This was uncovered as part of the API integration testing initiative. The output of the test is as follows:
AssertionError: User "<anon>": Action "createaccount" returned error code "internal_api_error_DBQueryError": [e157f2f8b7882298c9335dfa] Exception caught: A database query error has occurred. This may indicate a bug in the software.!
This appears to happen "sometimes" when creating multiple user accounts in quick succession/in parallel.
[exception] [86a30c127f24cc294a52e10b] /mediawiki/api.php Wikimedia\Rdbms\DBQueryError from line 1596 of /var/www/mediawiki/includes/libs/rdbms/database/Database.php: A database query error has occurred. Did you forget to run your application's database schema updater after upgrading?
Query: INSERT IGNORE INTO `user` (user_name,user_password,user_newpassword,user_email,user_email_authenticated,user_real_name,user_token,user_registration,user_editcount,user_touched) VALUES ('Fiona rP1u6F7lZW','','','',NULL,'','6df48babcce9b00c103ba4518fedb8b8','20190814204515','0','20190814204515')
Function: User::addToDatabase
Error: 1213 Deadlock found when trying to get lock; try restarting transaction (db-master)
#0 /var/www/mediawiki/includes/libs/rdbms/database/Database.php(1567): Wikimedia\Rdbms\Database->getQueryExceptionAndLog(string, integer, string, string)
#1 /var/www/mediawiki/includes/libs/rdbms/database/Database.php(1156): Wikimedia\Rdbms\Database->reportQueryError(string, integer, string, string, boolean)
#2 /var/www/mediawiki/includes/libs/rdbms/database/Database.php(2132): Wikimedia\Rdbms\Database->query(string, string)
#3 /var/www/mediawiki/includes/user/User.php(4186): Wikimedia\Rdbms\Database->insert(string, array, string, string)
#4 /var/www/mediawiki/includes/libs/rdbms/database/Database.php(3956): User->{closure}(Wikimedia\Rdbms\DatabaseMysqli, string)
#5 /var/www/mediawiki/includes/libs/rdbms/database/DBConnRef.php(68): Wikimedia\Rdbms\Database->doAtomicSection(string, Closure)
#6 /var/www/mediawiki/includes/libs/rdbms/database/DBConnRef.php(630): Wikimedia\Rdbms\DBConnRef->__call(string, array)
#7 /var/www/mediawiki/includes/user/User.php(4212): Wikimedia\Rdbms\DBConnRef->doAtomicSection(string, Closure)
#8 /var/www/mediawiki/includes/auth/AuthManager.php(1423): User->addToDatabase()
#9 /var/www/mediawiki/includes/auth/AuthManager.php(1150): MediaWiki\Auth\AuthManager->continueAccountCreation(array)
#10 /var/www/mediawiki/includes/api/ApiAMCreateAccount.php(93): MediaWiki\Auth\AuthManager->beginAccountCreation(User, array, string)
#11 /var/www/mediawiki/includes/api/ApiMain.php(1589): ApiAMCreateAccount->execute()
#12 /var/www/mediawiki/includes/api/ApiMain.php(533): ApiMain->executeAction()
#13 /var/www/mediawiki/includes/api/ApiMain.php(504): ApiMain->executeActionWithErrorHandling()
#14 /var/www/mediawiki/api.php(86): ApiMain->execute()
#15 {main}