Jenkins sporadically reports failures of the mediawiki-extensions-hhvm job on innocuous, newly-rebased changes in mediawiki/core. For example https://integration.wikimedia.org/ci/job/mediawiki-extensions-hhvm/5936/consoleFull
There were 7 errors: 1) ApiFlowThankTest::testRequestWithoutToken RuntimeException: Cannot override frozen service "storage". /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/vendor/Pimple/Container.php:76 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Thanks/tests/ApiFlowThankIntegrationTest.php:93 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:132 2) ApiFlowThankTest::testInvalidRequest RuntimeException: Cannot override frozen service "storage". /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/vendor/Pimple/Container.php:76 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Thanks/tests/ApiFlowThankIntegrationTest.php:93 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:132 3) ApiFlowThankTest::testValidRequest RuntimeException: Cannot override frozen service "storage". /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/vendor/Pimple/Container.php:76 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Thanks/tests/ApiFlowThankIntegrationTest.php:93 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:132 4) ApiFlowThankTest::testRequestWithInvalidId RuntimeException: Cannot override frozen service "storage". /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/vendor/Pimple/Container.php:76 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Thanks/tests/ApiFlowThankIntegrationTest.php:93 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:132 5) ApiFlowThankTest::testRequestWithOwnId RuntimeException: Cannot override frozen service "storage". /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/vendor/Pimple/Container.php:76 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Thanks/tests/ApiFlowThankIntegrationTest.php:93 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:132 6) ApiFlowThankTest::testApiTestGroup RuntimeException: Cannot override frozen service "storage". /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/vendor/Pimple/Container.php:76 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Thanks/tests/ApiFlowThankIntegrationTest.php:93 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:132 7) ApiFlowThankTest::testMediaWikiTestCaseParentSetupCalled RuntimeException: Cannot override frozen service "storage". /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/vendor/Pimple/Container.php:76 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Thanks/tests/ApiFlowThankIntegrationTest.php:93 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:132
I'm not sure exactly when it started, but at least as early as build #5894. The failure rate graph shows a rise at around that time. https://gerrit.wikimedia.org/r/#/c/192608/ is presumably implicated, but perhaps was merged too early to be the whole story.