A typical trace is
Expectation (masterConns <= 0) by ApiMain::setRequestExpectations not met (actual: 1):
[connect to 10.64.32.25 (wikishared)]
#0 /srv/mediawiki/php-1.31.0-wmf.25/includes/libs/rdbms/TransactionProfiler.php(164): Wikimedia\Rdbms\TransactionProfiler->reportExpectationViolated()
#1 /srv/mediawiki/php-1.31.0-wmf.25/includes/libs/rdbms/loadbalancer/LoadBalancer.php(747): Wikimedia\Rdbms\TransactionProfiler->recordConnection()
#2 /srv/mediawiki/php-1.31.0-wmf.25/includes/libs/rdbms/database/DBConnRef.php(46): Wikimedia\Rdbms\LoadBalancer->getConnection()
#3 /srv/mediawiki/php-1.31.0-wmf.25/includes/libs/rdbms/database/DBConnRef.php(266): Wikimedia\Rdbms\DBConnRef->__call()
#4 /srv/mediawiki/php-1.31.0-wmf.25/extensions/ReadingLists/src/ReadingListRepository.php(187): Wikimedia\Rdbms\DBConnRef->select()
#5 /srv/mediawiki/php-1.31.0-wmf.25/extensions/ReadingLists/src/ReadingListRepository.php(316): MediaWiki\Extensions\ReadingLists\ReadingListRepository->isSetupForUser()
#6 /srv/mediawiki/php-1.31.0-wmf.25/extensions/ReadingLists/src/Api/ApiQueryReadingLists.php(83): MediaWiki\Extensions\ReadingLists\ReadingListRepository->getAllLists()
#7 /srv/mediawiki/php-1.31.0-wmf.25/includes/api/ApiQuery.php(249): MediaWiki\Extensions\ReadingLists\Api\ApiQueryReadingLists->execute()
#8 /srv/mediawiki/php-1.31.0-wmf.25/includes/api/ApiMain.php(1580): ApiQuery->execute()
#9 /srv/mediawiki/php-1.31.0-wmf.25/includes/api/ApiMain.php(542): ApiMain->executeAction()
#10 /srv/mediawiki/php-1.31.0-wmf.25/includes/api/ApiMain.php(513): ApiMain->executeActionWithErrorHandling()
#11 /srv/mediawiki/php-1.31.0-wmf.25/api.php(94): ApiMain->execute()
#12 /srv/mediawiki/w/api.php(3): include()
#13 {main}