This is blocking the re-enablement of the "assert no runtime errors" step in CI, which checks that mw-error.log remains empty. This was accidentally removed a few months ago (T246358).
This error ocurrs once on every build.
mw-deberror.log
Fri Mar 20 18:10:36 UTC 2020 32a44f17c9c0 wikidb-unittest_ Error connecting to [Null] as user [Null]: Got schema 'such'; not supported.
#0 /workspace/src/includes/libs/rdbms/database/DatabaseMysqlBase.php(126): Wikimedia\Rdbms\Database->newExceptionAfterConnectError(string)
#1 /workspace/src/includes/libs/rdbms/database/Database.php(322): Wikimedia\Rdbms\DatabaseMysqlBase->open(string, string, string, string, string, string)
#2 /workspace/src/includes/libs/rdbms/database/Database.php(306): Wikimedia\Rdbms\Database->doInitConnection()
#3 /workspace/src/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1329): Wikimedia\Rdbms\Database->initConnection()
#4 /workspace/src/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1209): Wikimedia\Rdbms\LoadBalancer->reallyOpenConnection(integer, Wikimedia\Rdbms\DatabaseDomain, array)
#5 /workspace/src/includes/libs/rdbms/loadbalancer/LoadBalancer.php(930): Wikimedia\Rdbms\LoadBalancer->getForeignConnection(integer, string, integer)
#6 /workspace/src/includes/libs/rdbms/loadbalancer/LoadBalancer.php(901): Wikimedia\Rdbms\LoadBalancer->getServerConnection(integer, string, integer)
#7 /workspace/src/includes/filebackend/filejournal/DBFileJournal.php(191): Wikimedia\Rdbms\LoadBalancer->getConnection(integer, array, string)
#8 /workspace/src/includes/filebackend/filejournal/DBFileJournal.php(61): DBFileJournal->getMasterDB()
#9 /workspace/src/includes/libs/filebackend/filejournal/FileJournal.php(111): DBFileJournal->doLogChangeBatch(array, string)
#10 /workspace/src/tests/phpunit/includes/filebackend/filejournal/DBFileJournalIntegrationTest.php(48): FileJournal->logChangeBatch(array, string)
#11 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestCase.php(1408): DBFileJournalIntegrationTest->testDoLogChangeBatch_exceptionDbConnect()
#12 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestCase.php(1028): PHPUnit\Framework\TestCase->runTest()
#13 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestResult.php(691): PHPUnit\Framework\TestCase->runBare()
#14 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestCase.php(756): PHPUnit\Framework\TestResult->run(DBFileJournalIntegrationTest)
#15 /workspace/src/tests/phpunit/MediaWikiIntegrationTestCase.php(418): PHPUnit\Framework\TestCase->run(PHPUnit\Framework\TestResult)
#16 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestSuite.php(597): MediaWikiIntegrationTestCase->run(PHPUnit\Framework\TestResult)
#17 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestSuite.php(597): PHPUnit\Framework\TestSuite->run(PHPUnit\Framework\TestResult)
#18 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestSuite.php(597): PHPUnit\Framework\TestSuite->run(PHPUnit\Framework\TestResult)
#19 /workspace/src/vendor/phpunit/phpunit/src/TextUI/TestRunner.php(621): PHPUnit\Framework\TestSuite->run(PHPUnit\Framework\TestResult)
#20 /workspace/src/vendor/phpunit/phpunit/src/TextUI/Command.php(200): PHPUnit\TextUI\TestRunner->doRun(PHPUnit\Framework\TestSuite, array, boolean)
#21 /workspace/src/tests/phpunit/phpunit.php(71): PHPUnit\TextUI\Command->run(array, boolean)
#22 /workspace/src/maintenance/doMaintenance.php(99): PHPUnitMaintClass->execute()
#23 /workspace/src/tests/phpunit/phpunit.php(130): require(string)
#24 {main}