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In the ticket SERVER-50423, we've added reconfigs to the upgrade/downgrade path that will change the name of the delay field name on replica set member configs. This behavior is hidden behind a feature flag. When the feature flag is enabled and we perform a reconfig on downgrade to change the field name from secondaryDelaySecs to slaveDelay, the test index_downgrade_fcv.js fails due to an invariant failure here. The error code is ReadConcernMajorityNotAvailableYet, which makes me suspect the index build component failed while attempting to establish a read concern majority cursor for the collection scan, as the system was in the middle of a reconfig. One potential solution is to busy-wait until read concern majority is available, which we seem to do here.
As a note, we have disabled the index_downgrade_fcv.js test for feature flag enabled variants in SERVER-50423. We should make sure to enable this test again once this ticket is closed.
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