Entity\EntityRedirect->__construct(Wikibase\Lexeme\Domain\Model\LexemeId, Wikibase\DataModel\Entity\ItemIdLexeme\Domain\Model\LexemeId)
#1 /srv/mediawiki/php-1.3337.0-wmf.1911/extensions/Wikibase/repo/includes/Content/EntityHandler.php(38471): Wikibase\Lib\Store\EntityContentDataCodec->decodeRedirect(string, NULL)
#2 /srv/mediawiki/php-1.3337.0-wmf.1911/includes/Revision/RevisionStore.php(1472208): Wikibase\Repo\Content\EntityHandler->unserializeContent(string, NULL)
#3 /srv/mediawiki/php-1.3337.0-wmf.1911/includes/Revision/RevisionStore.php(1634449): MediaWiki\Revision\RevisionStore->loadSlotContent(MediaWiki\Revision\SlotRecord, NULL, NULL, NULL, integer)
#4 [internal function]: Closure$MediaWiki\Revision\RevisionStore::loadSlotRecords->MediaWiki\Revision\{closure}(MediaWiki\Revision\SlotRecord)
#5 /srv/mediawiki/php-1.3337.0-wmf.1911/includes/Revision/SlotRecord.php(30724): call_user_func(Closure$MediaWiki\Revision\RevisionStore::loadSlotRecords;2106re, MediaWiki\Revision\SlotRecord)
#6 /srv/mediawiki/php-1.3337.0-wmf.1911/includes/Revision/api/ApiQueryRevisionRecordsBase.php(175481): MediaWiki\Revision\SlotRecord->getContent()
#7 /srv/mediawiki/php-1.3337.0-wmf.1911/includes//api/ApiQueryRevisionsBase.php(923): MediaWiki\390): ApiQueryRevision\RevisionRecord->getContent(stringsBase->extractSlotInfo(MediaWiki\Revision\SlotRecord, integer, UserNULL)
#8 /srv/mediawiki/php-1.3337.0-wmf.1911/includes/page/Articlapi/ApiQueryRevisionsBase.php(49314): ApiQueryRevision->getContent(integersBase->extractAllSlotInfo(MediaWiki\Revision\RevisionStoreRecord, Userinteger)
#9 /srv/mediawiki/php-1.3337.0-wmf.19/includes/page/Article.php(714): Article->fetchRevisionRecord(1/includes/api/ApiQueryRevisions.php(438): ApiQueryRevisionsBase->extractRevisionInfo(MediaWiki\Revision\RevisionStoreRecord, stdClass)
#10 /srv/mediawiki/php-1.3337.0-wmf.19/extensions/Wikibase/repo/includes/Act11/includes/api/ApiQueryRevisions/ViewEntityAction.php(79): Article->viewBase.php(62): ApiQueryRevisions->run()
#11 /srv/mediawiki/php-1.3337.0-wmf.19/extensions/Wikibase/repo11/includes/Actions/ViewEntityActionapi/ApiQuery.php(54): Wikibase\ViewEntityAction->showEntityPage333): ApiQueryRevisionsBase->execute()
#12 /srv/mediawiki/php-1.3337.0-wmf.1911/includes/MediaWikiapi/ApiMain.php(501721): Wikibase\ViewEntityAction->showApiQuery->execute()
#13 /srv/mediawiki/php-1.3337.0-wmf.1911/includes/MediaWikiapi/ApiMain.php(294): MediaWiki->perform702): ApiMain->executeAction(Article, Title)
#14 /srv/mediawiki/php-1.3337.0-wmf.1911/includes/MediaWikiapi/ApiMain.php(867): MediaWiki->performRequest673): ApiMain->executeActionWithErrorHandling()
#15 /srv/mediawiki/php-1.3337.0-wmf.19/includes/MediaWik11/api.php(517): MediaWiki->main90): ApiMain->execute()
#16 /srv/mediawiki/php-1.3337.0-wmf.19/index11/api.php(42): MediaWiki->run5): wfApiMain()
#17 /srv/mediawiki/w/indexapi.php(3): includrequire(string)
#18 {main}
```
-------
And an older stacktrace from a job runner
```name=trace from jobrunner,lines=10
#0 /srv/mediawiki/php-1.33.0-wmf.21/extensions/Wikibase/repo/includes/Content/EntityHandler.php(383): Wikibase\Lib\Store\EntityContentDataCodec->decodeRedirect(string, NULL)
#1 /srv/mediawiki/php-1.33.0-wmf.21/includes/Revision/RevisionStore.php(1472): Wikibase\Repo\Content\EntityHandler->unserializeContent(string, NULL)
#2 /srv/mediawiki/php-1.33.0-wmf.21/includes/Revision/RevisionStore.php(1634): MediaWiki\Revision\RevisionStore->loadSlotContent(MediaWiki\Revision\SlotRecord, NULL, NULL, NULL, integer)
#3 [internal function]: Closure$MediaWiki\Revision\RevisionStore::loadSlotRecords(MediaWiki\Revision\SlotRecord)
#4 /srv/mediawiki/php-1.33.0-wmf.21/includes/Revision/SlotRecord.php(307): call_user_func(Closure$MediaWiki\Revision\RevisionStore::loadSlotRecords;828, MediaWiki\Revision\SlotRecord)
#5 /srv/mediawiki/php-1.33.0-wmf.21/includes/Revision/RevisionRecord.php(175): MediaWiki\Revision\SlotRecord->getContent()
#6 /srv/mediawiki/php-1.33.0-wmf.21/includes/Revision.php(923): MediaWiki\Revision\RevisionRecord->getContent(string, integer, NULL)
#7 /srv/mediawiki/php-1.33.0-wmf.21/includes/page/WikiPage.php(809): Revision->getContent(integer, NULL)
#8 /srv/mediawiki/php-1.33.0-wmf.21/extensions/CirrusSearch/includes/Sanity/Checker.php(192): WikiPage->getContent()
#9 /srv/mediawiki/php-1.33.0-wmf.21/extensions/CirrusSearch/includes/Sanity/Checker.php(167): CirrusSearch\Sanity\Checker->checkIfRedirect(WikiPage)
#10 /srv/mediawiki/php-1.33.0-wmf.21/extensions/CirrusSearch/includes/Sanity/Checker.php(133): CirrusSearch\Sanity\Checker->checkExisitingPage(string, integer, WikiPage, array)
#11 /srv/mediawiki/php-1.33.0-wmf.21/extensions/CirrusSearch/includes/Job/CheckerJob.php(214): CirrusSearch\Sanity\Checker->check(array)
#12 /srv/mediawiki/php-1.33.0-wmf.21/extensions/CirrusSearch/includes/Job/Job.php(100): CirrusSearch\Job\CheckerJob->doJob()
#13 /srv/mediawiki/php-1.33.0-wmf.21/extensions/EventBus/includes/JobExecutor.php(65): CirrusSearch\Job\Job->run()
#14 /srv/mediawiki/rpc/RunSingleJob.php(77): JobExecutor->execute(array)
#15 {main}
```
### Impact
* Some pages cannot be viewed, e.g. <https://test.wikidata.org/wiki/Q193394> (user is shown unhelpful system error, triggers fatal error, and emits HTTP 500).
* Some API requests fail
* Some jobs are are aborted mid-way, including `CirrusSearch\Job\CheckerJob`.